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- ID:
- ivo://CDS.VizieR/J/MNRAS/472/2963
- Title:
- Metallicities of Pristine stars
- Short Name:
- J/MNRAS/472/2963
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Pristine survey is a narrow-band, photometric survey focused around the wavelength region of the CaII 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 ([Fe/H]<=-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 sample consists of 205 stars in the magnitude range 14<V<18. Applying the photometric selection criteria cuts the sample down to 149 stars, and from these we report a success rate of 70 per cent for finding stars with [Fe/H]<=-2.5 and 22 per cent for finding stars with [Fe/H]<=-3.0. These statistics compare favourably with other surveys that search for extremely metal-poor stars, namely an improvement by a factor of ~4-5 for recovering stars with [Fe/H]<=-3.0. In addition, Pristine covers a fainter magnitude range than its predecessors and can thus probe deeper into the Galactic halo.
- ID:
- ivo://CDS.VizieR/J/A+A/354/169
- Title:
- Metal-poor field stars abundances
- Short Name:
- J/A+A/354/169
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have determined Li, C, N, O, Na, and Fe abundances, and ^12^C/^13^C isotopic ratios for a sample of 62 field metal-poor stars in the metallicity range -2<=[Fe/H]<=-1. Stars were selected in order to have accurate luminosity estimates from the literature, so that evolutionary phases could be clearly determined for each star. We further enlarged this dataset by adding 43 more stars having accurate abundances for some of these elements and similarly well defined luminosities from the literature.
- ID:
- ivo://CDS.VizieR/J/AJ/108/538
- Title:
- Metal-poor MS stars UBV photometry
- Short Name:
- J/AJ/108/538
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- From analysis of a photometrically selected sample of 175 metal-poor field stars with main sequence gravities (hereafter BMPs) and UBV colors blueward of the most metal-poor globular cluster main-sequence turnoffs, 0.15<(B-V)_0_<0.35, and properties of the candidate lists of the HK objective prism survey, we calculate the space density and a suitably defined specific frequency of BMPs within ~2kpc of the Sun. We consider two cases. If we adopt the luminosities and color distribution of globular cluster blue stragglers (hereafter BS) for BMPs, the BMP space density, is ~350kpc^-3^, from which we obtain a specific frequency S(BMP)~8, a value 9 times larger than that of BSs in globular clusters. From this result, we conclude that cluster-type BSs are but a minor component of the field BMPs and that the remainder must be of a different nature. If we adopt luminosities of metal-poor models halfway between the Zero Age Main Sequence and the phase of core hydrogen exhaustion, we obtain a space density of 450(+300,-150)kpc^-3^ and a specific frequency S(BMP)=10(+5,-3). From a subsample of 107 BMPs with available radial velocities we derive a galactic rotation of 128km/s and an isotropic velocity dispersion of {sigma}~90km/s, values intermediate between those of halo and thick-disk populations. From analysis of a larger sample of stars on 0.15<(B-V)0<0.35 binned by a crude line-blanketing parameter, we find that our results are insensitive to adopted BMP selection criteria: none of these subsamples of A- and early F-type stars above the galactic plane possess disk kinematics. The region of the UBV two-color diagram occupied by BMPs could be populated by metal-deficient, main-sequence gravity stars with ages substantially younger than those of the metal-poor halo. Because we cannot imagine how or where the observed local population of BMPs could have been produced within our galaxy during the past 3 to 10Gyr, we suggest that BMPs are the bluest members of metal-poor intermediate-age main sequences accreted, probably, from dwarf spheroidal satellites of the Milky Way during the past 10Gyr. We discuss observational consequences of this suggestion.
- ID:
- ivo://CDS.VizieR/J/ApJ/703/2177
- Title:
- Metal-poor MS turnoff stars summary
- Short Name:
- J/ApJ/703/2177
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identify 10 -- seven for the first time -- elements of cold halo substructure (ECHOS) in the volume within 17.5kpc of the Sun in the inner halo of the Milky Way. Our result is based on the observed spatial and radial velocity distribution of metal-poor main-sequence turnoff (MPMSTO) stars in 137 Sloan Extension for Galactic Understanding and Exploration lines of sight. We show that all of our detections are statistically significant and that we expect no false positives. These ECHOS represent the observable stellar debris of ancient merger events in the stellar accretion history of the Milky Way, and we use our detections and completeness estimates to infer a formal upper limit of 0.34^+0.02^_-0.02_ on the fraction of the MPMSTO population in the inner halo that belong to ECHOS.
- ID:
- ivo://CDS.VizieR/J/ApJ/781/40
- Title:
- Metal-poor stars from HES survey. II. Spectroscopy
- Short Name:
- J/ApJ/781/40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on the discovery of seven low-metallicity stars selected from the Hamburg/ESO Survey, six of which are extremely metal-poor (EMP, [Fe/H]{<=}-3.0), with four having [Fe/H]{<=}-3.5. Chemical abundances or upper limits are derived for these stars based on high-resolution (R~35000) Magellan/MIKE spectroscopy, and are in general agreement with those of other very and extremely metal-poor stars reported in the literature. Accurate metallicities and abundance patterns for stars in this metallicity range are of particular importance for studies of the shape of the metallicity distribution function of the Milky Way's halo system, in particular for probing the nature of its low-metallicity tail. In addition, taking into account suggested evolutionary mixing effects, we find that six of the program stars (with [Fe/H]{<=}-3.35) possess atmospheres that were likely originally enriched in carbon, relative to iron, during their main-sequence phases. These stars do not exhibit overabundances of their s-process elements, and hence may be, within the error bars, additional examples of the so-called CEMP-no class of objects.
- ID:
- ivo://CDS.VizieR/J/AJ/145/13
- Title:
- Metal-poor stars from SDSS/SEGUE. I. Abundances
- Short Name:
- J/AJ/145/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Chemical compositions are determined based on high-resolution spectroscopy for 137 candidate extremely metal-poor (EMP) stars selected from the Sloan Digital Sky Survey (SDSS) and its first stellar extension, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). High-resolution spectra with moderate signal-to-noise (S/N) ratios were obtained with the High Dispersion Spectrograph of the Subaru Telescope. Most of the sample (approximately 80%) are main-sequence turnoff stars, including dwarfs and subgiants. Four cool main-sequence stars, the most metal-deficient such stars known, are included in the remaining sample. Good agreement is found between effective temperatures estimated by the SEGUE stellar parameter pipeline, based on the SDSS/SEGUE medium-resolution spectra, and those estimated from the broadband (V - K)_0_ and (g - r)_0_colors. Our abundance measurements reveal that 70 stars in our sample have [Fe/H] < -3, adding a significant number of EMP stars to the currently known sample. Our analyses determine the abundances of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) in addition to Fe. The fraction of carbon-enhanced metal-poor stars ([C/Fe] > +0.7) among the 25 giants in our sample is as high as 36%, while only a lower limit on the fraction (9%) is estimated for turnoff stars. This paper is the first of a series of papers based on these observational results. The following papers in this series will discuss the higher-resolution and higher-S/N observations of a subset of this sample, the metallicity distribution function, binarity, and correlations between the chemical composition and kinematics of extremely metal-poor stars.
- ID:
- ivo://CDS.VizieR/J/AJ/154/52
- Title:
- Metal-poor stars from SDSS/SEGUE. I Unevolved stars
- Short Name:
- J/AJ/154/52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present elemental abundances for eight unevolved extremely metal-poor (EMP) stars with T_eff_>5500K, among which seven have [Fe/H]{<}-3.5. The sample is selected from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration (SDSS/SEGUE) and our previous high-resolution spectroscopic follow-up with the Subaru Telescope. Several methods to derive stellar parameters are compared, and no significant offset in the derived parameters is found in most cases. From an abundance analysis relative to the standard EMP star G64-12, an average Li abundance for stars with [Fe/H]<-3.5 is A(Li)=1.90, with a standard deviation of {sigma}=0.10dex. This result confirms that lower Li abundances are found at lower metallicity, as suggested by previous studies, and demonstrates that the star-to-star scatter is small. The small observed scatter could be a strong constraint on Li-depletion mechanisms proposed for explaining the low Li abundance at lower metallicity. Our analysis for other elements obtained the following results: (i) a statistically significant scatter in [X/Fe] for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in [Na/Fe] with a separation of ~0.8dex, (ii) an absence of a sharp drop in the metallicity distribution, and (iii) the existence of a CEMP-s star at [Fe/H]{simeq}-3.6 and possibly at [Fe/H]{simeq}-4.0, which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase.
- ID:
- ivo://CDS.VizieR/J/AJ/142/188
- Title:
- Metal-poor stars from the HES using CH G-band
- Short Name:
- J/AJ/142/188
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We describe a new method to search for metal-poor candidates from the Hamburg/ESO objective-prism survey (HES) based on identifying stars with apparently strong CH G-band strengths for their colors. The hypothesis we exploit is that large overabundances of carbon are common among metal-poor stars, as has been found by numerous studies over the past two decades. The selection was made by considering two line indices in the 4300{AA} region, applied directly to the low-resolution prism spectra. This work also extends a previously published method by adding bright sources to the sample. The spectra of these stars suffer from saturation effects, compromising the index calculations and leading to an undersampling of the brighter candidates. A simple numerical procedure, based on available photometry, was developed to correct the line indices and overcome this limitation. Visual inspection and classification of the spectra from the HES plates yielded a list of 5288 new metal-poor (and by selection, carbon-rich) candidates, which are presently being used as targets for medium-resolution spectroscopic follow-up. Estimates of the stellar atmospheric parameters, as well as carbon abundances, are now available for 117 of the first candidates, based on follow-up medium-resolution spectra obtained with the SOAR 4.1m and Gemini 8m telescopes. There are eight newly discovered stars with [Fe/H] < -3.0 in our sample, including two with [Fe/H] < -3.5.
- ID:
- ivo://CDS.VizieR/J/A+A/640/A25
- Title:
- Metal-poor stars limb-darkening coefficients
- Short Name:
- J/A+A/640/A25
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Benchmark stars are crucial as validating standards for current as well as future large stellar surveys of the Milky Way. However, the number of suitable metal-poor benchmark stars is currently limited, owing to the difficulty in determining reliable effective temperatures (Teff) in this regime. We aim to construct a new set of metal-poor benchmark stars, based on reliable interferometric effective temperature determinations and a homogeneous analysis. The aim is to reach a precision of 1% in Teff, as is crucial for sufficiently accurate determinations of the full set of fundamental parameters and abundances for the survey sources. We observed ten late type metal-poor dwarf and giants: HD 2665, HD 6755, HD 6833, HD 103095, HD 122563, HD 127243, HD 140283, HD 175305, HD 221170 and HD 224930. Only three of the ten stars (HD 103095, HD 122563 and HD 140283) have previously been used as benchmark stars. For the observations, we used the high angular resolution optical interferometric instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb darkening models and determined effective temperatures directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities (log(g)) were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE and FIES spectrographs and estimated metallicities ([Fe/H]) from a 1D non-LTE abundance analysis of unblended lines of neutral and singly ionized iron. We inferred Teff to better than 1% for five of the stars (HD 103095, HD 122563, HD 127243, HD 140283 and HD 224930). The effective temperatures of the other five stars are reliable to between 2-3%; the higher uncertainty on the Teff for those stars is mainly due to their having a larger uncertainty in the bolometric fluxes. We also determined log(g) and [Fe/H] with median uncertainties of 0.03dex and 0.09dex, respectively. This study presents reliable and homogeneous fundamental stellar parameters for ten metal-poor stars that can be adopted as a new set of benchmarks. The parameters are based on our consistent approach of combining interferometric observations, 3D limb darkening modelling and spectroscopic observations. The next paper in this series will extend this approach to dwarfs and giants in the metal-rich regime.
