We exploit the recent observations of extremely metal-poor (EMP) stars in the Galactic halo and investigate the constraints on the IMF of the stellar population that left these low-mass survivors of [Fe/H]<-2.5 and the chemical evolution that they took part in. A high-mass IMF with the typical mass~10Msun and the overwhelming contribution of low-mass members of binaries to the EMP survivors are derived from the statistics of carbon-enriched EMP stars with and without the enhancement of s-process elements (Komiya et al. 2007). We first examine the analysis to confirm their results for various assumptions on the mass-ratio distribution function. As compared with the uniform distribution, the increase or decrease function of the mass ratio gives a higher- or lower-mass IMF, and a lower-mass IMF results for the independent distribution with the both members in the same IMF, but the derived ranges of typical mass differ less than by a factor of two and overlap for the extreme cases. Furthermore, we prove that the same constraints are placed on the IMF from the surface density of EMP stars estimated from the surveys and the chemical evolution consistent with the metal yields of theoretical supernova models. We then apply the derived high-mass IMF with the binary contribution to show that the observed MDF of EMP stars can be reproduced not only for the shape but also for the number of EMP stars. In particular, the scarcity of stars below [Fe/H]<-4 is naturally explained in terms of the hierarchical structure formation, and there is no indication of significant changes in the IMF for the EMP Population. The present study indicates that 3 HMP stars of [Fe/H]<-4 are the primordial stars that were born as the low-mass members of binaries before the host clouds were polluted by their own supernovae.
We present a low metallicity map of the Milky Way consisting of $\sim$111,000 giants with $-3.5 \lesssim$ [Fe/H] $\lesssim -$0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to $\sim$7kpc from the solar neighborhood and cover the main Galactic stellar populations, including the thick disk and the inner halo. Notably, this map can reliably differentiate metallicities down to [Fe/H] $\sim -3.0$, and thus provides an unprecedented view into the ancient, metal-poor Milky Way. Among the more metal-rich stars in our sample ([Fe/H] $> -2.0$…
Massive disk galaxies like our Milky Way should host an ancient, metal-poor, and centrally concentrated stellar population. This population reflects the star formation and enrichment in the few most massive progenitor components that coalesced at high redshift to form the proto-Galaxy. While metal-poor stars are known to reside in the inner few kiloparsecs of our Galaxy, current data do not yet provide a comprehensive picture of such a metal-poor "heart" of the Milky Way. We use information from Gaia DR3, especially the XP spectra, to construct a sample of 2 million bright (BP $<15.5$ mag) gia…
We explore the kinematics and orbital properties of a sample of 323 very metal-poor stars in the halo system of the Milky Way, selected from the high-resolution spectroscopic follow-up studies of Aoki et al. and Yong et al. The combined sample contains a significant fraction of carbon-enhanced metal-poor (CEMP) stars (22% or 29%, depending on whether a strict or relaxed criterion is applied for this definition). Barium abundances (or upper limits) are available for the great majority of the CEMP stars, allowing for their separation into the CEMP-$s$ and CEMP-no sub-classes. A new method to ass…
Dwarf spheroidal galaxies (dSph) orbiting the Milky Way are complex objects often with complicated star formation histories and internal dynamics. In this work, we search for stellar substructures in four of the classical dSph satellites of the Milky Way: Sextans, Carina, Leo I, and Leo II. We apply two methods to search for stellar substructure: the minimum spanning tree method, which helps us to find and quantify spatially connected structures, and the "brute-force" method, which is able to find elongated stellar substructures. We detected the previously known substructure in Sextans, and al…
In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower-mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disk. However, recent observations of stars in the MW show that metal-poor ([Fe/H] < -2) stars are preferentially on prograde orbits with respect to the disk. Using the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor…
The extent to which turbulence mixes gas in the face of recurrent infusions of fresh metals by supernovae (SN) could help provide important constraints on the local star formation conditions. This includes predictions of the metallicity dispersion amongst metal poor stars, which suggests that the interstellar medium was not very well mixed at these early times. The purpose of this {\it Letter} is to help isolate, via a series of numerical experiments, some of the key processes that regulate turbulent mixing of SN elements in galactic disks. We study the gas interactions in small simulated patc…
We present a chemo-dynamical analysis of low-resolution ($R \sim 1300$) spectroscopy of stars from the AAOmega Evolution of Galactic Structure (AEGIS) survey, focusing on two key populations of carbon-enhanced metal-poor (CEMP) stars within the disk system of the Milky Way: a mildly prograde population ($L_z < 1000\,$kpc$\,$km$\,$s$^{-1}$) and a strongly prograde ($L_z > 1000\,$kpc$\,$km$\,$s$^{-1}$) population. Based on their chemical and kinematic characteristics, and on comparisons with similar populations found in the recent literature, we tentatively associate the former with an ex-situ i…
We report the analysis of high-resolution, high-S/N spectra of an extremely metal-poor, extremely C-rich red giant, Seg 1-7, in the Segue 1 system - described in the literature alternatively as an unusually extended globular cluster or an ultra-faint dwarf galaxy. The radial velocity of Seg 1-7 coincides precisely with the systemic velocity of Segue 1, and its chemical abundance signature of [Fe/H] = -3.52, [C/Fe] = +2.3, [N/Fe] = +0.8, [Na/Fe] = +0.53, [Mg/Fe] = +0.94, [Al/Fe] = +0.23 and [Ba/Fe] < -1.0 is similar to that of the rare and enigmatic class of Galactic halo objects designated CEM…
The Pristine survey uses narrow-band photometry to derive precise metallicities down to the extremely metal-poor regime ([Fe/H] < -3), and currently consists of over 4 million FGK-type stars over a sky area of $\sim 2~500\, \mathrm{deg}^2$. We focus our analysis on a subsample of $\sim 80~000$ main sequence turnoff stars with heliocentric distances between 6 and 20 kpc, which we take to be a representative sample of the inner halo. The resulting metallicity distribution function (MDF) has a peak at [Fe/H] = -1.6, and a slope of $Δ$(LogN)/$Δ[Fe/H] = 1.0 \pm 0.1$ in the metallicity range of -3.4…
Although there is consensus that metal-rich stars in the Milky Way bulge are formed via secular evolution of the thin disc, the origin of their metal-poor counterparts is still under debate. Two different origins have been invoked for metal-poor stars: they might be classical bulge stars or stars formed via internal evolution of a massive thick disc. We use N-body simulations to calculate the kinematic signature given by the difference in the mean Galactocentric radial velocity ($Δ\rm V_{\rm GC}$) between metal-rich stars ([Fe/H] $\ge$ 0) and moderately metal-poor stars (-1.0 $\le$ [Fe/H] $<$ …
Carbon monoxide (CO) is one of the primary coolants of gas and an easily accessible tracer of molecular gas in spiral galaxies but it is unclear if CO plays a similar role in metal poor dwarfs. We carried out a deep observation with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak in a nearby extremely metal poor galaxy, Sextans A, with 7% Solar metallicity. A weak CO J=1-0 emission is seen, which is already faint enough to place a strong constraint on the conversion factor (a_CO) from the CO luminosity to the molecular gas mass that is derived from the spatially reso…
Extremely metal-poor, high-ionizing starbursts in the local Universe provide unique laboratories for exploring in detail the physics of high-redshift systems. Also, their ongoing star-formation and haphazard morphology make them outstanding proxies for primordial galaxies. Using integral field spectroscopy, we spatially resolved the ISM properties and massive stars of two first-class low metallicity galaxies with Wolf-Rayet features and nebular HeII emission: Mrk178 and IZw18. In this review, we summarize our main results for these two objects.
The dawn is quiet II: Gaia XP constraints on the Milky Way's proto-Galaxy from very metal-poor MDF tails
The Ohio State University（俄亥俄州立大学）；Center for Cosmology and AstroParticle Physics (CCAPP)（宇宙学与亚原子物理中心）；Institut de Ciencies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB)（巴塞罗那大学宇宙科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚太空研究所）；The University of Oklahoma（奥克拉荷马大学）
We present our detailed spectroscopic analysis of the chemical composition of four red giant stars in the halo globular cluster NGC 6426. We obtained high-resolution spectra using the Magellan2/MIKE spectrograph, from which we derived equivalent widths and subsequently computed abundances of 24 species of 22 chemical elements. For the purpose of measuring equivalent widths, we developed a new semi-automated tool, called EWCODE. We report a mean Fe content of [Fe/H] = -2.34$\pm$0.05 dex (stat.) in accordance with previous studies. At a mean $α$-abundance of [(Mg,Si,Ca)/3 Fe] = 0.39$\pm$0.03 dex…
We analyze the global structure of the Milky Way (MW)'s stellar halo including its dominant subcomponent, Gaia-Sausage-Enceladus (GSE). The method to reconstruct the global distribution of this old stellar component is to employ the superposition of the orbits covering over the large MW's space, where each of the orbit-weighting factor is assigned following the probability that the star is located at its currently observed position. The selected local, metal-poor sample with ${\rm [Fe/H]}<-1$ using {\it Gaia} EDR3 and SDSS DR16 shows that the global shape of the halo is systematically rounder …
We use Gaia DR2 astrometric and photometric data, published radial velocities and MESA models to infer distances, orbits, surface gravities, and effective temperatures for all ultra metal-poor stars ($\FeH<-4.0$ dex) available in the literature. Assuming that these stars are old ($>11\Gyr$) and that they are expected to belong to the Milky Way halo, we find that these 42 stars (18 dwarf stars and 24 giants or sub-giants) are currently within $\sim20\kpc$ of the Sun and that they map a wide variety of orbits. A large fraction of those stars remains confined to the inner parts of the halo and wa…
Globular clusters can form inside their host galaxies at high redshift when gas densities were higher and gas-rich mergers were common. They can also form inside lower-mass galaxies that have since been accreted and tidally disrupted, leaving their globular cluster complement bound to higher-mass halos. We argue that the age-metallicity-specific orbital energy relation in a galaxy's globular cluster system can be used to identify its origin. Gas-rich mergers should produce tightly bound systems in which metal-rich clusters are younger than metal-poor clusters. Globular clusters formed in massi…
We present a study of nearby dwarf galaxies selected from the ALFALFA blind HI survey. A primary goal of the project was to utilize a non-standard selection method with the hope of detecting previously unrecognized extremely metal-poor (XMP) galaxies. The study was motivated by the recent discovery of two XMP galaxies $-$ Leo P and Leoncino $-$ which were both originally found via the ALFALFA survey. We have obtained narrowband H$α$ images for 42 dwarf systems, many of which are located in the local void in front of the Pisces-Perseus Supercluster. Spectra for eleven of the best candidates res…
We report the discovery of the Little Cub, an extremely metal-poor star-forming galaxy in the local Universe, found in the constellation Ursa Major (a.k.a. the Great Bear). We first identified the Little Cub as a candidate metal-poor galaxy based on its Sloan Digital Sky Survey photometric colors, combined with spectroscopy using the Kast spectrograph on the Shane 3-m telescope at Lick Observatory. In this letter, we present high-quality spectroscopic data taken with the Low Resolution Imaging Spectrometer at Keck Observatory, which confirm the extremely metal-poor nature of this galaxy. Based…
The Galactic bulge of the Milky Way is made up of stars with a broad range of metallicity, -3.0 < [Fe/H] < 1 dex. The mean of the Metallicity Distribution Function (MDF) decreases as a function of height z from the plane and, more weakly, with galactic radius. The most metal rich stars in the inner Galaxy are concentrated to the plane and the more metal poor stars are found predominantly further from the plane, with an overall vertical gradient in the mean of the MDF of about -0.45 dex/kpc. This vertical gradient is believed to reflect the changing contribution with height of different populat…
We have developed a chemo-dynamical approach to assign 36,010 metal-poor SkyMapper stars to various Galactic stellar populations. Using two independent techniques (velocity and action space behavior), $Gaia$ EDR3 astrometry, and photometric metallicities, we selected stars with the characteristics of the "metal-weak" thick disk population by minimizing contamination by the canonical thick disk or other Galactic structures. This sample comprises 7,127 stars, spans a metallicity range of $-3.50<${\metal}~$<-0.8$, and has a systematic rotational velocity of $\langle V_ϕ\rangle=154$\,km\,s$^{-1}$ …
We have used N-body simulations for the Milky Way to investigate the kinematic and structural properties of the old metal-poor stellar halo in the barred inner region of the Galaxy. We find that the extrapolation of the density distribution for bulge RR Lyrae stars, $ρ\sim r^{-3}$, approximately matches the number density of RR Lyrae in the nearby stellar halo. We follow the evolution of such a tracer population through the formation and evolution of the bar and box/peanut bulge in the N-body model. We find that its density distribution changes from oblate to triaxial, and that it acquires slo…
In this work, we study the chemical compositions and kinematic properties of six metal-poor stars with [Fe/H] $< -2.5$ in the Galactic halo. From high-resolution (R $\sim$~110,000) spectroscopic observations obtained with the Lick/APF, we determined individual abundances for up to 23 elements, to quantitatively evaluate our sample. We identify two carbon-enhanced metal-poor stars (J1630+0953 and J2216+0246) without enhancement in neutron-capture elements (CEMP-no stars), while the rest of our sample stars are carbon-intermediate. By comparing the light-element abundances of the CEMP stars with…
The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during th…
We explore the origin of a population of stars recently detected in the inner parsec of the Milky Way Nuclear Cluster (NC), which exhibit sub-solar metallicity and a higher rotation compared to the dominant population. Using state-of-the-art $N$-body simulations, we model the infall of a massive stellar system into the Galactic center, both of Galactic and extra-galactic origin. We show that the newly discovered population can either be the remnant of a massive star cluster formed a few kpc away from the Galactic center (Galactic scenario) or be accreted from a dwarf galaxy originally located …
Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M>100Msun) formed in minihalos at redshift z>20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely-metal poor (EMP) stars in the Milky Way halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose a new solution to the problem in which the IMF of second-generation stars formed at z>10, before reionization, is deficien…
A few rare halo giants in the range [Fe/H] $\simeq -2.9\pm0.3$ exhibit {\it r}-process element abundances that vary as a group by factors up to [$r$/Fe] $\sim80$, relative to those of the iron peak and below. Yet, the astrophysical production site of these {\it r}-process elements remains unclear. We report initial results from four years of monitoring the radial velocities of 17 {\it r}-process-enhanced metal-poor giants to detect and characterise binaries in this sample. We find three (possibly four) spectroscopic binaries with orbital periods and eccentricities that are indistinguishable fr…
We report on the result of an ongoing campaign to determine chemical abundances in extremely metal poor (EMP) turn-off (TO) stars selected from the Sloan Digital Sky Survey (SDSS) low resolution spectra. This contribution focuses principally on the largest part of the sample (18 stars out of 29), observed with UVES@VLT and analyzed by means of the automatic abundance analysis code MyGIsFOS to derive atmosphere parameters and detailed compositions. The most significant findings include i) the detection of a C-rich, strongly Mg-enhanced star ([Mg/Fe]=1.45); ii) a group of Mn-rich stars ([Mn/Fe]>…
The extremely metal-poor stars are the direct descendants of the first generation stars. They carry the chemical signature of the pristine Universe at the time they formed, shortly after the Big Bang. We aim to derive information about extremely metal-poor stars from their observed spectra. Four extremely metal-poor stars were selected from the Sloan Digital Sky Survey (SDSS) and observed during the guaranteed observing time of X-Shooter. The X-Shooter spectra were analysed using an automatic code, MyGIsFOS, which is based on a traditional analysis method. It makes use of a synthetic grid comp…
Extremely metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. In spite of the ongoing formation of stars in these galaxies, the presence of molecular gas (which is known to provide the material reservoir for star formation in galaxies, such as our Milky Way) remains unclear. Here, we report the detection of carbon monoxide (CO), the primary tracer of molecular gas, in a galaxy with 7% solar metallicity, with additional detections in two gal…
The Pristine survey is a narrow-band, photometric survey focused around the wavelength region of the Ca II H & K absorption lines, designed to efficiently search for extremely metal-poor stars. In this work, we use the first results of a medium-resolution spectroscopic follow-up to refine the selection criteria for finding extremely metal-poor stars ($\textrm{[Fe/H]} \leq -3.0$) in the Pristine survey. We consider methods by which stars can be selected from available broad-band and infrared photometry plus the additional Pristine narrow-band photometry. The spectroscopic sample presented in th…
We present the first measurement of the metallicity gradient in extremely metal-poor galaxies (EMPGs). With Subaru/Faint Object Camera And Spectrograph (FOCAS) Integral Field Unit (IFU), we have observed a nearby, low-mass EMPG, HSC J1631+4426, whose oxygen abundance and stellar mass are known to be 12+log(O/H) $=6.9$ and $\log_{10}(M_*/{\rm M}_\odot)=5.8$, respectively. The measured metallicity gradient is $-0.36 \pm 0.04$ dex kpc$^{-1}$ corresponding to $-0.049 \pm 0.006$ dex R$_\mathrm{e}^{-1}$ for the continuum effective radius of $R_\mathrm{e} = 0.14$ kpc. Our observation has successfully…
We present high-resolution, high-S/N spectra of an extremely metal- poor giant star Boo-1137 in the "ultra-faint" dwarf spheroidal galaxy (dSph) Bootes I (absolute magnitude Mv ~ -6.3). With [Fe/H] = -3.7, this the most metal-poor star yet identified in an ultra-faint dSph. Comparison of relative abundances, [X/Fe], for some 15 elements with those of the extremely metal-poor giants of the Galactic halo shows Boo-1137 is "normal" with respect to C and N, the odd-Z elements Na and Al, the Fe-peak elements, and the n-capture elements Sr and Ba, in comparison with the bulk of the halo with [Fe/H] …
LTE abundances of light elements in extremely metal-poor (EMP) stars have been previously derived from high quality spectra. New derivations, free from the NLTE effects, will better constrain the models of the Galactic chemical evolution and the yields of the very first supernovae. The NLTE profiles of the magnesium and potassium lines have been computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI and adjusted to the observed lines in order to derive the abundances of these elements. The NLTE corrections for magnesium and potassium are in good agre…
The dwarf irregular galaxy HIPASS J1131-31 was discovered as a source of HI emission at low redshift in such close proximity of a bright star that we call it Peekaboo. The galaxy resolves into stars in images with Hubble Space Telescope, leading to a distance estimate of 6.8+-0.7 Mpc. Spectral optical observations with the Southern African Large Telescope reveal HIPASS J1131-31 to be one of the most extremely metal-poor galaxies known with the gas-phase oxygen abundance 12+log(O/H) = 6.99+-0.16 dex via the direct [OIII] 4363 line method and 6.87+-0.07 dex from the two strong line empirical met…
We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find the half-light radius of Segue 3 is 26" +/- 5" (2.1 +/- 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere M_V = 0.0 +/- 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0 +1.5/-0.4 Gyr …
We model the history of Galactic r-process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way (MW) type halo. We assume that all r-process sources are neutron star mergers (NSMs) with a power law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to to better compute the properties of metal poor (MP) and carbon enhanced metal poor (CEMP) stars, along with statistics of their r-process enhanced subclasses. Our simulations underpredict the cumulative ratios of r-process enhanced MP and CEMP stars (MP-r, CEMP-r…
The lowest metallicity massive stars in the Local Universe with Z~(Zo/50-Zo/30) are the crucial objects to test the validity of assumptions in the modern models of very low-metallicity massive star evolution. These models, in turn, have major implications for our understanding of galaxy and massive star formation in the early epochs. DDO68-V1 in a void galaxy DDO68 is a unique extremely metal-poor massive star. Discovered by us in 2008 in the HII region Knot3 with Z = Zo/35 [12+log(O/H)=7.14], DDO68-V1 was identified as an LBV star. We present here the LBV lightcurve in V band, combining own n…
We search for signs of rotation in the subsystems of the Milky Way and M31 that are defined by their satellite galaxies, their globular cluster populations, and their BHB stars. A set of simple distribution functions are introduced to describe anisotropic and rotating stellar populations embedded in dark haloes of approximate Navarro-Frenk-White form. The BHB stars in the Milky Way halo exhibit a dichotomy between a prograde rotating, comparatively metal-rich component ([Fe/H] > -2) and a retrograde rotating, comparatively metal-poor ([Fe/H] < -2) component. The prograde metal-rich population …
(Abridged) Carbon-enhanced metal-poor (CEMP) stars in the halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over 30000 calibration stars from the Sloan Digital Sky Survey (SDSS). Using our present criterion that low-metallicity stars exhibiting [C/Fe] ratios ("carbonicity") in excess of [C/Fe]$ = +0.7$ are considered CEMP stars, the global frequency of CEMP stars in the halo system for \feh\ $< -1.5$ is 8%; for \feh\ $< -2.0$ it is 12%; for \feh\ $<-2.5$ it is 20%. We also confirm a signi…
Carbon-Enhanced Metal-Poor Star Candidates in the Milky Way from J-PLUS and S-PLUS
University of Notre Dame（圣母大学）；Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements (JINA-CEE)（核天体物理联合研究所——元素演化中心 (JINA-CEE)）；University of Chinese Academy of Sciences（中国科学院大学）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Chungnam National University（忠南国立大学）；NSF NOIRLab（美国国家光学红外天文研究实验室）
We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW) mass M_vir ~ 10^12.1 M_sun) halos using a suite of 45 zoom-in, dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z=0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M_star ~ 10^8-10^10 M_sun. Halos with more quiescent accretion histories t…
We present the results of a detailed abundance analysis of one of the confirmed building blocks of the Milky Way stellar halo, a kinematically-coherent metal-poor stellar stream. We have obtained high resolution and high S/N spectra of 12 probable stream members using the MIKE spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the 2dCoude spectrograph on the Smith Telescope at McDonald Observatory. We have derived abundances or upper limits for 51 species of 46 elements in each of these stars. The stream members show a range of metallicity (-3.4 < [Fe/H] < -1.5) but ar…
We study a complete, colour-selected sample of double-degenerate binary systems containing extremely low mass (ELM) <0.25 Msol white dwarfs (WDs). We show, for the first time, that Milky Way disk ELM WDs have a merger rate of approximately 4 x 10^(-5)/yr due to gravitational wave radiation. The merger end-product depends on the mass ratio of the binary. The ELM WD systems that undergo stable mass transfer can account for >3% of AM CVn stars. More importantly, the ELM WD systems that may detonate merge at a rate comparable to the estimated rate of underluminous SNe, rare explosions estimated to…
We present a new spectroscopic study of the faint Milky Way satellite Sagittarius II. Using multi-object spectroscopy from the Fibre Large Array Multi Element Spectrograph, we supplement the dataset of Longeard et al. (2020) with 47 newly observed stars, 19 of which are identified as members of the satellite. These additional member stars are used to put tighter constraints on the dynamics and the metallicity properties of the system. We find a low velocity dispersion of SgrII v = 1.7 +/- 0.5 km s-1, in agreement with the dispersion of Milky Way globular clusters of similar luminosity. We conf…
The chemical abundances of a galaxy's metal-poor stellar population can be used to investigate the earliest stages of its formation and chemical evolution. The Magellanic Clouds are the most massive of the Milky Way's satellite galaxies and are thought to have evolved in isolation until their recent accretion by the Milky Way. Unlike the Milky Way's less massive satellites, little is know about the Magellanic Clouds' metal-poor stars. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) and archival data to target nine LMC and four SMC giants for high-resolution…
The early chemical evolution of the Galaxy and the Universe is vital to our understanding of a host of astrophysical phenomena. Since the most metal-poor Galactic stars (with metallicities down to [Fe/H]\sim-5.5) are relics from the high-redshift Universe, they probe the chemical and dynamical conditions of the Milky Way and the origin and evolution of the elements through nucleosynthesis. They also provide constraints on the nature of the first stars, their associated supernovae and initial mass function, and early star and galaxy formation. The Milky Way's dwarf satellites contain a large fr…
Local extremely metal-poor (XMP) galaxies are of particular astrophysical interest since they allow us to look into physical processes characteristic of the early Universe, from the assembly of galaxy disks to the formation of stars in conditions of low metallicity. Given the luminosity-metallicity relationship, all galaxies fainter than Mr < -13 are expected to be XMPs. Therefore, XMPs should be common in galaxy surveys. However, they are not, because several observational biases hamper their detection. This work compares the number of faint XMPs in the SDSS-DR7 spectroscopic survey with the …
Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12+log(O/H)< ~7.3] objects, with blue colours of outer parts. The colours indicate the ages of the oldest visible stellar population of one to a few Gyr. They all are intrinsically faint, mostly Low Surface Brightness dwarfs, with M_B range of -9.5 to -14 mag. Thus, their finding is a subject of the severe observational selection. The recent advancement in search for su…
