- ID:
- ivo://CDS.VizieR/J/ApJ/855/83
- Title:
- Abundances of very metal-poor stars in Sagittarius
- Short Name:
- J/ApJ/855/83
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Sagittarius (Sgr) is a massive disrupted dwarf spheroidal galaxy in the Milky Way halo that has undergone several stripping events. Previous chemical studies were restricted mainly to a few, metal-rich ([Fe/H]>~-1) stars that suggested a top-light initial mass function (IMF). Here we present the first high-resolution, very metal-poor ([Fe/H]=-1 to -3) sample of 13 giant stars in the main body of Sgr. We derive abundances of 13 elements, namely C, Ca, Co, Fe, Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th, that challenge the interpretation based on previous studies. Our abundances from Sgr mimic those of the metal-poor halo, and our most metal-poor star ([Fe/H]~-3) indicates a pure r-process pollution. Abundances of Sr, Pb, and Th are presented for the first time in Sgr, allowing for age determination using nuclear cosmochronology. We calculate ages of 9+/-2.5Gyr. Most of the sample stars have been enriched by a range of asymptotic giant branch (AGB) stars with masses between 1.3 and 5M_{sun}_. SgrJ190651.47-320147.23 shows a large overabundance of Pb (2.05dex) and a peculiar abundance pattern best fit by a 3M_{sun}_ AGB star. Based on star-to-star scatter and observed abundance patterns, a mixture of low- and high-mass AGB stars and supernovae (15-25M_{sun}_) is necessary to explain these patterns. The high level (0.29+/-0.05dex) of Ca indicates that massive supernovae must have existed and polluted the early ISM of Sgr before it lost its gas. This result is in contrast with a top-light IMF with no massive stars polluting Sgr.
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- ID:
- ivo://CDS.VizieR/J/A+A/601/A10
- Title:
- A grid of MARCS model atmospheres for S stars
- Short Name:
- J/A+A/601/A10
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing asymptotic giant branch. A grid of MARCS model atmospheres has been computed for S stars, covering the range 2700<=Teff(K)<=4000, 0.50<=C/O<0.99, 0<=logg<=5, [Fe/H]=0., -0.5dex, and [s/Fe]= 0, 1, and 2 dex (where the latter quantity refers to the global overabundance of s-process elements). The MARCS models make use of a new ZrO line list. Synthetic spectra computed from these models are used to derive photometric indices in the Johnson and Geneva systems, as well as TiO and ZrO band strengths. A method is proposed to select the model best matching any given S star, a non-trivial operation since the grid contains more than 3500 models covering a five-dimensional parameter space. The method is based on the comparison between observed and synthetic photometric indices and spectral band strengths, and has been applied on a vast subsample of the Henize sample of S stars. Our results confirm the old claim by Piccirillo (1980MNRAS.190..441P) that ZrO bands in warm S stars (Teff > 3200K) are not caused by the C/O ratio being close to unity, as traditionally believed, but rather by some Zr overabundance. The TiO and ZrO band strengths, combined with V-K and J-K photometric indices, are used to select Teff, C/O, [Fe/H] and [s/Fe]. The Geneva U-B_1 and B_2-V_1 indices (or any equivalent) are good at selecting the gravity. The defining spectral features of dwarf S stars are outlined, but none is found among the Henize S stars. More generally, it is found that, at Teff=3200K, a change of C/O from 0.5 to 0.99 has a strong impact on V-K (2mag). Conversely, a range of 2 mag in V-K corresponds to a 200K shift along the (Teff, V-K) relationship (for a fixed C/O value). Hence, the use of a (Teff, V-K) calibration established for M stars will yield large errors for S stars, so that a specific calibration must be used, as provided in the present paper. Using the atmospheric parameters derived by our method for the sample of Henize S stars, we show that the extrinsic-intrinsic dichotomy among S stars reveals itself very clearly as a bimodal distribution in the effective temperatures. Moreover, the increase of s-process element abundances with increasing C/O ratios and decreasing temperatures is apparent among intrinsic stars, confirming theoretical expectations.
- ID:
- ivo://CDS.VizieR/J/A+A/314/191
- Title:
- Atmospheric parameters in metal-poor stars. I
- Short Name:
- J/A+A/314/191
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present atmospheric parameters for about 300 stars of different chemical composition, whose spectra will be used to study the galactic enrichment of Fe and light elements. These parameters were derived using an homogeneous iterative procedure, which considers new calibrations of colour-T_eff_ relations for F, G and K-type stars based on Infrared Flux Method (IRFM) and interferometric diameters for population I stars, and the Kurucz (1992) model atmospheres. We found that these calibrations yield a self-consistent set of atmospheric parameters for T_eff_>4400K, representing a clear improvement over results obtained with older model atmospheres. Using this T_eff_ -scale and Fe equilibrium of ionization, we obtained very low gravities (implying luminosities incompatible with that expected for RGB stars) for metal-poor stars cooler than 4400K; this might be due either to a moderate Fe overionization (expected from statistical equilibrium calculations) or to inadequacy of Kurucz models to describe the atmospheres of very cool giants. Our T_eff_ scale is compared with other scales recently used for metal-poor stars; it agrees well with those obtained using Kurucz (1992) models, but it gives much larger T_eff_'s than those obtained using OSMARCS models (Edvardsson et al. 1993). This difference is attributed to the different treatment of convection in the two sets of models. For the Sun, the Kurucz (1992) model appears to be preferable to the OSMARCS ones because it better predicts the solar limb darkening; furthermore, we find that our photometric T_eff_ 's for metal-poor stars agree well with both direct estimates based on the IRFM, and with T_eff_'s derived from H{alpha} wings when using Kurucz models.
- ID:
- ivo://CDS.VizieR/J/A+A/356/238
- Title:
- Atmospheric parameters in metal-poor stars. III.
- Short Name:
- J/A+A/356/238
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of the analysis of an extensive set of new and literature high quality data concerning Fe, C, N, O, Na, and Mg. This analysis exploited the T_eff_ scale determined in Gratton et al. (1996, Cat. <J/A+A/314/191>), and the non-LTE abundance corrections computed in Gratton et al. (1999, Cat. <J/A+A/350/955>). Results obtained with various abundance indices are discussed and compared.
- ID:
- ivo://CDS.VizieR/J/ApJ/826/110
- Title:
- Boo-127 and Boo-980 high-resolution spectra
- Short Name:
- J/ApJ/826/110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present chemical abundance measurements of two metal-poor red giant stars in the ultra-faint dwarf galaxy Bootes I, based on Magellan/MIKE high-resolution spectra. For Boo-980, with [Fe/H]=-3.1 , we present the first elemental abundance measurements, while Boo-127, with [Fe/H]=-2.0 , shows abundances in good agreement with previous measurements. Light and iron-peak element abundance ratios in the two Bootes I stars, as well as those of most other Bootes I members, collected from the literature, closely resemble those of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba are systematically lower than the main halo trend and also show a significant abundance spread. Overall, this is similar to what has been found for other ultra-faint dwarf galaxies. We apply corrections to the carbon abundances (commensurate with stellar evolutionary status) of the entire sample and find 21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13% without using the carbon correction. We reassess the metallicity distribution functions for the CEMP stars and non-CEMP stars, and confirm earlier claims that CEMP stars might belong to a different, earlier population. Applying a set of abundance criteria to test to what extent Bootes I could be a surviving first galaxy suggests that it is one of the earliest assembled systems that perhaps received gas from accretion from other clouds in the system, or from swallowing a first galaxy or building block type object. This resulted in the two stellar populations observable today.
- ID:
- ivo://CDS.VizieR/J/ApJ/652/1585
- Title:
- Bright metal-poor stars from HES survey
- Short Name:
- J/ApJ/652/1585
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a sample of 1777 bright (9<B<14) metal-poor candidates selected from the Hamburg/ESO Survey (HES). Despite saturation effects present in the red portion of the HES objective-prism spectra, the data were recoverable and quantitative selection criteria could be applied to select the sample. Analyses of medium-resolution (~2{AA}) follow-up spectroscopy of the entire sample, obtained with several 24m class telescopes, yielded 145 new metal-poor stars with metallicity [Fe/H]<-2.0, of which 79 have [Fe/H]<-2.5 and 17 have [Fe/H]<-3.0. We also obtained C/Fe estimates for all of these stars.
- ID:
- ivo://CDS.VizieR/J/AJ/109/2828
- Title:
- CaII H and K filter photometry II.
- Short Name:
- J/AJ/109/2828
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A catalog of 1990 stars on the Caby system is presented. The sample includes stars covering an extensive range in spectral type, luminosity class, and metallicity, as well as apparent magnitude. The hk indices are on the standard system defined in Anthony-Twarog et al. (1991AJ....101.1902A), while the V and (b-y) indices have been transformed to the system of Olsen (1993A&AS..102...89O), superseding the values in the original catalog of standard stars.
- ID:
- ivo://CDS.VizieR/J/AJ/130/2804
- Title:
- Carbon abundances in metal-poor stars
- Short Name:
- J/AJ/130/2804
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We develop and test a method for the estimation of metallicities ([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhanced metal-poor (CEMP) stars based on the application of artificial neural networks, regressions, and synthesis models to medium-resolution (1-2{AA}) spectra and J-K colors. We calibrate this method by comparison with metallicities and carbon abundance determinations for 118 stars with available high-resolution analyses reported in the recent literature.
- ID:
- ivo://CDS.VizieR/J/A+A/622/L4
- Title:
- Carbon and oxygen in metal-poor halo stars
- Short Name:
- J/A+A/622/L4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Carbon and oxygen are key tracers of the Galactic chemical evolution; in particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor halo stars could be a signature of nucleosynthesis by massive Population III stars. We reanalyse carbon, oxygen, and iron abundances in 39 metal-poor turn-off stars. For the first time, we take into account 3D hydrodynamic effects together with departures from local thermodynamic equilibrium (LTE) when determining both the stellar parameters and the elemental abundances, by deriving effective temperatures from 3D non-LTE H{beta} profiles, surface gravities from Gaia parallaxes, iron abundances from 3D LTE FeII equivalent widths, and carbon and oxygen abundances from 3D non-LTE CI and OI equivalent widths. We find that [C/Fe] stays flat with [Fe/H], whereas [O/Fe] increases linearly up to 0.75dex with decreasing [Fe/H] down to -3.0dex. Therefore [C/O] monotonically decreases towards decreasing [C/H], in contrast to previous findings, mainly because the non-LTE effects for OI at low [Fe/H] are weaker with our improved calculations.
- ID:
- ivo://CDS.VizieR/J/ApJ/833/20
- Title:
- Carbon-enhanced metal-poor (CEMP) star abundances
- Short Name:
- J/ApJ/833/20
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate anew the distribution of absolute carbon abundance, A(C)=log{epsilon}(C), for carbon-enhanced metal-poor (CEMP) stars in the halo of the Milky Way, based on high-resolution spectroscopic data for a total sample of 305 CEMP stars. The sample includes 147 CEMP-s (and CEMP-r/s) stars, 127 CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently employed [Ba/Fe] criterion. We confirm previous claims that the distribution of A(C) for CEMP stars is (at least) bimodal, with newly determined peaks centered on A(C)=7.96 (the high-C region) and A(C)=6.28 (the low-C region). A very high fraction of CEMP-s (and CEMP-r/s) stars belongs to the high-C region, while the great majority of CEMP-no stars resides in the low-C region. However, there exists complexity in the morphology of the A(C)-[Fe/H] space for the CEMP-no stars, a first indication that more than one class of first-generation stellar progenitors may be required to account for their observed abundances. The two groups of CEMP-no stars we identify exhibit clearly different locations in the A(Na)-A(C) and A(Mg)-A(C) spaces, also suggesting multiple progenitors. The clear distinction in A(C) between the CEMP-s (and CEMP-r/s) stars and the CEMP-no stars appears to be as successful, and likely more astrophysically fundamental, for the separation of these sub-classes as the previously recommended criterion based on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its application to present and future large-scale low- and medium-resolution spectroscopic surveys.
