Description
Latest results of large surveys, relation between kinematics and chemical abundances among very iron-poor stars, the questions of 1D-3D model atmospheres, LTE, NON LTE, the CEMP stars in external galaxies, are at least some CEMP He-rich? Detection of the companion of CEMP-s stars?
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...
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...
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...
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...
