It has been over a quarter century that the class of metal-poor stars known as carbon-enhanced metal-poor (CEMP) stars was first recognized. In that time, astronomers have recognized a number of sub-classes, apparently due to a variety of nucleosynthesis pathways. Most recently, the sub-class formerly referred to as CEMP-r/s has been shown to be due to the so-called intermediate...
At the time of writing there are fourteen stars known to have an iron content below 3.16 e-5 that of the Sun. Twelve out of fourteen of these stars are extremely enhanced in carbon, so that their total metal content Z is larger than 1e-3 that of the Sun, although their iron content may be lower than 1.e-7 that of the Sun. I will summarize extant observations of these very iron poor stars. New...
Stellar abundance measurements are heavily model dependent, and for 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). Systematic errors imparted by the use of 1D LTE modelling tend to grow towards lower metallicities, and are thus especially significant for CEMP stars. ...
We recently announced the discovery of SMSS 1605-1443, which has the lowest detected iron abundance of any star at [Fe/H] = -6.2 (1D LTE). It is strongly carbon enhanced with [C/Fe] ~ 4, but otherwise exhibits a perfectly normal halo star abundance pattern with no detection of neutron capture elements. Assuming a single enrichment event, we find good matches to predictions for Population III...
In the Big Bang Nucleosynthesis (BBN), hydrogen, helium, and small traces of lithium and beryllium, were produced. A few million years after BBN, the first stars were born. Important questions about star formation, galactic evolution, and the yields of the first supernovae can be answered from the study of these first stars and their descendants. The most chemically primitive...
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...
I will present the results of a Bayesian derivation of stellar parameters, distances, and orbits for all ultra metal-poor stars (UMPs, [Fe/H] $< -4$) available in the literature, as well as for the extremely metal-poor stars (EMPs, [Fe/H]$<-3$) observed by Pristine survey, a unique spectrophotometric survey based on a narrow-band Ca H$\&$K filter that aims to detect and analyse EMPs stars. The...
We present a study of the metallicity gradient in the outer halo component of the Milky Way's dual halo system, using metallicities from SDSS DR15 and several other datasets along with high-precision astrometry from Gaia DR2. It has previously been recognized that the outer halo has one of the most metal-poor stellar populations in our Galaxy (peaking around [Fe/H] = -2.2). In this work, we...
Carbon-enhanced metal-poor stars belong to the second generation of stars to form in the Universe; as such, they are a valuable insight into nuclear processes and stellar environments that predate the formation of the Milky Way. At present, approximately one hundred and fifty CEMP stars have been studied via high-resolution spectroscopy. We have observed over 200 new CEMP stars at the South...
Molecular lines are frequently the only available carbon and oxygen abundance indicators in CEMP stars. However, formation of molecular lines in the atmospheres of CEMP stars is prone to the influence of convection and NLTE. These effects have to be properly taken into account when aiming at reliable abundance estimates. In this contribution we present an overview of the current status in the...
The atmospheres of low-mass stars preserve information on the chemical and physical conditions of their birth gas clouds. Studying chemical abundances in the oldest, most metal-poor stars provides an observational window into the early Universe through which we can infer the properties and details of the earliest chemical enrichment events (e.g., supernovae, AGB stars).
The most metal-poor...
Dwarf galaxies are the most common type of galaxy in the Universe. The Local Group is the only place where we are able to obtain an unobscured view of early star formation and chemical enrichment in galaxies from abundance measurements in individual stars by using moderate- to highresolution spectroscopy. Recent measurements of [Ce/Fe] in different dwarf spheroidal galaxies, suggest possible...
The elements locked up in old, metal-poor stars carry a wealth of information on the properties of the early Universe and how it evolved. Stellar abundances are fossil records of the physical conditions in the interstellar medium and of the progenitors that created the material the low-mass stars formed from. All heavy elements show a large star-to-star abundance scatter at low metallicities,...
Understanding the origin of CEMP-no stars is crucial for our understanding of physical processes in the early universe. CEMP-no stars do not show any s-process enhancement and therefore cannot easily be explained by transfer of carbon and s-process elements from a binary AGB companion like the CEMP-s stars. This has been supported by the large radial velocity program by Hansen et al. (2016),...
The elemental abundances in many metal-poor stars show enhancements of carbon and as well neutron-capture elements: CEMP-s, CEMP-r and CEMP-r/s stars. In several radial velocity (RV) projects we have tried to uncover the origin of CEMP stars with enhancements of s- and/or r-process elements. The orbital elements of those that are binaries seem to give hints about the origin of the...
We derive for the first time the multiplicity of the first stars from the abundance patterns of extremely metal-poor (EMP) stars in the Milky Way. Based on theoretical models of the chemical yields of the first supernovae, we train decision trees to classify EMP stars. This machine learning-based approach predicts if a certain abundance pattern is consistent with supernova enrichment by one or...
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...
The last 30 years of galactic archaeological studies have provided many lines of evidence that CEMP-no stars are direct descendants of the very first stars. In particular, the last few years have been very thrilling because the recent observational studies have shown that there are likely multiple pathways to form CEMP-no stars based on distinctly different CEMP-no groups exhibited in the...
Stellar halo of the Milky Way is believed to have formed through hierarchical mergers of small stellar systems such as dwarf galaxies. By studying the orbits and chemistry of very metal-poor halo stars, we can decipher the merger events of ancient galaxies, as well as their chemical properties. We applied a novel clustering method, StarGO, to the largest bright very metal-poor star catalog...
In this work, we study the chemical compositions and kinematics of six metal-poor stars in the Galactic Halo. These stars were selected from the LAMOST survey and were followed up with high-resolution (R $\sim$~110,000) with the Lick/APF. By investigating the chemical compositions and kinematics of this sample, we identified two carbon-enhanced metal-poor stars (J1630+0953 and J2216+0246)...
Carbon-Enhanced Metal-Poor (CEMP) stars are among the most important objects for constraining the formation and evolution of the first stellar populations in the Galaxy. CEMP stars with enhancements in slow neutron-capture process (s-process) elements (CEMP-s stars, objects with [Fe/H] < -2, [C/Fe] > 0.7 and [Ba/Fe] > 1) are a significant fraction (as high as 25%) of all metal-poor stars. Of...
Gamma-ray bursts can be seen to very high redshifts, and the bright power-law continua of their afterglows provide ideal backlights for absorption lines studies. I will review what we have learnt from GRBs about evolving chemical abundances in the early universe; and consider the implications of the high HI column densities seen on the sight lines to GRBs for the escape fraction of ionizing...
I will review the current standard model of how the first stars formed at the end of the cosmic dark ages, and how they impacted the subsequent history of the universe. The Population III formation process is intricately linked to the particle physics nature of dark matter, and can thus serve as an astrophysical laboratory to probe this elusive component of the cosmic matter-energy content. I...
The accretion disks around Population III (PopIII) stars are known to fragment under fast H$_2$ cooling.
Based on the vertical disk structure, we study the optical depth for H$_2$ line cooling under an accretion disk geometry. With the physically motivated optical depth, we find that cooling in the inner disk with $r < 10 {\rm\ AU}$ is attenuated significantly due to the large surface...
It is well known that the adiabatic index of H2 varies as its rovibrational modes are excited as a function of temperature. For the formation of the first stars at redshifts 20 and above, this variation can be significant because the primordial molecular clouds where the first stars form and the material present in accretion shocks near the first protostars can reach high temperatures where...
As of now, the initial mass function (IMF) of metal-free stars is one of the key unsolved problems in the early Universe. Of particular interest is whether there are low-mass population III (Pop III) stars that survive until today. To determine how many such stars we should find in the Milky Way, we model Pop III star formation and feedback in the progenitors of Milky Way-like haloes. Assuming...
Stellar nucleosynthesis defines a sequence of abundances: - pure products of H-burning; - pure products of He-burning; - products of He-burning mixed into the H-burning region (class 2); - products of class 2 mixed in He-burning (class 3); - products of class 3 again mixed in H-burning zone (class 4). CEMP-no stars with enough observed data are distributed in classes 2, 3 and 4. Spinstars...
The study of the long-dead early generations of massive stars is crucial in order to obtain a complete picture of the chemical evolution of the Universe. The nature of these stars can be inferred indirectly by investigating the origin of the low-mass extremely metal-poor stars observed in our neighborhood, some of which are almost as old as the Universe. The material forming these low-mass...
Mass loss is expected to be very low at very low metallicities, even more for the first generations of stars. A lower mass loss induces a lower angular momentum lost during the main sequence, resulting in faster rotators. As the surface velocity increases, due to internal angular momentum transport from the core to the surface, the first stars reach critical velocity at their surface way...
Understanding the nature of the first stars is essential to decipher the chemical abundance patterns in the most iron-poor CEMP stars. Due to their zero-metallicity nature, Pop III stars were structurally quite different than stars of higher metallicity. Namely, interactions between H- and He-burning layers have been recorded in the literature for both rotating and non-rotating stellar models....
Most C-enhanced metal poor stars show substantial enhancements of heavy n-capture elements, mostly of second-peak elements such as Ba, La and Eu. We report on a comprehensive simulation program involving 3D hydrodynamic simulations, 1D stellar evolution and nucleosynthesis simulations, galactic chemical evolution simulations as well as novel approaches to neutron-capture nucleosynthesis...
Understanding the nature of the first stars and their explosive deaths is key to understanding the early universe and the evolution of high-redshift galaxies. With new facilities such as JWST we may soon have the first observations of the earliest stellar populations, but to understand these observations we will require detailed theoretical models. Using the Geneva stellar evolution code, we...
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...
Theoretical investigations have long indicated that supernovae of the first stars would explode in an asymmetric fashion, mainly driven by their fast rotations. However, insufficient observational evidence has prevented in-depth studies. Ultra metal-poor stars ([Fe/H]<-5) encode information about their First progenitor star, such as the explosion mechanism, through the relative...
Large observation campaigns of metal-poor stars in Galactic halo and dwarf galaxies have revealed that there are stars with higher carbon abundance relative to the solar one ([C/Fe] > 0.7; carbon-enhanced metal-poor [CEMP] stars). It has been considered that long-lived or low-mass CEMP stars form through fragmentation of their parent clouds induced by gas cooling of carbon grains. Carbon...
JWST will uncover a vast population of low-luminosity galaxies at Cosmic Dawn that is responsible for most of reionization. We present predictions on this high-redshift population, focusing on their physical properties and role during reionization. We use two suites of high-resolution cosmological simulations -- the Renaissance Simulations and the Tempest Simulations -- that sample different...
The Local Group holds the living fossils of the first stars and galaxies. Still, these precious relics are extremely rare, and hence very difficult to catch. In this talk I will discuss the most recent observational findings and theoretical predictions for present-day metal-poor stars, underlying the links with the properties of the first stars. In particular, I will show that chemical...
We investigate the star formation histories and chemical evolution of isolated analogs of Local Group untra-faint 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...
Cosmic reionization corresponds to the period in the history of the Universe during which the predominantly neutral intergalactic medium was ionised by the emergence of the first luminous sources. I will first briefly describe what we know about cosmic reionisation from observations, and explain the actual limitations/challenges of state-of-the art simulations of this phenomenon.
In...
The carbon enhanced metal poor (CEMP) stars are of immense importance as they likely carry signatures of first few generations of stars in the Universe. At the observational front there has been a tremendous advance in our ours understanding of CEMP stars, and recently theoretical studies have provided new insights. Nevertheless, the origin of CEMP stars, and in particular that of their...
Stars are complex three-dimensional (3D) objects involving many physical processes: convection, rotation, magnetic fields. In this talk, I will review the efforts to constrain the physical ingredients of 1D stellar evolution models of the first stars using multi-D (magneto-)hydrodynamic simulations.
Thanks to space missions, measurements of solar-like oscillations are now available for a large sample of stars. This has led to the precise characterization of the global and internal properties of these stars, and in particular of their internal rotation. In this talk, we will first discuss how these asteroseismic measurements can help us progress in the modelling of angular momentum...
We derive stellar masses and precise chemical abundances for 26 halo red giants, among which 5 and 17 turned out to be [Fe/H]<-2 and <-1, respectively, by combining high-resolution spectroscopy and asteroseismology. We selected the 26 stars in the Kepler field based on radial velocity and metallicity estimated from spectroscopic surveys and confirmed that they show halo-like kinematics using...
The Sun holds a special place in stellar astrophysics, we use it as reference for the determination of the chemical abundances of all stars in the Milky Way. Since 1962, the field of helioseismology has provided unprecedented ways of testing the internal structure of solar models. Soon after the discovery of neutrino-oscillation and the end of the solar neutrino problem, the revision of the...
Binary stellar systems are unique astrophysical laboratories for the study of black holes (BHs). Accretion of matter from a binary companion gives rise to X-ray emission, bringing them to the X-ray binary phase and making them visible on Gpc scales, while the recent gravitational wave observations enabled us to witness the last few seconds of the lives of coalescing binary BHs. X-ray binaries...
Supermassive stars (SMSs) are candidates for being the progenitors of the most massive quasars discovered recently at high redshift. The viability of this formation channel (direct collapse) depends on the properties of the progenitor, whose evolution is dominated by rapid accretion. I will present the most recent models of SMSs, that include accretion and rotation, and discuss their...
I will review (some of) the latest developments in chemo-dynamical numerical simulations of galaxies, particularly focusing on the early formation stages.
To understand the formation and evolution of Milky Way galaxy, Galactic Chemical Evolution (GCE) simulations are performed using N-body Monte Carlo technique. Our GCE models predict the elemental abundance gradients of C, N, O, Mg, Si, Ca, Ti, Fe, and Zn using the revised solar abundance of value 0.0143. The galaxy is radially divided into eight annular rings of 2 kpc width each at a distance...
We present observations and theoretical models regarding the formation of the Galactic bulge. Our main interest concerns the blue horizontal branch and moderately metal-poor globular clusters, that might be among the oldest objects in the Galaxy. We also present abundances, in particular of heavy elements, that could provide evidence for early s-process nucleosynthesis in early rotating massive stars.
Stellar clusters are the key for understanding of galaxy formation and evolution because they host number of old stars. We determine the atmospheric parameters of two members of a new stellar cluster Gaia 2. The low resolution spectra of the stars were obtained at TÜBİTAK National Observatory, using the RTT150 telescope (with 1.5 m). We announce the first results in the present study.
We have recently investigated the origin of chemical signatures observed in the oldest star of our Galaxy by means of a stochastic chemical evolution model. The elements we have studied are carbon, nitrogen and oxygen and furthermore neutron-capture elements. We have found that rotating massive stars are a promising way to explain several signatures observed in these fossil stars.
Analysing...
Carbon-enhanced metal-poor (CEMP) stars contribute to about 20% of the metal-poor stars below [Fe/H] < -2.0. The origin of carbon in these stars could be due to AGB mass transfer in a binary system. These class of objects (CEMP-s) also show enhancement of s-process elements, and most of them show radial velocity variations indicating the presence of a companion. However, the class of CEMP...
The aim of this work is to study the chemical abundances of Milky Way halo stars which belong to the oldest stellar population of the Galaxy and detailed Chemical analysis of these populations can address several intriguing problems in the area of galaxy formation. For our present study, we have carried out high resolution spectroscopic survey using the Hanle Echelle Spectrograph (HESP) at 2m...
First I will show the initial mass function (IMF) of the first enrichment sources obtained with our abundance fitting analysis. We compare the elemental abundance patterns of about 200 extremely metal-poor (EMP) stars to our nucleosynthesis yields of primordial supernovae. As a result, the IMF spans 13-100 Msun with a peak at 25 Msun with a large fraction of hypernovae. The majority of the...
The study of the oldest and most metal-poor stars in our Galaxy promotes our
understanding of the cosmic chemical evolution and the beginning of Galaxy and
star formation. However, they are notoriously difficult to find, with only 5
stars of [Fe/H]<-5 having been detected to date. Thus, the spectrophotometric
data of ~1 billion stars which will become available with the third Gaia...
CEMP-s stars are thought to be the EMP equivalents of Pop. I barium stars or of Pop. ii CH stars, owing their chemical peculiarities to binary evolution. Their present-day companion is a CO white dwarf that had been the core of an AGB stars. We review the observations of different types of chemically peculiar stars originating from binary evolution, including those presumably polluted by a...
From low metallicity stars and the presence of radioactive isotopes in deep-sea sediments we know that the main r-process, producing the heaviest elements, is a rare event. The question remains whether neutron star mergers, via GW170817 the only proven r-process site, are the only contributors or also (a rare class of) supernovae, hypernovae/collapsars, as well as neutron star - black hole...
CEMP stars have increasingly been shown to be important for understanding Pop III stars, early chemical enrichment, and many neutron capture nucleosynthetic processes. However, currently there are only approximately 150 CEMP stars that have been studied in high resolution. While this number is sufficient for statistic of a single population of stars, CEMP stars exist in many varied subgroups...
Searching for the rare metal-poor stars requires fast and effective analysis methods on the vast spectral survey data. Here, we develop a deep learning network to search for metal-poor and carbon-enhanced metal-poor (CEMP) stars in low-resolution spectra. We train a deep convolutional neural network (CNN) on a synthesized stellar spectra grid with Teff ranging from 5000K to 7500K, log g...
Extremely metal-poor stars (EMPs, [Fe/H] < -3) are excellent tools for Galactic archaeologists to study the first stars and the early formation history of the Milky Way. A great diversity has been seen in the EMP stars, both in the medley of their chemical abundance ratios, as well as in their kinematics; a diversity suggestive of the variety of their nucleosynthetic origins and formation...
