- ID:
- ivo://CDS.VizieR/J/ApJ/847/142
- Title:
- Ultra-metal-poor stars LTE and NLTE abundances
- Short Name:
- J/ApJ/847/142
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new ultra-metal-poor stars parameters with [Fe/H] < -4.0 based on line-by-line non-local thermodynamic equilibrium (NLTE) abundances using an up-to-date iron model atom with a new recipe for non-elastic hydrogen collision rates. We study the departures from LTE in their atmospheric parameters and show that they can grow up to ~1.00dex in [Fe/H], ~150K in Teff and ~0.5dex in logg toward the lowest metallicities. Accurate NLTE atmospheric stellar parameters, in particular [Fe/H] being significantly higher, are the first step to eventually providing full NLTE abundance patterns that can be compared with Population III supernova nucleosynthesis yields to derive properties of the first stars. Overall, this maximizes the potential of these likely second-generation stars to investigate the early universe and how the chemical elements were formed.
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- ID:
- ivo://CDS.VizieR/II/180
- Title:
- uvby-beta photometry of metal-poor stars
- Short Name:
- II/180
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using the calibrations derived by Schuster and Nissen (=1989A&A...221...65S), the interstellar color excesses, E(b-y), and the metallicities, [Fe/H], have been determined for the 711 high velocity and metal-poor stars in the catalogue of uvby-beta photometry (=1988A&AS...73..225S). 220 of these listed in table1 turn out to be halo stars according to the criterion [Fe/H] <= -1.0. About 15% of the halo stars have colors that are significantly affected by interstellar reddening, i.e. E(b-y) > 0.025. The distributions in the c0-(b-y)0 diagram of the halo stars grouped according to [Fe/H], have well defined turn-off points. From the Teff values of these points a minimum age of 18-20 Gyr for the halo stars is determined. However, this age may be 2-3 Gyr too high due to a possible systematic error in Teff for the stellar models used. Relative ages of 77 halo stars in the turn-off region of the c0-(b-y)0 diagram are derived using the isochrones VandenBerg and Bell (=1985ApJS...58..561V). Assuming that the O/Fe abundance ratio is constant in halo stars, evidence of a cosmic age scatter of +/-2.5Gyr at a given metallicity is found. Furthermore, the mean age of the halo stars is found to decrease smoothly by about 3 Gyr from [Fe/H]= -2.0 to -1.0. These results suggest that a pressure-supported slow uniform collapse controlled the formation and early evolution of the Galaxy. However, the scenario of Searle and Zinn (=1978ApJ...225..357S) is also compatible with the results.
- ID:
- ivo://CDS.VizieR/J/ApJ/900/106
- Title:
- Vanadium abundances for 255 metal-poor stars
- Short Name:
- J/ApJ/900/106
- Date:
- 13 Jan 2022 00:15:01
- Publisher:
- CDS
- Description:
- We present vanadium (V) abundances for 255 metal-poor stars derived from high-resolution optical spectra from the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We use updated VI and VII atomic transition data from recent laboratory studies, and we increase the number of lines examined (from one to four lines of VI, and from two to seven lines of VII). As a result, we reduce the V abundance uncertainties for most stars by more than 20% and expand the number of stars with V detections from 204 to 255. In the metallicity range -4.0<[Fe/H]{<}-1.0, we calculate the mean ratios [VI/FeI]=-0.10+/-0.01 ({sigma}=0.16) from 128 stars with >=2VI lines detected, [VII/FeII]=+0.13+/-0.01 ({sigma}=0.16) from 220 stars with >=2 VII lines detected, and [VII/VI]=+0.25+/-0.01 ({sigma}=0.15) from 119 stars. We suspect that this offset is due to departures from local thermodynamic equilibrium, and we recommend using [VII/FeII], which is enhanced relative to the solar ratio, as a better representation of [V/Fe]. We provide more extensive evidence for abundance correlations detected previously among scandium, titanium, and vanadium, and we identify no systematic effects in the analysis that can explain these correlations.
- ID:
- ivo://CDS.VizieR/J/A+A/608/A89
- Title:
- Very metal poor stars in MW halo
- Short Name:
- J/A+A/608/A89
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the non-local thermodynamic equilibrium (NLTE) abundances of up to 10 chemical species in a sample of 59 very metal-poor (VMP, -4<=[Fe/H]<~-2) stars in seven dwarf spheroidal galaxies (dSphs) and in the Milky Way (MW) halo. Our results are based on high-resolution spectroscopic datasets and homogeneous and accurate atmospheric parameters determined in Paper I. We show that once the NLTE effects are properly taken into account, all massive galaxies in our sample, that is, the MW halo and the classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax, reveal a similar plateau at [alpha/Fe]=0.3 for each of the alpha-process elements: Mg, Ca, and Ti. We put on a firm ground the evidence for a decline in alpha/Fe with increasing metallicity in the Bootes I ultra-faint dwarf galaxy (UFD), that is most probably due to the ejecta of type Ia supernovae. For Na/Fe, Na/Mg, and Al/Mg, the MW halo and all dSphs reveal indistinguishable trends with metallicity, suggesting that the processes of Na and Al synthesis are identical in all systems, independent of their mass. The dichotomy in the [Sr/Ba] versus [Ba/H] diagram is observed in the classical dSphs, similarly to the MW halo, calling for two different nucleosynthesis channels for Sr. We show that Sr in the massive galaxies is better correlated with Mg than Fe and that its origin is essentially independent of Ba, for most of the [Ba/H] range. Our three UFDs, that is Bootes I, UMa II, and Leo IV, are depleted in Sr and Ba relative to Fe and Mg, with very similar ratios of [Sr/Mg]=-1.3 and [Ba/Mg]=-1 on the entire range of their Mg abundances. The subsolar Sr/Ba ratios of Bootes I and UMa II indicate a common r-process origin of their neutron-capture elements. Sculptor remains the classical dSph, in which the evidence for inhomogeneous mixing in the early evolution stage, at [Fe/H]<-2, is the strongest.
- ID:
- ivo://CDS.VizieR/J/ApJ/788/180
- Title:
- Very metal-poor stars in the Milky Way's halo
- Short Name:
- J/ApJ/788/180
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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. (2013, J/AJ/145/13) and Yong et al (2013, J/ApJ/762/26). 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 subclasses. A new method to assign membership to the inner- and outer-halo populations of the Milky Way is developed, making use of the integrals of motion, and applied to determine the relative fractions of CEMP stars in these two subclasses for each halo component. Although limited by small-number statistics, the data suggest that the inner halo of the Milky Way exhibits a somewhat higher relative number of CEMP-s stars than CEMP-no stars (57% versus 43%), while the outer halo possesses a clearly higher fraction of CEMP-no stars than CEMP-s stars (70% versus 30%). Although larger samples of CEMP stars with known Ba abundances are required, this result suggests that the dominant progenitors of CEMP stars in the two halo components were different; massive stars for the outer halo, and intermediate-mass stars in the case of the inner halo.
- ID:
- ivo://CDS.VizieR/J/A+A/620/A187
- Title:
- Very metal-poor turn-off stars abundances
- Short Name:
- J/A+A/620/A187
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Extremely metal-poor stars are keys to understand the early evolution of our Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of metal-poor turn-off stars, whereas most of the previously known extremely metal-poor stars are giant stars. Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor turn-off stars as observed in metal-poor giant stars. We found a significant spread of the [alpha/Fe] ratios with several stars showing sub-solar [Ca/Fe] ratios. We could measure the abundance of strontium in 12 stars of the sample, leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium in two stars of the sample. One of the stars (SDSS J114424-004658) shows both very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1dex).
- ID:
- ivo://CDS.VizieR/J/MNRAS/340/304
- Title:
- [X/Fe] of Galactic disc F and G dwarfs
- Short Name:
- J/MNRAS/340/304
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Photospheric abundances are presented for 27 elements from carbon to europium in 181 F and G dwarfs from a differential local thermodynamic equilibrium (LTE) analysis of high-resolution and high signal-to-noise ratio spectra. Stellar effective temperatures (T_eff_) were adopted from an infrared flux method calibration of Stroemgren photometry. Stellar surface gravities (g) were calculated from Hipparcos parallaxes and stellar evolutionary tracks. Adopted T_eff_ and g values are in good agreement with spectroscopic estimates. Stellar ages were determined from evolutionary tracks. Stellar space motions (U, V, W) and a Galactic potential were used to estimate Galactic orbital parameters. These show that the vast majority of the stars belong to the Galactic thin disc.
