Stellar Archaeology as a Time Machine to the First Stars

3 Dec 2018, 08:30 → 7 Dec 2018, 18:00 Asia/Tokyo

Lecture Hall (Kavli IPMU)

Date:

3rd - 7th December 2018

Venue:  

Lecture Hall, Kavli IPMU 

Scientific Rationale:

The number of observed extremely metal poor stars has increased rapidly with new surveys and more powerful instruments in the last years. This wealth of observational data has the potential to constrain the nature of the first generations of stars and the chemical evolution of the Milky Way. Especially the recent focus on dwarf galaxies and progress in the theoretical modelling of the first supernovae has helped to connect EMP stars to the first stars in the Universe. However, many questions remain open in theory and observations.

The goal of this conference is to discuss and promote the connection between stellar archaeology and the first stars. We want to present systematically different approaches in constraining the nature of the first stars, such as stellar archaeology, numerical simulations, or gravitational waves to initiate synergetic discussion between the communities. What are common challenges, future observational perspectives, and what needs to be done in the next decade?

Being held in Japan, we also would like to encourage discussions on recent achievements and future prospects based on the Japanese 8.2m Subaru telescope, especially using the High Dispersion Spectrograph (HDS), Hyper Suprime-Cam (HSC) or Prime Focus Spectrograph (PFS) on the field of stellar archaeology.

Main Topics:

• EMP stars
• Pop III supernova yields
• Carbon enhanced metal-poor stars
• Numerical simulations of Pop III star formation
• Further Pop III constraints (GWs, supernovae, 21cm signal,…)
• Transition to Pop II formation (dust cooling, critical metallicity)
• s- and r-process nucleosynthesis
• Dwarf galaxies & Galactic chemical evolution
• 3D/NLTE analysis
• Science with the Subaru Telescope

We encourage especially young researchers and students to attend the conference.

 

Confirmed Invited Speakers: Anish Amarsi, Timothy Beers, Piercarlo Bonifacio, Volker Bromm, Gen Chiaki, Anna Frebel (remote), Alex Heger, Raphael Hirschi, Alexander Ji, Amanda Karakas, Tomoya Kinugawa, Evan Kirby, Chiaki Kobayashi, Haining Li, Vinicius Placco, Stefania Salvadori, Anna Schauer, Britton Smith, Else Starkenburg (remote), Eline Tolstoy

 

Phishing Alert: We did not authorise a travel agency to make hotel reservations for our participants. Please let us know if you are contacted by a third party regarding this conference.

 

Sponsors:

  

 

 

 

 

This workshop is supported in part by MEXT Grant-in-Aid for Scientific Research, Kiban A: No.17H01101 for Masashi Chiba, Keban A: No. 16H02168 for Wako Aoki. The workshop is also supported by 天文学振興財団

 

 

Our twitter account and facebook group are opened!

Please follow us and tweet with #TimeMachineIPMU !

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Monday 3 December

08:30 Breakfast & Registration

1 Welcome & Opening Remarks

Pop III Simulations: (Chair: Volker Bromm)

2 The formation of the first stars

In this talk, I will shortly review some basic properties of the first stars. These first luminous objects form out of metal-free gas clouds in so-called minihaloes at high redshift. However, large-scale effects such as streaming velocities and Lyman-Werner radiation can delay their formation. I will give an overview over the literature and show results from my own cosmological simulations.

Speaker: Dr Anna Schauer (University of Texas)

Schauer.pdf

3 Simulating the formation of the first stars

I summarize the remaining questions to uncover the first star formation and the initial mass function.

Speaker: Dr Shingo Hirano (Kyushu University)

hirano.pdf

4 Ionization degree and magnetic diffusivities in the low-metallicity star formation

Magnetic field changes the angular momentum in a star-forming cloud by magnetic braking and driving an outflow. It affects the formation of binaries and multiple stellar systems. Since the coupling between the gas and magnetic field depends on the ionization degree of a cloud, the accurate calculation of the ionization degree is needed in star-formation. We calculate the thermal and chemical evolution of primordial star-forming clouds by using a chemical network in which reverse reactions are considered for all the forward reactions. Considering reverse reactions only for 1/5 of the forward reactions, previous studies can not calculate the ionization degree in the transition from chemical non-equilibrium to equilibrium. We find that at $\sim 10^{14}-10^{18}\ {\rm cm}^{-3}$, the ionization degree becomes 10-100 times higher than that obtained in the previous studies. This is due to the lithium photoionization by thermal photons, which is missed in the previous studies. We also calculate the magnetic diffusivities and discuss the implication of our results.

Speaker: Nakauchi Daisuke

Archaeology_nakauchi_submit.pdf

11:00 Break

Pop III Simulations: (Chair: Gen Chiaki)

5 Feedback and Fragmentation: key processes in high-mass star formation across the cosmic time

I present our resent efforts to understand key processes in the Pop III and low-metallicity star formation. One is the protostellar UV feedback, which is known to be strong enough to halt the accretion onto stars for Pop III cases. However, our knowledge on the UV feedback with non-zero metallicity is actually limited. Our recent radiation-hydrodynamic simulations show that, for cases with Z=Zsun, the UV feedback plays totally the opposite effect, i.e., the UV feedback rather enhances the mass accretion (Kuiper & Hosokawa 2018). I explain why.

The other process is the disk fragmentation, which potentially leads to the formation of binary stellar systems. 3D simulations show that the disk fragmentation easily occurs but merger of the fragments also easily occurs. Although the survival rate of the fragments is important, it is uncertain which physical processes control it. To understand this, we have performed a suite of 3D simulations to follow the orbital evolution of fragments in a well controlled manner (Chon & Hosokawa in prep.). The simulations show great diversity of the outcome, i.e., the merger or long-term survival as binaries, depending on different settings. I show that such a diversity is actually well understood with simple analytical considerations.

Speaker: Dr Takashi Hosokawa (Kyoto university)

hosokawa.pdf

6 Radiative feedback and final stellar masses in low-metallicity environments

In massive star formation, the radiative feedback is a key process in general because it potentially halts the accretion flow to limit the stellar masses. Previous studies show that the UV feedback, the dynamical expansion of an HII region and photoevaporation of a circumstellar disk, plays such a role in the primordial star formation(e.g.,McKee & Tan 2008; Hosokawa et al.2016). However, it is unknown how effects of the radiative feedback vary with increasing the metallicity. Semi-analytic models predict that the presence of dust grains qualitatively modifies how the radiative feedback operates (e.g.,Fukushima et al.2018).

In this talk, I present our recent 2D radiation-hydrodynamic simulations of the massive star formation at various metallicities, aiming to understand the variation of the radiative feedback strength and resulting final stellar masses. Our early results already show some interesting features. The UV feedback becomes weak with increasing the metallicity, owing to the dust attenuation of ionizing photons. The radiation pressure exerted on dust grains gets stronger instead to become the dominant feedback at $Z>~0.01Z_{\odot}$. However, strength of the radiation-pressure seems limited for cases where the mass accretion occurs through the circumstellar disk. The resulting star formation efficiency thus may be higher with the higher metallicities.

Speaker: Hajime Fukushima (Kyoto University)

Fukushima.pdf

7 The very young first galaxy SED: Contribution from Pre-Main-Sequence Stars

It is expected that the first galaxies will be observed by the next generation telescopes, such as NASA's James Webb Space Telescope (JWST). The early star-forming galaxies are expected to brighten in the rest frame near-infrared. Understanding their spectra accurately is important to prepare future observations. In the conventional calculations of spectra of galaxies, Pre-main-sequence (PMS) stars are not considered. In nearby galaxies, this assumption does not matter. However, PMS stars in the first galaxies, which do not contain heavy elements, can be directly observed. For young first galaxies, the contribution from PMS stars can be significant.
We calculate the spectral energy distribution (SED) of the first galaxies, which contain PMS stars, by calculating contribution from PMS stars by combining the stellar evolution code MESA and the spectra model BT-Settl model, and contribution from MS stars by using the stellar population synthesis code PEGASE. We estimate the SEDs for various models with different IMFs, ages, and star formation histories. We find that the contribution from PMS increases the AB magnitude by 1 in mid-infrared, and PMS stars' contribution can be significant over 0.5 Myr after a star-formation episode. Thus, we reveal the importance of PMS in the first galaxy SED.

Speaker: Hiroto Mitani

Hiroto_Mitani.pdf

12:30 Break

EMP Stars: Observation: (Chair: Miho Ishigaki)

8 Extremely metal-poor stars: new entries and new insights

In the last seven years the search and analysis of extremely metal-poor (EMP) stars has witnessed an efervescent activity by various groups that adopted different successful strategies to find large numbers of EMP stars. Suffice to say that the number of stars with metallicity below -4.5 has increased almost by a factor of five since 2010. In my review I will try to capture this exceptional activity from the observational side and the new insights (and problems !) that the newdiscoveries pose. I will also outline some of the future persepctives in the field that rest on future instruments and surveys.

Speaker: Prof. Piercarlo Bonifacio (Observatoire de Paris)

9 LAMOST/Subaru project: searching for metal-poor stars, moving groups and alpha-deficient stars

Metal-poor stars in the Milky Way is of special importance to understand the history of Galactic evolution and early nucleosynthesis. While various groups of stars showing peculiarities in kinematics and/or chemical composition are evidence of the merging process of the halo, and unusual events such as particular nucleosynthesis environments. However, these stars are rare. With its large aperture and high spectrum acquiring rate, LAMOST is capable to obtain millions of spectra of Galactic stars, and hence provides an unprecedented opportunity to identify such rare objects in the Milky Way. Through the ongoing LAMOST survey, a large sample of candidates of metal-poor stars and stars with abnormal kinematics and/or abundance patterns has been obtained. Follow-up observations with Subaru telescope have been carried out to explore detailed abundance pattern for part of the sample. In this talk, we report progress of the LAMOST/Subaru project on exploring the early evolution and formation of the Galaxy through metal-poor stars, moving groups and alpha-deficient stars. We have established a catalogue of ten thousand bright very metal-poor stars ([Fe/H] < -2.0) stars including 670 candidates of extremely metal-poor ([Fe/H] < -3.0) and ultra metal-poor ([Fe/H] < -4.0) stars.

Speaker: Dr Haining Li (NAOC)

TimeMachine_LAMOST-Subaru_v1.pdf

10 The recent discovery of two extremely metal-poor dwarf stars in the Galactic halo

The most ancient stars witnessed the assembly of the Galaxy, and they are key for studying the chemical history of the Milky Way. Extremely metal-poor stars offer an opportunity to learn about low-mass star formation, Galactic evolution and supernovae yields. During the last few years we have been exploring the SDSS and LAMOST low-resolution spectroscopic surveys to identify stars at [Fe/H] < -3. Using medium-resolution spectroscopy with ISIS at WHT and OSIRIS at GTC we have followed-up more than 60 promising candidates. Six stars were confirmed to belong to the regime [Fe/H] < -4. We have in particular discovered two very primitive dwarf stars, both with Teff~6200 K: SDSS J0815+4729 at [Fe/H]<-5 and SDSS J0023+0307 at [Fe/H]<-6. The mere existence of these two stars provides new constraints on the properties of first stars and demonstrates that our methodology is highly efficient identifying metal-poor candidates from large spectroscopic surveys.

Speaker: David D. Aguado (Instituto de Astrofísica de Canarias)

talk_tokyo_aguado.pdf

15:35 Break

EMP Stars: Observation: (Chair: Haining Li)

11 An Ultra Metal-poor Star Near the Hydrogen-burning Limit

While theoretical simulations of Population III star formation show that protostellar disks can fragment, it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period $P=34.757\pm0.010$ d in the ultra metal-poor (UMP) single-lined spectroscopic binary system 2MASS J18082002--5104378. The secondary star 2MASS J18082002--5104378 B has a mass $M_{2}=0.14_{-0.01}^{+0.06}~M_{\odot}$, placing it near the hydrogen-burning limit for its composition. The 2MASS J18082002--5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002--5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars to explore the formation of low-mass Population III stars via disk fragmentation. We find evidence that the survival of low-mass secondaries around solar-mass UMP primaries implies the survival of solar-mass secondaries around Population III primaries with masses $10~M_{\odot}\leq~M_{\ast}\leq~100~M_{\odot}$.

Speaker: Prof. Kevin Schlaufman (Johns Hopkins University)

schlaufman.pdf

12 LAMOST-Subaru study of Li-rich stars in the Milky way disk and halo: Is Li-rich episode(s) in giants universal ?

Lithium (Li), a big-bang product that is sensitive to temperature in stellar interiors, is found to be enriched in 1$\%$ of giants in Milky Way and dwarf-spheroidals which pose challenges in low mass star's evolution. A systematic search for Li-strong stars using LAMOST spectra doubled the existing catalog of Li-rich stars, and spans super-solar metallicity to beyond [Fe/H]$\sim$-3 in different Galactic populations. Abundance analysis of selected candidates in disk and halo, using Subaru HDS spectra, confirmed Li-richness. The sample on HR-diagram occupied from subgiant branch to all along RGB, indicates multiple origins for Li anomaly. Existence of Li-rich subgiants suggest high Li in post-first-drege-up giants may preserved from its progenitor, however, internal Li production followed by extra-mixing and/or binary mergers are favorable for giants close to luminosity bump. Be abundances derived from Subaru near-UV spectra suggests no clear correlation with Li, putting constraints on pollution from planet-ingestion. Absence of anomaly in other elements put constraints on pollution from binary companions. We will discuss possible Li origins for our sample and focus on whether any common mechanism that could explain Li enrichment in giants spanning large metallicity range, which provide constraints on Li contribution from metal-poor giants toward Galactic Li enrichment.

Speaker: Dr Yerra Bharat Kumar (NAOC, Beijing)

Bharat_TimeMachine_IPMU_Tokyo2018.pdf

13 The lithium isotopic ratio of the metal-poor spectroscopic binary CS 22876-032: the cosmological Li problem.

The discovery of the 7Li abundance plateau in metal-poor dwarf stars by Spite & Spite (1982) was considered as a signature of the nucleosynthesis in the Early Universe. However, independent determination of the baryonic density from the CMB fluctuations implies a factor of 3−4 larger primordial Li abundance. More recent observations have exhibited even a lithium abundance meltdown and increased scatter towards lower metallicities.

The 6Li isotope was announced by Asplund et al. (2006) to show a plateau at a lower level, but even much larger than expected from the standard theory. However, Cayrel et al. (2007) demonstrated, using 3D-NLTE hydrodynamical simulations, that the 670.8nm Li line asymmetry associated with 6Li was probably caused by convective flows (see also Steffen et al. 2012; Lind et al. 2013).

We got UVES high-resolution (at R~110,000) and high-quality (S/N~580) spectrum of the EMP binary CS22876-032 ([Fe/H] ~ -3.7), allowing us to to investigate the 6Li/7Li at about 0.5 dex below the previous attempts in EMPs.

In this talk I will show a brief summary of cosmological Li problem and the results and the implications of the analysis of the 6Li/7Li in the EMP binary CS 22876-032 using 3D-NLTE spectral synthesis tools.

Speaker: Jonay I. Gonzalez-Hernandez (Instituto de Astrofísica de Canarias)

tokyo_Li6_jonayf.pdf

14 3D non-LTE abundances in metal-poor stars

Stellar abundance measurements are heavily model dependent, and for late-type
stars, and especially for metal-poor late-type stars, the accuracy is often
limited by the use of one-dimensional (1D) hydrostatic model atmospheres and
the assumption of local thermodynamic equilibrium (LTE). Recently it has
become feasible to relax both assumptions simultaneously, i.e. to perform 3D
non-LTE spectroscopic analyses. In this talk I shall discuss the physics of
this more accurate method and review recent developments, including our latest
results for carbon, oxygen, and iron abundances in the metal-poor halo.

Speaker: Dr Anish Amarsi (MPIA)

amarsi.pdf

15 Observing the Pristine Galaxy

The lowest metallicity stars that still exist today probably carry the imprint of very few supernova. As such, they represent our best observational approach to understand the First Stars. In this talk I will review the early (chemical) evolution of the Milky Way system from both modeling and observational perspectives. In particular, I will present results of the Pristine survey, a Franco-Canadian photometric narrow-band survey designed to efficiently decompose the metallicity structure of the Milky Way halo. I will show how we can use this great discriminatory power to hunt for the very rare extremely metal-poor stars (bearers of the chemical imprint of the first stars) and greatly improve our study of metal-poor satellites in the halo.

Speaker: Dr Else Starkenburg (AIP)

16 Welcome Reception

Tuesday 4 December

08:30 Breakfast

Supernova Yields: (Chair: Alexey Tolstov)

17 Nucleosynthesis in massive and very massive Pop III stars

Understanding the early chemical evolution of the Galaxy requires to understand the nucleosynthesis in the supernovae of the first generations of stars. The properties of these massive stars and of their explosions determine the nucleosynthesis patterns we expect. Unfortunately, direct observation of these supernovae will remain difficult except for some special cases. Conversely, indeed, the fossil records in ultra-metal poor stars may be used to derive properties of the first stars and their supernovae. Abundance trends in the early galaxy can give us clues about explosion properties of early generations of stars. In my talk I will aim to give a review of supernova nucleosynthesis yields from single massive and very massive Pop III stars. I will include results from recent studies on hypernovae yields for Pop III stars as well as for pair-instability supernovae.

Speaker: Alex Heger

IPMU-20181204-Heger.pdf

18 Supernova nucleosynthesis

After the Big Bang nucleosynthesis, the first heavy element enrichment in the Universe was made by a supernova (SN) explosion of a population (Pop) III star (Pop III SN). The abundance ratios of elements produced from Pop III SNe are recorded in abundance patterns of extremely metal-poor (EMP) stars. The observations of the increasing number of EMP stars have made it possible to statistically constrain the explosion properties of Pop III SNe. I briefly summarize the supernova nucleosynthesis of Pop III SNe inferred from the observations of EMP stars.

Speaker: Nozomu Tominaga

tominaga.pdf

19 The initial mass function of the first stars inferred from elemental abundances in extremely metal-poor stars

We develop the code to fit observed elemental abundance patterns with the supernova yield models of the first (metal-free) stars. The yield models of first-star's masses in the range 13-100Msun with several different explosion energies are calculated based on the mixing-fallback model to approximately take into account the mixing and fallback of elements in aspherical explosions. We use this code to fit elemental abundance patterns of more than 200 extremely metal-poor ([Fe/H]<-3) stars compiled from literature. The results suggest that the mass function of the first stars that have contributed to the first chemical enrichment is peaked at ~ 25Msun with smaller contributions from lower-mass first stars. I will discuss their implications, limitations and the application to an expanded sample of extremely metal-poor stars.

Speaker: Miho Ishigaki (IPMU, the Univ. of Tokyo)

mishigaki_FirstStars2.pdf

10:55 Break

Supernova Yields: (Chair: Nozomu Tominaga)

20 Models for Unusual Supernovae and Possible Connections to Extremely Metal-Poor Stars

It is remarkable that quite a large diversity of supernova properties have been observed (e.g., very faint and superluminous supernovae, weak and extremely energetic explosions). Some unusual supernovae might have important influences in Pop III era. We examine the origin and nucleosynthesis properties of such unusual supernovae, and compare them with the abundance patterns observed in extremely metal-poor stars.

Speaker: Dr Ken'ichi Nomoto (Kavli IPMU, University of Tokyo)

First_nomoto.pdf

21 Photometric indicators of first superovae: radiation hydrodynamics simulations

Nowadays in the nearby Universe astronomers detect about 10 supernovae (SNe) per day. All these SNe lead to the formation of metal-rich stars in billions of years after the explosion. In the coming years in the distant Universe or metal-free gas pockets we expect the detection of the first SNe. Their progenitors are zero-metal, compact stars and the explosion leads to the formation of metal-poor stars in next generation. The first SN explosions have strong dynamical, thermal, and chemical feedback on the formation of subsequent stars and evolution of galaxies. The question is how to identify first SNe? How do they explode? Using the observed abundance patterns of the metal poor stars we perform radiation hydrodynamics simulations to find the difference between the first and normal SNe. We find that first SNe are usually bluer, shorter and fainter. The peculiarities of the color evolution can be used as easy-to-use indicator of the first SNe by current and future surveys.

Speaker: Alexey Tolstov (Kavli IPMU, Th University of Tokyo)

Tolstov-TimeMachineIPMU2018.pdf

22 Nucleosynthesis yield of primordial supernovae by bipolar jet-induced explosion

Recent stellar nucleosynthesis yield survey of massive star has demonstrated the needs of highly aspherical explosion in massive stars, i.e. bipolar jet-induced explosion [Grimmett et al., MNRAS 479, 495 (2018)]. Such explosion model is known to be the candidate for explaining the carbon-enhanced metal-poor stars. However, so far there is very little understanding to explore or constrain the properties of such jet model. In this presentation we will report our recent progress on the simulation results and nucleosynthesis yields of the bipolar jet-induced supernovae based on our array of two-dimensional hydrodynamics models with nucleosynthesis. By using the massive star models of masses from 40 – 80 $M_{{\rm sun}}$ as the progenitors, we study how the nucleosynthesis yield depends on the energetics of the jet, including the energy injected by the jet, the jet duration, the jet geometry and so on. The applications to the carbon-enriched metal-poor stars, and the influences of this model on the odd-number elements (e.g. K, Cl, Sc), are discussed.

Speaker: Dr Shing Chi Leung (Kavli IPMU, The University of Tokyo)

Presentation_SATM2018_Leung2.pdf

12:30 Break

s-/r-process: (Chair: Takuma Suda)

23 The s-process in low and intermediate-mass stars

The chemical evolution of the Universe is governed by the nucleosynthesis contribution from stars, which in turn is determined primarily by the initial stellar mass. I will review models of the slow neutron capture process (the s-process) and stellar yields from single metal-poor stars up to about 8 solar mass. Stars in this mass range evolve to become cool red giants after the main sequence.
It is during the giant branches that these stars experience mixing events that change the surface composition, with significant enrichments in carbon and heavy elements synthesized by the s-process. While the qualitative picture of the s-process is well known, there are major uncertainties that affect stellar yields. These problems are particularly problematic for metal-poor stars owing to a
lack of observational constraints. I will discuss some of these uncertainties and also highlight areas where progress has been made.

Speaker: Amanda Karakas (Monash University)

japan-karakas-2018.pdf

24 Stellar evolution at low metallicity and in metal-free environment

In this talk, I will review the evolution of stars at low metallicity (Z) and in metal-free environments. I will focus on massive stars, rotation and mass loss and their dependence on metallicity. I will also review key uncertainties, both stellar and nuclear. Finally, I will present results on the (not always) weak s process in massive low-Z rotating stars.

Speaker: Raphael Hirschi (Keele University, IPMU)

hirschi_Tokyo_Time_Machine_2018.pdf

25 Impact of Neutron Star Merger and Supernova Nucleosynthesis on the Element Genesis and Neutrino Physics

GW170817/SSS17a was an event of the century that opened a new window to multi-messenger astronomy and astrophysics. Optical and near-infrared emissions among them suggest that their total energy release is consistent with radiative decays of theoretical prediction of r-process nuclei although no specific r-process element was identified. Core-collapse supernovae (both MHD Jet- and ν-SNe) are viable candidates for the r-process. MHD Jet-SNe explain the universality in the observed elemental r-process abundance pattern in metal poor stars. Neutron star merger (NSM), on the other hand, could not contribute to the early Galaxy for cosmologically long merging time-scale for slow GW radiation. Nevertheless, NSM is still a possible explanation for the solar-system r-process abundance. We propose a novel solution to this twisted problem by carrying out NSM and SN r-process nucleosynthesis calculations in Galactic chemo-dynamical evolution.
We also discuss the impact on neutrino oscillation physics. Heavy elements originate from many processes such as r-, s-, νp-processes. We find that νp-process operates with amounts of free neutrons via weak interactions due to the collective neutrino oscillations. Reaction flows can reach the production of abundant p-nuclei 94Mo, 96Ru, etc. This nucleosynthetic method turns out to be a unique probe indicating still unknown neutrino-mass hierarchy.

Speaker: Taka Kajino (NAOJ / UT / Being University)

Kajino-IPMU2018-subm.pdf

15:10 Break

s-/r-process: (Chair: Tadafumi Matsuno)

26 Neutron-capture Nucleosynthesis in Relics of the First Galaxies

Ultra-faint dwarf galaxies are ideal systems to perform stellar
archaeology, as each galaxy provides multiple metal-poor stars sampling an
independent burst of star formation and chemical enrichment from the early
universe. In this talk, I will discuss the neutron-capture element
signature in ultra-faint dwarf galaxies, which chemically distinguish these
systems from most halo stars, globular clusters, and larger dwarf galaxies.
The stars in these tiny galaxies display highly stochastic neutron-capture
element enrichment, with a neutron-capture element floor of unknown origin.
I will also show some early evidence that r-process elements provide a
means to link ultra-faint dwarf galaxies to the Milky Way's old stellar
halo.

Speaker: Dr Alexander Ji (Carnegie Observatories )

Ji_IPMUTimeMachine.pdf

27 s-Process Nucleosynthesis in the Progenitors of Carbon-Enhanced Metal-Poor Stars

The first stars in the universe must have provided chemical imprints on the surface of observed metal-poor stars with [Fe/H]<-2. One of the most important signatures is the abundances of carbon-enhanced metal-poor (CEMP) stars, which is defined as [C/Fe]>=0.7. The origin of a CEMP star and its subclasses such as CEMP-s and CEMP-no stars, divided by the enhancement of s-process elements, engages the keenest attention of those who work on first stars and stellar archaeology. However, in spite of the obvious importance of the role of the s-process in AGB stars at low-metallicity, theoretical models and their applications to CEMP stars with [Fe/H]<-2.5 are not well investigated, while it is established that metal-poor AGB stars undergo a proton ingestion episode that triggers the s-process. In this paper, three modes of the s-process, the convective 13C burning triggered by the proton ingestion episode, the convective 22Ne burning, and the radiative 13C burning, are considered, using a detailed nucleosynthesis code customised to compute the s-process in metal-poor AGB stars. We argue that the abundance distribution of neutron-capture elements can be interpreted as that of a contribution from the s-process, and hence, Ba in CEMP-s and CEMP-no stars share the same nucleosynthetic origin.

Speaker: Dr Takuma Suda (RESCEU, the Univ. of Tokyo)

suda.pdf

Poster Short Presentations: (Chair: Alexey Tolstov)

28 Chemical Enrichment by the First Supernovae with two AMR codes

The first stars forged the first metals inside their stellar cores and eventually ejected them to the primordial IGM medium through supernovae explosions. These metals significantly influence the next generation of star formation. Therefore, it is very important to understand how these metals chemically enriched the early universe. In this poster, we use two popular codes, ZEUS-MP and FLASH, and modify them to simulate the explosion process of the first supernovae in the minihalos. We find the mixing of SNe ejecta caused by the Rayleigh-Taylor instability due to the explosion energy and halo masses. Our results can help to understand the abundance pattern of some metal-poor stars that are thought to form after the first stars.

Speaker: Chia-Jung Hsu (Chalmers University of Technology)

01-onepage slide_Hsu.pdf

29 Formation of carbon-enhanced metal poor (CEMP) stars in the early Universe

We present here a three-dimesional hydrodynamical simulation for star formation. Our aim is to explore the effect of the metal-line cooling on the thermodynamics of the star-formation process. We explore the effect of changing the metallicty of the gas from $Z/Z_{\odot}=-4$ to $Z/Z_{\odot}=-2$. Furthermore, we explore the implications of using the observational abundance pattern of a set of CEMP-no stars, which have been considered as truly second-generation stars.

In order to pursue our aim, we modelled the microphysics by employing the public astrochemistry package KROME, using a chemical network which includes sixteen chemical species. We couple KROME with the fully three-dimensional hydrodynamical SPH code GRADSPH. With this framework we investigate the collapse of a metal-enhanced cloud, exploring the fragmentation process and the formation of stars.

We found that the metallicity has a clearly impact on the thermodynamics of the collapse, allowing the cloud to reach the CMB temperature floor for a metallicity $Z/Z_{\odot}=-2$, which is in agreement with previous work. As long as only metal line cooling is considered, our results support the metallicity threshold proposed by Bromm+2001, which will very likely regulate the first episode of fragmentation and potentially determine the masses of the resulting star clusters.

Speaker: Mr Patricio Fibla (Universidad de Concepcion)

02-Poster_presentation_pfibla.pdf

30 Searching for extremely metal-poor star candidates from LAMOST and SDSS survey

The very metal-poor stars are important in the Milky Way, which record the heavy element abundances produced in the first generations of stars, thus can help us to understand the earliest nucleosynthesis events. Thanks to the large sky surveys like HK survey, Hamburg/ESO survey, SDSS, RAVE, SkyMapper, the number of very metal-poor stars especially the Extremely Metal-Poor stars (EMP, [Fe/H]<-3) have been increased. The abundance analysis method based on the Equivalent Width (WD) of CaII K absorption line and stellar atmospheric model had been verified to be valid to search EMP candidates especially the Ultra Metal-Poor stars (UMP, [Fe/H]<-4]). Here we used this method to search for new EMP candidates from the LAMOST and SDSS low resolution spectra. We re-determined the [Fe/H] for all the stars with [Fe/H] < -2 measured by the LSP3 (Xiang et al. 2016). While for the SDSS sample, we re-calculated the [Fe/H] for the stars with [Fe/H] <-2.5 measured by SSPP. Here we present the EMP candidates found by us from LAMOST and SDSS. For those having high resolution spectroscopic analysis results, the agreements are very good.

Speaker: Prof. Huawei Zhang (Department of Astronomy, Peking University, China)

03-HuaweiZhang.pdf

MP_poster_Zhang.pdf

31 Where do Population III Stars Form? The Effects of Radiative Feedback and Self-Shielding on the Host Halo Mass Distribution

We perform a cosmological simulation with a comoving volume of 1 Mpc$^{3}$ to study the birthplaces of Population III stars, using the adaptive mesh refinement code Enzo. We investigate the distribution of host halo masses and its relationship to the Lyman-Werner background intensity. In our sample of 697 host halos, we find that 84% of them have masses below the Machecek et al. (2001) relation because of the inclusion of H2 self-shielding. In our simulation above a redshift of 12.5, the mean halo mass is time-independent and ~10$^{5.8}$ solar masses. Afterwards, it steadily rises above the Machacek et al. relation to a mean value of ~10$^{6.6}$ solar masses. Most of these halos form multiple Population III stars, with a median number of four, up to a maximum of 16. We also find that a few halos do form stars below the Machacek et al. relation but in a high Lyman-Werner radiation field with values up to ~50 J$_{21}$. Our results suggest that Population III star formation may be less affected by Lyman-Werner radiation feedback than previously thought and that Population III multiple systems are common

Speaker: Ms Danielle Skinner (Georgia Institute of Technology)

Poster_sample_Kavli_Skinner.pdf

32 Stellar Evolution of Pop III and EMP Binaries

Modern cosmological simulations suggest that one massive Pop-III star might form into multi stars due to the fragmentations of the star-forming cloud. Most of these stars are likely to develop into binaries instead of single isolated stars. In the case of close binaries, the interaction between two stars frequently occurs. It leads to drive a significant mass-transfer even affect the fate of the two stars. In this poster, I will show the results of our stellar evolution models of Pop-III/EMP binaries with MESA and discuss their physical properties. The results suggest that mass-transfer can dramatically affect the evolution track and the fate of these binaries. Therefore, the stellar feedback of Pop-III/EMP binaries may have a profound impact on the early Universe.

Speaker: Mr Sung-Han Tsai (ASIAA/NCU)

ipmu_1m_tsai.pdf

33 Implications of massive Population III binary systems for the ionizing photons and black hole binary mergers

Population III (Pop III) binary systems are expected to play a role in the re-ionization. Merging binary black hole (BBH) also can be formed by evolution of such binary system. We calculate the evolution of Pop III binary systems using the MESA code to investigate 1) the effect on the production of ionizing photons and 2) close BBH systems coalescing within the Hubble time. We consider the mass range of 25 ~ 100 solar masses for the primary star and initial mass ratios of q = 0.5 ~ 0.9. We find that the contribution of binary stars for the production of ionizing photons is not important compared to single stars. This is because envelope stripping by mass transfer is not significantly enough for the primary star to evolve blueward. However, formation of high mass X-ray binary systems in our simulation leaves a room of additional contribution to reionization. We also find that some binary systems in our simulations evolve into a close BBH system via stable mass transfer, without undergoing the common-envelope phase. We conclude that massive BBH systems (20 ~ 40 solar masses) as gravitational wave radiation sources can be produced from Pop III stars.

Speaker: Mr Hunchul Lee (Seoul National University)

06-1min_poster_Hunchul_Lee.pdf

34 Morphological Classification of Veiled CEMP Stars

We present a novel approach for categorical analysis of strongly carbon-enhanced metal-poor (CEMP) stars using medium-resolution (R~1,800) spectra. Analysis of cool (Teff ~ 4000 K) CEMP stars is largely inhibited by a strong depression of the underlying continuum (veiling) by extreme molecular bands, making normalization and fitting of metallic features such as Ca II H&K lines difficult.

Consequently, few metal-poor dwarf carbon stars are known, with which we can constrain the low-mass tail of the Pop II IMF. We present a new technique for spectral normalization of these stars, and explore a technique for assigning CEMP morphological types based on characteristic abundances seen in Group I, II, and III CEMP stars, using the Yoon-Beers diagram introduced in Yoon et al. (2016).

Speaker: Devin D. Whitten (University of Notre Dame)

07-Whitten_1min_slide.pdf

Whitten_A0_2018_final.pdf

35 Using Fluorine in Extremely Metal-Poor Stars to Test Population III Supernova Yields (Project)

Supernovae explosions of population III stars were the first enrichment mechanisms in the early universe, encoding chemical signatures in next-generation stars, where low-mass stars could have survived until now. These objects can reveal information of the cosmic chemical evolution and early formation stages of the Galaxy. Tominaga studied this developing models including Pop III SNe feedback yields well reproducing observations in EMPs.
This project aims study Fluorine abundances in EMP stars G77-61 and CS29498-2429. These, are predicted to hold enough Fluorine to be detected at these metallicities. High F abundances in EMP stars are due to the CNO cycle at the bottom of the H-layer during the shock wave propagation while exploding. The reaction takes place if the progenitor star has an N-rich layer near the core, carrying the synthesis at appropriate temperature. Therefore, F abundances could help to constrain N-enhancement in progenitors and explosion energy.
To obtain reliable spectra, this work will propose the first observations in this type of stars to detect F at these metallicities. Using CRIRES+, will allow measuring the HF molecular line at 2.3μm obtaining reliable measurements and information from the first stages of our Galaxy, better understand its formation and early chemical evolution.

Speaker: Mr Aldo Mura Guzman (Australian National University)

08-MinuteTalk_Aldo_Mura.pdf

36 A Zoom-in Simulation of the Accretion Flow surrounding a POP III Protostar

Population III stars are known to form in a warm environment due to a lack of coolant in the early Universe. The warm nature leads to a high accretion rate and a formation of a massive primordial accretion disk. A massive disk is known to be gravitationally unstable. In previous zoom-in cosmological numerical simulation, a spiral structure emerges out of the disk. The dense region then gradually collapses into several gravitationally bound clumps. To better understand the fragmentation process, we have developed a Poisson solver in the cylindrical coordinate. The numerical scheme allows us to have a closer look on the disk physics. I will present the numerical scheme that have been developed, and a preliminary calculation of the accretion process around POP III protostar.

Speaker: Wei-Ting Liao (University of Illinois)

09-wtliao_poster_talk.pdf

37 Children of the First Stars: Birth from a Population III Supernova

We present evidence for triggered star formation following a supernova of a metal-free (Population III) star, providing a direct connection between a single Population III star and a subset of extremely metal-poor stars. We simulate the formation and ensuing radiative and supernova feedback of several Population III stars in a cosmological volume with the adaptive mesh refinement code, Enzo. In the vicinity of one of the stars at redshift 15, the blastwave from a $3 \times 10^{52}$ erg supernova passes a molecular cloud of mass 770 $M_{\odot}$. Existing only 11 pc from the star, the cloud survives the blast and is not completely photo-evaporated. After the star explodes, the blastwave rapidly shock-heats the diffuse gas, increasing the external pressure, crushing the cloud into collapse. At the end of the simulation, 374 kyr after the SN, the cloud is gravitationally unstable and resolved with a maximum resolution of 4 AU. The inner 550 $M_{\odot}$ of the cloud exceeds the Bonnor-Ebert mass suggesting it will continue to collapse and form stars. We confirm the blastwave induces the collapse by performing another simulation without a supernova. As the photo-evaporative flow does not abate, the cloud is not pressure confined and continues to expand.

Speaker: Corey Brummel-Smith (Georgia Institute of Technology)

10-Brummel-Smith_Corey_single_slide.pdf

TriggeredSF_poster_final_Brummel-Smith.pdf

38 Magnesium enhanced stars of the Milky Way Galaxy

It is well known that magnesium is largely produced by massive stars which explode as core-collapse supernovae at the end of their evolutionary stage. We observed several magnesium-enhanced metal poor stars with the Gemini North 8m telescope and obtained high-resolution (R~42,000) spectroscopic data using the GRACES system. We measured the abundance ratios of alpha elements (e.g. Mg, Si) and some s-process elements (e.g. Y, Ba) of our targets and compared their abundance patterns with the nucleosynthesis yields from stellar models for different initial masses and various physical assumptions such as rapid rotation, and mixing and fall-back during supernova explosion. We find relatively high [Mg/Ni] ratios in our targets compared to other metal-poor stars. Further discussion is needed to explain our observation results.

Speaker: Hye-Eun Jang (Seoul National University)

11-Poster_Magnesium_Jang.pdf

39 Investigating the Nature of First-Generation Stars By Chemical Abundance Analysis of Ultra Metal-Poor Stars

Chemical abundance ratios of ultra metal-poor (UMP; [Fe/H] < -4.0) stars hold the key to understanding the nature of the first generation of stars born in the early Universe, as well as the nucleosynthesis processes associated with their evolution. UMP stars are believed to be true second generation stars and, despite their importance, only about two dozen have been discovered thus far. In an effort to search for additional such stars, we selected UMP candidates from low-resolution spectra (R ~ 2000) from the Sloan Digital Sky Survey, and obtained high-resolution (R ~ 40,000) spectra with Gemini/GRACES for the UMP candidates. In this study, we present for the UMP candidates results of chemical abundance analysis and comparison of measured abundance patterns with chemical yields predicted by a supernovae model in order to investigate their possible progenitors. Our results will be able to provide stringent constraints on the mass distribution of the first generation of stars, which are likely to be progenitors of the UMP objects.

Speaker: Ms Miji Jeong (Chungnam National University)

12-1min_miji jeong.pdf

40 Explosive nucleosynthesis in aspherical supernovae of Pop III stars

We have investigated explosive nucleosynthesis in core-collapse supernovae (SNe) of massive stars, based on two-dimensional hydrodynamic simulations of the SN explosion. Employing a simplified light-bulb scheme for neutrino transport and excising a central part of a proto-neutron star (PNS), we follow long-term evolution of the SN explosion over 1.0 second after the core bounce for 22 massive stars with the solar metallicity (from 10.8 to 40$M_\odot$) and 15 Pop III stars (from 10 to 40$M_\odot$). We adopt a PNS core model, with which we evaluate evolution of neutrino luminosities and temperatures as in Ugliano et al. 2012 and tune two parameters of the PNS core model for solar metallicity stars, so that a star with $\sim 20 M_\odot$ explodes as SN1987A-like, that means an explosion energy of $\sim 10^{51} \rm ergs$ and a Ni56 mass of $\sim 0.07 M_\odot$.

For the Pop III stars, we find IMF-averaged abundances of SN ejecta of the stars well reproduce averaged abundances of observed in metal-poor stars (Cayrel et al. 2004).
In particular, K, which is underproduced in previous theoretical evaluation based on 1D models, is abundantly produced in our model and the IMF-averaged abundances of K is comparable to the observed abundance.

Speaker: SHINICHIRO FUJIMOTO (National Institute of Technology, Kumamoto College)

Fujimoto-poster.pdf

41 The Assembly History of the Galactic Stellar Halo Traced by Carbon-Enhanced Metal-Poor Stars

We present an analysis of the kinematic properties of the Galactic halo stars, using over 100,000 main sequence turnoff (MSTO) stars observed in Sloan Digital Sky Survey. After separating the MSTO stars into an inner-halo region (IHR) and outer-halo region (OHR), based on the spatial variation of their [C/Fe], we find that stars in the OHR show a clear retrograde motion of -49 km/s and a more spherical distribution, while stars in the IHR exhibit zero net rotation (-3 km/s) with a much more radially biased distribution. Moreover, after classifying carbon-enhanced metal-poor (CEMP) stars among the MSTO sample into CEMP-s and CEMP-no objects by their absolute carbon abundances, we examine the spatial distributions of the fractions of CEMP-no and CEMP-s stars and the kinematics of each sub-class. The CEMP-no stars are the majority sub-class of CEMP stars in the OHR (~65%), and the minority sub-class in the IHR (~44%). The CEMP-no stars in each halo region exhibit slightly higher counter-rotation and a more spherical distribution of orbits than the CEMP-s stars. These distinct characteristics provide strong evidence that numerous low-mass satellite galaxies have donated stars to the OHR, while more-massive dwarf galaxies provided the dominant contribution to the IHR.

Speaker: Prof. Young Sun Lee (Chungnam National University)

14-YSLEE_1minute_talk.pdf

SA_YSLEE_Final.pdf

42 NLTE Analyses of high-Resolution H-band Spectra on Neutral and singly-Ionized Calcium

Calcium is one of the $\alpha$-elements. It can be used to analyze star formation, the Galactic structures, and the chemical enrichment of the Galaxy. APOGEE aims to explore red giants across all components of the Milky Way with providing high resolution and high S/N spectra. It has obtained more than 5,000,000 spectra for over 150,000 stars. We updated calcium atomic model with collisional rates from quantum-mechanical computations. We investigated the reliability of our model and the NLTE effects on optical and $H$-band Ca I lines. The study found that our model can give consistent abundances between optical and $H$-band Spectra. The NLTE effects of optical lines differ from line to line and NLTE effects on $H$-band lines in our parameter spaces are negligible.

Speaker: zeming zhou

15-Zeming Zhou -- NLTE Analysis of high-Resolution H-band Spectra.pdf

43 Study the neutron-capture processes based on the odd isotopic fractions of barium

Stars with enhanced n-capture elements are very important for our understanding on the characteristics of n-capture processes under environments with different metallicities. The odd isotopic fractions of barium could directly show the relative contributions from the s- and r-processes, even maybe from the i-process. In this talk, I will show some work on the odd isotopic fractions of barium determination for the r-II, r/s, and s-only stars. The characteristics of r- and s-processes will be discussed, and we also want to show some light on the i-process.

Speaker: Prof. Wenyuan Cui (Department of Physics, Hebei Normal University, China)

19-Cui-isotope-tokyo.pdf

poster_Tokyo_2018_1202_Cui.pdf

44 Studying the impact of IMF on element yields of Population III stars

The first stars in the Universe (Population III) are thought to be the
responsible for synthesizing the first heavy elements, thereby creating
the chemical conditions from which the second generation of stars has formed. One of the aims of
current investigations is to better constraint the Initial Mass Function
of Pop III stars from the observed abundance patterns of extremely
metal poor stars, which is essential to model their influence on cosmic
history. We present a set of predictions for the abundance ratios of C,
O, N, Ba, Eu and CNO yields, assuming different types of IMFs. We will
briefly discuss the parameter space that is consistent with the extremely
metal poor stars known so far.

Speaker: Camila Olave (Universidad de Concepción)

17-stellararcheologyslideOlave.pdf

45 Manganese as an Archaeological Tool to Study Type Ia Supernovae in Dwarf Galaxies

Ancient, metal-poor dwarf galaxies provide some of the best links to the earliest nucleosynthesis in the nearby Universe. Stellar abundances in dwarf galaxies can be used to estimate nucleosynthetic yields from supernovae, which can in turn be used to distinguish among different physical models of these supernovae. In particular, manganese is a sensitive probe of the density of the white dwarf progenitors of thermonuclear supernovae (Type Ia SNe). In this work, we report preliminary work using Keck/DEIMOS medium-resolution spectroscopy to measure manganese abundances in classical dSph galaxies. We present initial validation of our measurement technique by testing on globular clusters, and we discuss potential implications for Type Ia supernova physics. We tentatively conclude that the majority of Type Ia SNe in ancient dwarf galaxies exploded below the Chandrasekhar mass.

Speaker: Mithi Alexa de los Reyes (Caltech)

18-delosReyes_KavliIPMU_gong.pdf

46 Dust structure and ISM interaction around C-Rich AGB star

Speaker: Lochan Khanal

lochan slide in pdf.pdf

16:45 Break & Poster Viewing

47 Public Outreach Event

Wednesday 5 December

08:30 Breakfast

48 Remote presentation: Anna Frebel

Transition to Pop II, Zcrit: (Chair: Shingo Hirano)

49 External Enrichment as a Pathway to Metal-Poor Star Formation

The physical conditions required for forming the first low mass stars are now understood to be a critical amount of heavy elements and some degree of turbulence to seed fragmentation. However, the transition from Population III to Population II has yet to be characterized in a global context. The process of external enrichment, in which a star-less halo is promptly enriched by the blast-wave from a nearby Pop III supernova, has been shown to be a viable channel for forming extremely metal-poor stars that trace a single progenitor. This is necessary for explaining the origins of the most metal-poor stars observed in the Milky Way. Beyond these rare events, what role does external enrichment play as structure formation and the gradual buildup of metals in the Universe continue? In this talk, I will present results from simulations extending past the first external enrichment event. I will discuss the physical conditions in metal-enriched star-forming environments that follow, including regions in which the metallicity exceeds the traditional gas-phase critical metallicity. I will also present measurements of the rates of external enrichment by single and multiple progenitors and discuss the consequences for metal-poor star searches within our own Galaxy.

Speaker: Dr Britton Smith (University of California, San Diego)

BrittonSmith.pdf

50 The first supernovae and the origins of metal-poor stars

Metals from Population III (Pop III) supernovae led to the formation of less massive Pop II stars in the early universe, altering the course of evolution of primeval galaxies and cosmological reionization. There are a variety of scenarios in which heavy elements from the first supernovae were taken up into second-generation stars, but cosmological simulations only model them on the largest scales. In this talk, we present small-scale, high-resolution simulations of the chemical enrichment of a primordial halo by a nearby supernova after partial evaporation by the progenitor star. We find that ejecta from the explosion crash into and mix violently with ablative flows driven off the halo by the star, creating dense, enriched clumps capable of collapsing into Pop II stars. Metals may mix less efficiently with the partially exposed core of the halo, so it might form either Pop III or Pop II stars. Both Pop II and III stars may thus form after the collision if the ejecta do not strip all the gas from the halo. The partial evaporation of the halo prior to the explosion is crucial to its later enrichment by the supernova.

Speaker: Dr Ke-Jung (Ken) Chen (ASIAA)

IPMU_first_Chen_Ken.pdf

51 Metal Mixing in Primordial Minihaloes and the Abundances in EMP Stars

There have been many attempts to investigate the first stars by analysing extremely metal-poor stars. Often, these studies rely on the assumption that the abundances observed in old, extremely metal-poor stars directly correspond to the yields of a single Pop III supernova (SN). We run high-redshift cosmological simulations to test this assumption. In these simulations we model the formation of the first stars, radiative and SN feedback and follow the metals produced in the first SNe to self-consistently predict the metal abundances in the second generation of stars. In this talk, I will discuss several characteristic cases with single and multiple selected SNe that give insights into the systematic effects of inhomogeneous metal mixing. I will also present preliminary results from simulations currently in progress that aim to provide a large self-consistent catalogue of simulated second generation stars.

Speaker: Mr Mattis Magg (ITA/ZAH University of Heidelberg)

presentation_MMagg.pdf

11:00 Break

EMP Stars: Theory: (Chair: Anna Schauer)

52 The first matter cycle in the Universe: from Population III to extremely metal-poor stars

We investigate the formation of extremely metal-poor (EMP) stars that are observed in the Galactic halo and neighboring ultra-faint dwarf galaxies. Their low metal abundances (${\rm [Fe/H]} < -3$) indicate that their parent clouds were enriched by a single or several supernovae (SNe) from the first (Pop III) stars. In this study, we perform numerical simulations of the formation sequence of a EMP star through the feedback effects from a Pop III star. We for the first time employ a metal/dust properties calculated consistently with the progenitor model. In a minihalo (MH) with mass $1.77\times 10^{6} \ {\rm M}_{\bigodot}$, a Pop III star with mass $13 \ {\rm M}_{\bigodot}$ forms at redshift $z=12.1$. After its SN explosion, the shocked gas falls back into the central MH internally enriching itself. The metallicity in the recollapsing region is $2.6\times 10^{-4} \ {\rm Z}_{\bigodot}$ (${\rm [Fe/H]} = -3.42$). The recollapsing cloud undergoes cooling by HD, CO, and OH molecules and heating along with H$_2$ formation. Eventually by grain growth and dust cooling, knotty filaments appear in the central 100 au region with the help of turbulence driven by the SN, leading to the formation of low-mass EMP stars surviving until the present day.

Speaker: Gen Chiaki (Georgia Tech)

181205TimeMachine_Chiaki.pdf

53 DWARF GALAXIES - NOW AND THEN

Dwarf galaxies were the most common type of systems in the Early Universe. The first stars were likely hosted by dwarf galaxies, which might have provided the bulk of ionizing photons driving the early phases of reionization. In the Local Group, metal-poor dwarf galaxies hosting 13 billion years old stars are extremely common. What can we learn from detailed observations of stars in these ancient companions of the Milky Way?
By presenting the most recent theoretical and observational findings, I will show how chemical abundance studies of ancient stars in both ultra-faint and classical dwarf galaxies allow us to probe different range of the unknown mass distribution of the first stars. Further, I will show how these studies can provide insightful connections between local dwarf galaxies and more distant Damped Lyman-Alpha systems.

Speaker: Dr Stefania Salvadori (University of Florence - Department of Physics and Astronomy)

54 Metal pollution of low-mass Population III stars through accretion of interstellar objects

We calculate accretion mass of interstellar objects (ISOs) like Oumuamua onto low-mass population III stars (Pop. III survivors), and estimate surface pollution of Pop. III survivors. An ISO number density estimated from the discovery ofOumuamua is so high ($\sim 0.2$ au$^{-3}$) that Pop.~III survivors have chances at colliding with ISOs about $10^5$ times per $1$~Gyr. `Oumuamua itself would be sublimated near Pop.~III survivors, since it has small size, $\sim 100$ m. However, ISOs with size $\ge 3$ km would reach the Pop. III survivor surfaces. Supposing an ISO cumulative number density with size larger than $D$ is $n \propto D^{-\alpha}$, Pop. III survivors can accrete ISO mass $\ge 10^{-16}M_\odot$, or ISO iron mass $\ge 10^{-17}M_\odot$, if $\alpha < 4$. This iron mass is larger than the accretion mass of interstellar medium (ISM) by several orders of magnitude. Taking into account material mixing in a convection zone of Pop.~III survivors, we obtain their surface pollution is typically [Fe/H] $\le -8$ in most cases, however the surface pollution of Pop.~III survivors with $0.8M_\odot$ can be [Fe/H] $\ge -6$ because of the very shallow convective layer. We first show the importance of ISOs for the metal pollution of Pop.~III survivors.

Speaker: Dr Ataru Tanikawa (The University of Tokyo)

tanikawa.pdf

55 Chemical enrichment and stellar population age

The supernovae of first stars produce metals that change the star formation mode in the early Universe from a top-heavy to the present-day IMF. However, the efficiency and governing processes of metal mixing are still unknown. By analysing cosmological simulation of the first galaxies, we find a strong correlation between metal mixing efficiency and time from star formation, whereas the total stellar mass in the halo only shows a weak correlation. The enrichment of a halo proceeds efficiently and after 10 million years already 50% of the hydrogen is mixed with metals on average. I will present a simple analytical prescription of metal mixing and demonstrate how it can be used to improve the sub-grid physics of semi-analytical simulations of EMP star formation and the chemical evolution of Milky Way progenitors.

Speaker: Yuta Tarumi

Yuta_Tarumi.pdf

56 The impact of metal depletion on convective mixing prescriptions in 1D stellar evolution models

1D stellar structure and evolution codes (SSECs) are widely used in astrophysics to determine fundamental stellar properties. Since our theoretical knowledge is incomplete, it is necessary to calibrate these codes empirically, especially free parameters such as the convective mixing length, or $\alpha_{\text{MLT}}$. Historically, we have relied on the Sun for this task. This has resulted in the ad hoc adoption of solar-valued parameters in models of stars with highly non-solar properties, including stars with very different chemical compositions.

With greater availability of high-precision observational data, however, we are recently able to calibrate SSECs using direct measurements of other stars.
My recent work presents empirical mixing length calibrations six stars with metallicities below [Fe/H]$\le-2.3$.
We find that sub-solar mixing lengths are required to reproduce the observed properties of our sample, suggesting a need to better constrain the relationship between $\alpha_{\text{MLT}}$ and abundance. These findings call into question the use of current stellar evolution databases, all of which assume solar-valued mixing lengths in their isochrones regardless of metallicity specification.
Our results likewise emphasize the need for more high-precision measurements of low-metallicity stars so that they may be used as calibrating agents for the next generation of highly accurate 1D stellar models.

Speaker: Dr Meridith Joyce (Australian National University)

JapanStellarArchaeology_Joyce.pdf

57 Social Event

Thursday 6 December

08:30 Breakfast

Further Pop III Constraints: (Chair: Tilman Hartwig)

58 Empirical Constraints for the Theory of Pop III Star Formation

Understanding the formation of the first (Pop III) stars at the end of the cosmic dark ages is one of the outstanding problems in modern cosmology. Based on numerical simulations, an increasingly detailed theoretical framework has emerged for how this happened. The key challenge now is to test our ideas with frontier observations over the next decade. The James Webb Space Telescope promises in-situ observations of hyper-energetic Pop III supernovae and possibly gamma-ray bursts. Other powerful probes are provided by 21cm cosmology, highlighted by the recent EDGES discovery of a global absorption spectral feature, and the unresolved cosmic infrared background. Ideally complementary are a number of fossil signatures, scrutinized in our local neighbourhood. Prime among them is the chemical abundance pattern left behind by Pop III nucleosynthesis in old, metal-poor stars. Finally, binary black hole remnants of massive Pop III progenitors may be detectable by LIGO through their gravitational wave signal.

Speaker: Prof. Volker Bromm (The University of Texas, Austin)

59 The remnants of first stars for gravitational wave sources

Using our population synthesis code, we found that the typical chirp mass of binary black holes (BH-BHs) whose origin is the first star (Pop III) is ~30Msun with the total mass of ~60Msun so that the inspiral chirp signal as well as quasi normal mode (QNM) of the merging black hole are interesting targets of LIGO,VIRGO and KAGRA (Kinugawa et al.2014 and 2016). The detection rate of the coalescing Pop III BH-BHs is ~ 180 events/yr (SFR_p/(10^{-2.5} Msun /yr/Mpc^3))([f_b/(1+f_b)]/0.33)Err_sys in our standard model where SFR_p, f_b and Err_sys are the peak value of the Pop III star formation rate, the binary fraction and the systematic error with Err_sys=1 for our standard model, respectively. Furthermore, we found that the chirp mass has a peak at ~30Msun in most of parameters and distribution functions (Kinugawa et al.2016). This result predicted the gravitational wave events like GW150914 and LIGO paper said ‘recently predicted BBH total masses agree astonishingly well with GW150914 and can have sufficiently long merger times to occur in the nearby universe (Kinugawa et al. 2014)’ (Abbot et al. ApJL 818,22 (2016)).
Thus, there is a good chance to check indirectly the existence of Pop III massive stars by gravitational waves.

Speaker: Dr Tomoya Kinugawa (The University of Tokyo)

KinugawaTimeMachine.pdf

60 Observability of low mass Pop III survivors in the Milky Way and dwarf galaxies

We study the number and the distribution of low mass Pop III stars in the Milky Way. In our model, hierarchical formation of dark matter minihalos and Milky Way sized halos are followed by high resolution cosmological simulations. We model the Pop III formation in H2 cooling minihalos without metal under UV radiation of the Lyman-Werner bands. Assuming a Kroupa IMF from 0.15 to 1.0 Msun for low mass Pop III stars, as a working hypothesis, we constrain the theoretical models in reverse by current and future observations.

We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, and the required number of dwarf galaxies to find one survivor is less than ten at 100 kpc for the tip of redgiant stars. Provided that available observations have not detected any survivors, the formation models of low mass Pop III stars with more than ten per minihalos are already excluded.

Speaker: Tomoaki Ishiyama

ishiyama.pdf

11:05 Break

Further Pop III Constraints: (Chair: Timothy Beers)

61 Aspherical supernova explosions in the First Milky Way progenitors

Elements heavier than hydrogen and helium were first produced in the universe within the first stars. After a few million years, these presumably massive stars exploded as the first supernovae, ejecting the newly forged elements. Theoretical investigations have long indicated that such supernovae would explode in an asymmetric fashion, but insufficient observational evidence has prevented in-depth studies. I will report on the first ever detection of a zinc line in the UV spectrum of an ancient hyper metal-poor second-generation star, HE 1327$-$2326 ([Fe/H]$=-5.20$), from which an abundance ratio of [Zn/Fe] = $0.80\pm0.25$ was obtained. Producing this large relative amount of zinc requires a high-entropy explosion environment, which could be achieved e.g., during an aspherical explosion with a bipolar outflow of a first star progenitor. Aspherical explosions in the early universe are in line with suggested fast rotational velocities of the First stars (Meynet 2006). I will discuss the consequent significant implications of such explosions on the the chemical enrichment across the early universe.

Speaker: Rana Ezzeddine (Massachusetts Institute of Technology/JINA-CEE)

RanaEzzeddine_Firststars-IPMU-Tokyo-122018.pdf

62 CAIUS: SYNTHETIC OBSERVATIONS USING A ROBUST RADIATIVE TRANSFER PIPELINE

As astronomers peer ever-deeper into the high-redshift Universe, a bevy of astrophysical questions on early star, black hole, and galactic structure formation await on the precipice of their elucidation. However, to truly and more completely decipher the first faint images from the Cosmic Dawn, a new generation of diagnostic and predictive tools is needed to bridge the gap between the state-of-the-art in astrophysical theory and observation. To this end, we have developed the CAIUS Monte Carlo radiative transfer pipeline, which takes cosmological simulations and produces synthetic observations and diagnostics for infrared space telescopes. Using our tools, we produce James Webb Space Telescope diagnostics for a direct-collapse black hole scenario as well as for a statistically significant sample of star-forming galaxies at z = 15. Our studies have found previously unexplored trends in both the evolution of the radiative environment in the early Universe as well as in the ways objects might be discerned and in particular, hope that we might soon observe the formation of a massive black hole.

Speaker: Dr Kirk Barrow (Georgia Institute of Technology)

BarrowIPMUpress.pdf

63 Growth of intermediate mass black holes in the first star clusters

We study the evolutions of eight primordial star clusters of Sakurai et al. (2017) after runaway stellar collisions and formation of massive stars and intermediate-mass black holes (IMBHs). Performing N-body simulations for ∼15Myr to follow star cluster dynamics with dark matter (DM) dynamics, we find that stars intrude into the IMBH and can cause tidal disruption events. By the TDEs, the IMBHs grow to 700−2500 Msun where the diversity is due to the difference of parent halo properties. We also find that the DM can affect the cluster evolutions by stripping stars from the outer part of the clusters. The stripping is caused by motion of the DM, causing increase of the DM density within the clusters and increase of velocity of the outer stars. By stripping massive stars, the DM can decrease the TDE rates. We compare our TDE rates with those of previous works. We discuss fates of the clusters and the IMBHs. In order for the IMBHs to become supermassive as observed at z>~6, an external mass supply by galaxy major mergers or cosmic flows is necessary, otherwise the IMBHs will remain as they are.

Speaker: Yuya Sakurai (Georgia Institute of Technology)

Sakurai.pdf

CEMP Stars: Theory: (Chair: Timothy Beers)

64 Low-mass primordial stars: i-process nucleosynthesis during core-flash proton ingestion episodes?

Models of zero metallicity and extremely metal-poor stars show that they evolve differently to their more metal-rich counterparts. In particular they suffer violent proton-ingestion episodes (PIEs) that lead to extreme carbon enrichment at the surface. The fresh carbon has a fundamental effect on their further evolution, and can be transferred to binary companions, producing CEMP stars. As first suggested by Fujimoto et al. (1990), the carbon enrichment may also be accompanied by s-process products produced during the PIEs, thus making the stars intrinsic CEMP-s stars. More recent models have shown that the neutron densities can become high enough to trigger the i-process. In this talk I will describe the evolution of a primordial star from the main sequence through to the thermally-pulsing asymptotic branch phase, with a particular focus on our new calculations of the i-process nucleosynthesis that occurs during the core helium flash proton ingestion episode. I will also briefly present a summary of the evolutionary outcomes we have found in our grid of zero metallicity and extremely metal-poor models. We have calculated the nucleosynthetic yields for these stars, with the caveat that they suffer from many uncertainties.

Speaker: Simon Campbell (Monash University Australia)

Campbell.pdf

65 Carbon-enhanced metal-poor stars as a consequence of inhomogeneous metal mixing

I will present a novel scenario for the formation of carbon-enhanced metal-poor (CEMP) stars. Carbon enhancement at low stellar metallicities is usually considered a consequence of faint or other exotic supernovae. A simple analytical estimate of cooling times in low-metallicity gas demonstrates a natural bias to the formation of CEMP stars as a consequence of inhomogeneous metal mixing: carbon-enhanced gas has a shorter cooling time and can form stars prior to a possible nearby pocket of carbon-normal gas, in which star formation is then suppressed due to energetic photons from the carbon-enhanced protostars. I will demonstrate that this is a natural formation mechanism for CEMP stars from carbon-normal supernovae, if inhomogeneous metal mixing provides carbonicity differences of at least one order of magnitude separated by >10pc.

Speaker: Tilman Hartwig (IPMU, the Univ. of Tokyo)

TimeMachine_TilmanHartwig_pdf.pdf

13:00 Break

CEMP Stars: Observation: (Chair: Wako Aoki)

66 CEMP Stars as Probes of First-Star Nucleosynthesis and Galaxy Assembly

Over the course of the past few decades, it has become clear that the class of metal-poor stars known as carbon-enhanced metal-poor (CEMP) stars are powerful probes of a number of areas of interest to contemporary astrophysics. In this contribution, I review the multiple lines of evidence that demonstrate the association of CEMP-no stars (which do not exhibit neutron-capture element enhancements) with the nucleosynthesis products of the very first stars, their likely birth place in low-mass mini-halos, and (once accreted by the halo) their role as tracers of the outer-halo population of the Galaxy. The CEMP-$s$ stars (which exhibit enhancements of the heavy $s$-process elements), by contrast, are likely to have been born in more massive mini-halos, and serve as tracers of the inner-halo population. The well-known increasing frequency of CEMP-no stars (and newly recognized relative constancy of CEMP-$s$ stars) with declining metallicity, and the identification of the primary groups in the Yoon-Beers diagram of $A$(C) vs. [Fe/H], provide the means to explore these associations in more detail, and to constrain numerical models of the formation of the Milky Way.

Speaker: Prof. Timothy Beers (University of Notre Dame)

Beers_CEMP_IPMU_2018.pdf

67 The Mass Distribution of the First Stars revealed by Abundance Pattern Matching of Ultra Metal-Poor Stars

In this talk, I will present a Monte Carlo approach to finding suitable stellar progenitors for Ultra Metal-Poor (UMP) stars, based on the discovery of new UMP stars in the Galactic Halo. UMP stars are thought to be formed from gas clouds polluted by the very first (Population III) stars to be born after the Big-Bang. These Pop. III stars are thought to be massive and short-lived, ending their lives in explosive events such as supernova type II. By studying the detailed chemical abundance patterns of UMP stars, it is possible to infer the main characteristics of their Pop. III progenitors, such as frequency, mass distribution, and explosion energies. Results suggest that at least two types of progenitors are needed at the lowest metallicities, to account for the observed chemical abundances of UMP stars in the Milky Way. These results place important constraints on the initial mass function at early times, as well as models of the chemical evolution of the Galaxy and the Universe. I will also present preliminary results from two new photometric surveys, aiming to find additional UMP stars in the Galaxy.

Speaker: Dr Vinicius Placco (University of Notre Dame)

Placco_ipmu2018.pdf

68 An abundance of very metal-poor stars in the Solar Neighborhood

The number of well-studied very and extremely metal-poor stars is gradually increasing, but still remains limited by the large search volumes necessary to identify them. Here, I will present evidence for a cornucopia of metal-poor dwarf stars (well) within 1 kpc, the so-called dwarf carbon (dC) stars, with order of 1000 already known from the Sloan Digital Sky Survey. The prototype and only well-studied member is a halo star with [Fe/H] = - 4.0 at only 78 pc. Even in the absence of full modeling of their carbon molecular-dominated spectra, their space motions have a distinct halo component, and which may dominate the population. Furthermore, their locus in an HR diagram makes it clear they are Population II stars. I will present ongoing work to identify additional stars, study their kinematical and spatial properties, and understand their fundamental nature. It is plausible the dC stars are the most abundant carbon-enhanced metal-poor stars in the Galaxy and hence a clear focus for stellar archaeology.

Speaker: Jay Farihi (University College London)

Farihi_talk.pdf

16:05 Break

CEMP Stars: Observation: (Chair: Vinicius Placco)

69 The Origin of CEMP-no Groups in the Milky Way: Connection to the Satellite Dwarf Galaxies

One of the primary goals of Galactic Archaeology is to understand the nature of the first generations of stars, their chemical enrichment, and their contribution to Galactic halo formation. However, only their direct descendants, the CEMP-no stars, are available for indirect studies of their properties. Yoon et al. (2016) claimed that there could be multiple pathways to form the halo CEMP-no stars, due to the complex morphology present in their A(C)-[Fe/H] space, comprising at least two distinctly different groups (Group II and Group III).

In this talk, I will provide two important recent results regarding the origin of the CEMP-no groups, along with a new discovery of a CEMP-no Group III star in an ultra-faint dwarf galaxy, Canes Venatici. First, I demonstrate that the bifurcated behavior of the CEMP-no stars into Group II and Group III stars is also present among stars in satellite dwarf galaxies, consistent with the idea that the field CEMP-no stars are the disrupted remains of numerous low-mass mini-halos. Secondly, I will show that understanding this morphology requires not only various nucleosynthetic pathways, but also relies on knowledge of the masses of their host mini-halos, i.e., the dilution of the yields from the first stars.

Speaker: Jinmi Yoon (University of Notre Dame)

stell-arch-yoon.pdf

70 The binarity of dwarf carbon stars and their possible role as main sequence counterparts to CEMP stars

Contrary to expectations, the most abundant carbon stars in the Galaxy are long-lived,
main-sequence stars. The origin of these dwarf carbon (dC) stars is an astrophysical
curiosity that is 40 years(!) old, and the mechanisms for enhancing their observed C/O
above unity are poorly constrained. Intriguingly, a significant fraction of the dC stars
have clear halo kinematics, and are thus almost certainly related to the carbon enhanced,
metal-poor (CEMP) stars observed in the Galactic halo.

We will present a search for evolved binary companions via radial velocity
measurements of these chemically peculiar dwarf stars, all of which are currently in the
solar neighbourhood. Over several years, we observed a few dozen dC stars with the
ISIS spectrograph on the WHT, and 22 stars with sufficient data are consistent with a
100% binary fraction. We hypothesise these main-sequence stars are essentially
CEMP-r or CEMP-s stars of relatively low mass, and are ancient and relatively pristine sites for stellar archaeology.

Speaker: Mr Lewis Whitehouse (University College London)

71 Structural Patterns of CEMP Stars in the Thick Disk of the Galaxy

We present results on the analysis of kinematic and chemical-abundance patterns of stars included in the AAOmega Evolution of Galactic Structure program (AEGIS) and LAMOST DR3. We examine this combined dataset for evidence of carbon-enhanced metal-poor (CEMP) stars associated with the inner halo, thick disk, and metal-weak thick disk (MWTD) of the Galaxy. Of special interest are the CEMP-s stars (which exhibit overabundances in s-process elements) and the CEMP-no stars (which have no neutron-capture element overabundances). By studying the kinematics of CEMP-s and CEMP-no stars in the inner halo and disk system, we aim to better constrain the formation history of the MWTD. The study of structure in the disk system and possible evidence of a past merger (that may have given rise to the thick disk) is an especially pertinent topic after the release of Gaia DR2, and this dataset provides a unique opportunity to examine this.

Speaker: Sarah Dietz (University of Notre Dame)

10_min_talk_Dietz.pdf

72 Conference Banquet

Friday 7 December

08:30 Breakfast

Dwarf Galaxies: (Chair: Kohei Hayashi)

73 Chemo-dynamical evolution of dwarf galaxies and future large spectroscopic surveys

I will give an overview of recent work on the spectroscopic chemical abundance surveys of dwarf galaxies in and around the Milky Way. I will make use of recent results from Gaia DR2 and show how they enhance our understanding of dwarf galaxies, both large and small, combined with ongoing VLT follow-up projects. I will assess how this fits in with an overall view of the chemo-dynamical evolution of dwarf galaxies. I will finally take a forward look to (near) future large European spectroscopic surveys.

Speaker: Prof. Eline Tolstoy (Kapteyn Astronomical Institute, University of Groningen )

tolstoy-ipmu-ga-dec18.pdf

74 Galactic and cosmic chemical evolution from carbon to uranium

Elemental abundances and isotopic ratios of stars in the Milky Way Galaxy have provided stringent constraints on the formation and evolutionary history of the Galaxy. It is now possible to apply this Galactic archaeology approach for other galaxies where elemental abundances of stellar populations can be measured within galaxies with IFU surveys. I will give an overview of Galactic and cosmic chemical enrichment using my chemodynamical simulations. At the early stage of galaxy formation, chemical enrichment took place inhomogeneously. With the inhomogeneous enrichment, stars with long delay-times such as asymptotic giant branch (AGB) stars and neutron star mergers (NSMs, the gravitational wave source) can contribute at low metallicities. We then reproduce the observed N/O-O/H relations with AGB stars only, without rotation of stars. However, we find that NSMs alone are unable to explain the observed Eu abundances, but may be able to together with magneto-rotational supernovae. At the very begging of the galaxy formation, the abundance fitting analysis showed that the chemical enrichment sources are different (faint supernovae/hypernovae), and I will discuss their contribution to galactic chemical evolution.

Speaker: Prof. Chiaki Kobayashi (University of Hertfordshire)

kobayashi.pdf

11:00 Break

Dwarf Galaxies: (Chair: Chiaki Kobayashi)

75 Chemical Evolution of Dwarf Satellites: Before and After Subaru/PFS

The dwarf galaxies in the Local Group are excellent laboratories for studying the creation of the elements (nucleosynthesis) and the build-up of those elements over time (chemical evolution). The galaxies' proximity permits spectroscopy of individual stars, from which detailed elemental abundances can be measured. Their small sizes and, in some cases, short star formation lifetimes imprinted chemical histories that are easy to interpret relative to larger, more complex galaxies, like the Milky Way.

I will briefly review the current state-of-the-art in using dwarf galaxies to study nucleosynthesis and galactic chemical evolution. I will focus on results obtained from multiplexed spectrographs, like Keck/DEIMOS. Then, I will discuss plans for the Subaru Prime Focus Spectrograph to study dwarf galaxies. The combination of PFS's field of view (1.3 deg$^2$) and multiplexing (2394 fibers) will aid in the search for the first stars and revolutionize our understanding of the chemical evolution of these important galaxies.

Speaker: Prof. Evan Kirby (Caltech)

Kirby_FirstStars_2018Dec7.pdf

76 Connecting the first galaxies with ultra faint dwarfs in the Local Group: their assembly probed with absorption spectroscopy

We investigate the star formation histories and chemical evolution of isolated analogs of Local Group untrafaint dwarf galaxies (UFDs) and gas-rich, low-mass dwarfs. We perform a suite of cosmological hydrodynamic zoom-in simulations to follow their evolution from the era of the first generation of stars down to z = 0. We confirm that reionization, combined with supernova (SN) feedback, is primarily responsible for the truncated star formation in UFDs. In this talk, we will show the importance of Population III stars, with their intrinsically high Carbon yields and the associated external metal enrichment, in producing low-metallicity stars and carbon-enhanced metal-poor (CEMP) stars. We will also discuss whether the progenitors of local, gas-rich dwarf galaxies (M_star ~ 10^6 solar mass) could possibly be detected as Damped Lyman-alpha Absorbers (DLAs) over cosmic time. Specifically, since we explicitly consider the contribution of heavy element enrichment from the first stars to the build-up of metals in dwarf galaxies, we can test the scenario that very metal-poor DLAs could contain the unique signature of Pop~III nucleosynthesis.

Speaker: Myoungwon Jeon (KyungHee University)

MyoungwonJeon.pdf

77 Dwarf galaxy formation and enrichment during reionization

JWST will uncover a vast population of low-luminosity galaxies at Cosmic Dawn that is responsible for most of reionization. We present predictions for this high-redshift population, using two suites of high-resolution cosmological simulations -- the Renaissance Simulations and the Tempest Simulations -- that sample different large-scale environments. The Tempest Simulations specifically focus on the progenitors of a Milky Way like galaxy. Using a sample of over 3,000 resolved galaxies along with the formation of 10,000 massive Population III stars, we show that the luminosity function flattens above a UV magnitude of -14 and that the faintest galaxies may be the ancestors of ultra-faint dwarfs. Metals from Population III supernova are the primary source of metals in a fraction of the most metal-poor galaxies. Star formation in low-luminosity galaxies is extremely bursty as the gas reservoir is easily disrupted by internal feedback, resulting in a large spread in galaxy relationships, such as the mass-metallicity, SFR-stellar mass, and stellar mass-halo mass relations. This inefficient star formation ultimately leads to high mass-to-light ratios, similar to local ultra-faint dwarfs, even at high redshifts.

Speaker: John Wise (Georgia Institute of Technology)

IPMU2018-Wise.pdf

78 Indentification of low-mass accreted galaxies with metal-poor stars

The Galactic halo is expected to contain signatures of past galaxy accretions. In fact, the kinematics of halo stars in the Gaia DR2 clearly shows the presence of a massive accreted galaxy. We search for signatures of other less-massive galaxies through the combination of chemical abundances and kinematics of metal-poor stars by using the data from the SAGA database, LAMOST DR4, and APOGEE. From the LAMOST DR4, we first show that the distribution of metal-poor stars in the phase space dramatically changes as a function of metallicity, indicating the occurrence of multiple accretions of galaxies with different metallicity. Abundance ratios of individual elements from the SAGA database demonstrate that the over-density of stars on extreme retrograde orbits, which is only seen at low-metallicity, is caused by an accretion of less-massive dwarf galaxy; they form a distinct low-$\alpha$ elements sequence in the abundance space. Finally, we present results of a trial to identify this population in APOGEE through a statistical approach that combines stellar kinematics and chemical abundances and discuss future prospects to identify relatively low-mass accreted galaxies.

Speaker: Tadafumi Matsuno (SOKENDAI)

20181207Matsuno.pdf

13:00 Break

Dwarf Galaxies: (Chair: Eline Tolstoy)

79 Pop III Supernova feedback on The Formation of The First Galaxies

Modern cosmological simulations suggest that the hierarchical assembly of dark matter halos provided the gravitational wells that allowed the primordial gases to form stars and galaxies inside them. The first galaxies comprised of the first systems of stars gravitationally bound in dark matter halos are naturally recognized as the building blocks of early Universe. To understand the formation of the first galaxies, we use an adaptive mesh refinement (AMR) cosmological code, ENZO to simulate the formation and evolution of the first galaxies. We first model an isolated galaxy by considering much microphysics such as gas dynamics, self-gravity, star formation, stellar feedback, and primordial gas cooling. To examine the effect of Pop III supernovae feedback to the first galaxy formation, we set up the initial temperature, density, and metallicity distributions of Pop III supernova bubbles by assuming different IMF of Pop III stars. Our results suggest that star formation in the first galaxies is sensitive to yields and energetics of the first supernovae. Therefore, our study can provide a channel to correlate the populations of the first stars and supernovae to star formation inside these first galaxies which may be soon observed by the JWST.

Speaker: Ms Li-Hsin Chen (Academia Sinica Institute of Astronomy and Astrophysics / National Taiwan University)

Li-Hsin Chen_20181207[Stellar Archaeology Conference].pdf

80 Detailed Elemental Abundances in the M31 Stellar Halo: Low-Resolution Resolved Stellar Spectroscopy

The stellar halo and tidal streams of M31 provide an essential counterpoint to the same structures around the Galaxy. While Galactic measurements of [Fe/H] and [$\alpha$/Fe] have been made, little is known about the detailed chemical abundances of the M31 system. To make progress with existing telescopes, we apply spectral synthesis to low-resolution spectroscopy (R $\sim$ 2500 at 7000 Angstroms) across a wide spectral range (4500 $<$ $\lambda$ $<$ 9100 Angstroms). We have obtained deep spectra of red giant branch stars (RGB) in the tidal streams and smooth halo of M31 using the DEIMOS 600ZD grating, resulting in higher signal-to-noise per spectral resolution element (S/N $\sim$ 30 Angstrom$^{-1}$). By applying our technique to RGB stars in Galactic globular clusters with existing measurements from higher-resolution spectroscopy, we demonstrate that our technique reproduces previous measurements derived from higher resolution spectra over a more limited spectral range (6300 - 9100 Angstroms) using the DEIMOS 1200G grating. For the first time, we present measurements of [Fe/H] and [$\alpha$/Fe] of sufficient quality and sample size to construct quantitative models of galactic chemical evolution in the M31 system. We also discuss this work in the context of future Subaru/PFS spectra in M31's stellar halo.

Speaker: Ivanna Escala (Caltech)

escala.pdf

81 Statistical properties of substructures around MW-size halos and their implications for the formation of stellar streams

Substructures such as stellar streams are the important tracers that record how the host halos accreted progenitor galaxies. To investigate the relationship between structural properties of substructures and orbits of their progenitors, we combine semi-analytic models with a high-resolution cosmological N-body simulation and analyze the statistical properties of substructures around Milky Way size halos. Using`Particle Tagging' method, we embed stellar components in progenitor halos and trace the phase-space distribution of the substructures down to z=0. Additionally, we use a semi-analytic model to assign stellar masses to the progenitors and successfully reproduce the stellar mass function of observed dwarf galaxies around the Milky Way and the Andromeda galaxy. We characterize structural properties of the substructures such as length and thinness at z=0 and explore the relationship between them and the orbits of progenitors. We find that the length and thinness of substructures vary smoothly as the redshift when the host halos accrete their progenitors. For substructures observed like streams at z=0, a large part of the progenitors is accreted by their host halos at redshift 0.5 < z < 2. We also find that the distributions of length and thinness of substructures vary smoothly as pericenter and apocenter of the progenitors.

Speaker: Mr Yu Morinaga (Chiba University)

morinaga.pdf

82 Metal Mixing in Galaxies as a Probe of Understanding Signatures from First Stars

Understanding the metal mixing in galaxies is a cornerstone to extract the signatures of first stars. Recent high-dispersion spectroscopic observations significantly widen our understanding of elemental abundances in metal-poor stars. Second generation stars inherit the abundances of nucleosynthetic signatures from first stars mixed in the interstellar medium. Here we show that abundances of heavy elements can be a tracer of metal mixing in the early universe. We performed a series of N-body/smoothed hydrodynamics simulations from the scale of star formation to the Milky Way. We find that the timescale of metal mixing is ~ 40 Myr by using our high-resolution simulations of the enrichment of r-process elements and zinc in dwarf galaxies. This efficiency of metal mixing is consistent with the value constrained in our simulations of the scale of star formation. We also discuss the prospects by using state-of-the-art cosmological zoom-in simulations and future facilities such as Subaru PFS and TMT.

Speaker: Yutaka Hirai (RIKEN)

morinaga.pdf

83 Chemical characterization of the Tucana II, Tucana III, and Sagittarius II dwarf galaxies using SkyMapper photometry

Ultra-faint dwarf galaxies are some of the oldest systems ($\sim$13Gyr) in the Milky Way halo. Studying the metallicities of their stars can place strong constraints on models of early chemical enrichment. Spectroscopy only permits the detailed chemical characterization of a handful of stars per system. This under-sampling has led to open questions such as whether the most metal-poor stars ([Fe/H]$\,<-$4.0) also exist in these systems.

I will present the first metallicity analysis of the Tucana$\,$II, Sagittarius$\,$II, and Tucana$\,$III ultra-faint dwarf galaxies based on deep narrow-band SkyMapper photometry. This new technique uses a narrow ‘v’ imaging filter that can yield simultaneous metallicity measurements down to g~22, sampling the full red giant branch of these systems. We have found new members in all three systems and evidence of tidal features in one system. We further obtained high-resolution spectra for two newly identified members of Tucana$\,$II, confirming it to be another typical ancient dwarf galaxy.

Implications are that we can produce spatially complete, magnitude-limited metallicity distributions of the most metal-poor members ([Fe/H]$\,<-$2.0) of these systems. A complete sampling of their most metal-poor stars is crucial for modeling element formation, metal mixing, and improving our understanding of the building blocks of Milky Way-sized galaxies.

Speaker: Anirudh Chiti (MIT)

Chiti_IPMU.pdf

84 Closing Remarks: Wako Aoki & Naoki Yoshida

20181207time_machine-closing-aoki.pdf

NaokiYoshida.pdf