- ID:
- ivo://CDS.VizieR/J/ApJ/847/127
- Title:
- Abundance analyses of V652 Her and HD 144941
- Short Name:
- J/ApJ/847/127
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low C/He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC. Defining atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of spectroscopic indicators including HeI and HeII line profiles, and the ionization equilibrium of ion pairs such as CII/CIII and NII/NIII. The various indicators provide a consistent set of atmospheric parameters: Teff=25000+/-300K, logg=3.10+/-0.12(cgs), and {xi}=13+/-2km/s are provided for V652 Her, and Teff=22000+/-600K, logg=3.45+/-0.15(cgs), and {xi}=10km/s are provided for HD 144941. In contrast to the non-LTE analyses, the LTE analyses-LTE atmospheres and an LTE line analysis-with the available indicators do not provide a consistent set of atmospheric parameters. The principal non-LTE effect on the elemental abundances is on the neon abundance. It is generally considered that these extreme helium stars with their very low C/He ratio result from the merger of two helium white dwarfs. Indeed, the derived composition of V652 Her is in excellent agreement with predictions by Zhang & Jeffery (2012MNRAS.419..452Z), who model the slow merger of helium white dwarfs; a slow merger results in the merged star having the composition of the accreted white dwarf. In the case of HD 144941, which appears to have evolved from metal-poor stars, a slow merger is incompatible with the observed composition but variations of the merger rate may account for the observed composition. More detailed theoretical studies of the merger of a pair of helium white dwarfs are to be encouraged.
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- ID:
- ivo://CDS.VizieR/J/ApJ/865/129
- Title:
- Abundance analysis of HD 222925
- Short Name:
- J/ApJ/865/129
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed abundance analysis of the bright (V=9.02), metal-poor ([Fe/H]=-1.47+/-0.08) field red horizontal-branch star HD222925, which was observed as part of an ongoing survey by the R-Process Alliance. We calculate stellar parameters and derive abundances for 46 elements based on 901 lines examined in a high-resolution optical spectrum obtained using the Magellan Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38<=Z<=90; their abundance pattern is a close match to the solar r-process component. The distinguishing characteristic of HD222925 is an extreme enhancement of r-process elements ([Eu/Fe]=+1.33+/-0.08, [Ba/Eu]=-0.78+/-0.10) in a moderately metal-poor star, so the abundance of r-process elements is the highest ([Eu/H]=-0.14+/-0.09) in any known r-process-enhanced star. The abundance ratios among lighter (Z<=30) elements are typical for metal-poor stars, indicating that production of these elements was dominated by normal Type II supernovae, with no discernible contributions from Type Ia supernovae or asymptotic giant branch stars. The chemical and kinematic properties of HD 222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a high-yield r-process event before being disrupted by interaction with the Milky Way.
- ID:
- ivo://CDS.VizieR/J/ApJ/838/90
- Title:
- Abundance analysis of 9 very metal-poor stars
- Short Name:
- J/ApJ/838/90
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have performed a differential line-by-line chemical abundance analysis, ultimately relative to the Sun, of nine very metal-poor main-sequence (MS) halo stars, near [Fe/H]=-2dex. Our abundances range from -2.66<=[Fe/H]<=-1.40dex with conservative uncertainties of 0.07dex. We find an average [{alpha}/Fe]=0.34+/-0.09dex, typical of the Milky Way. While our spectroscopic atmosphere parameters provide good agreement with Hubble Space Telescope parallaxes, there is significant disagreement with temperature and gravity parameters indicated by observed colors and theoretical isochrones. Although a systematic underestimate of the stellar temperature by a few hundred degrees could explain this difference, it is not supported by current effective temperature studies and would create large uncertainties in the abundance determinations. Both 1D and <3D> hydrodynamical models combined with separate 1D non-LTE effects do not yet account for the atmospheres of real metal-poor MS stars, but a fully 3D non-LTE treatment may be able to explain the ionization imbalance found in this work.
- ID:
- ivo://CDS.VizieR/J/AJ/144/95
- Title:
- Abundance in stars of the outer galactic disk. IV.
- Short Name:
- J/AJ/144/95
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present radial velocities and chemical abundances for nine stars in the old, distant open clusters Be18, Be21, Be22, Be32, and PWM4. For Be18 and PWM4, these are the first chemical abundance measurements. Combining our data with literature results produces a compilation of some 68 chemical abundance measurements in 49 unique clusters. For this combined sample, we study the chemical abundances of open clusters as a function of distance, age, and metallicity. We confirm that the metallicity gradient in the outer disk is flatter than the gradient in the vicinity of the solar neighborhood. We also confirm that the open clusters in the outer disk are metal-poor with enhancements in the ratios [{alpha}/Fe] and perhaps [Eu/Fe]. All elements show negligible or small trends between [X/Fe] and distance (<0.02dex/kpc), but for some elements, there is a hint that the local (R_GC_<13kpc) and distant (R_GC_>13kpc) samples may have different trends with distance. There is no evidence for significant abundance trends versus age (<0.04dex/Gyr). We measure the linear relation between [X/Fe] and metallicity, [Fe/H], and find that the scatter about the mean trend is comparable to the measurement uncertainties. Comparison with solar neighborhood field giants shows that the open clusters share similar abundance ratios [X/Fe] at a given metallicity. While the flattening of the metallicity gradient and enhanced [{alpha}/Fe] ratios in the outer disk suggest a chemical enrichment history different from that of the solar neighborhood, we echo the sentiments expressed by Friel et al. that definitive conclusions await homogeneous analyses of larger samples of stars in larger numbers of clusters. Arguably, our understanding of the evolution of the outer disk from open clusters is currently limited by systematic abundance differences between various studies.
- ID:
- ivo://CDS.VizieR/J/MNRAS/454/1976
- Title:
- Abundance ratio for 5 local stellar associations
- Short Name:
- J/MNRAS/454/1976
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have observed high-dispersion echelle spectra of main-sequence stars in five nearby young associations - Argus, Carina-Near, Hercules-Lyra, Orion and Subgroup B4 - and derived abundances for elements ranging from Na to Eu. These are the first chemical abundance measurements for two of the five associations, while the remaining three associations are analysed more extensively in our study. Our results support the presence of chemical homogeneity among association members with a typical star-to-star abundance scatter of about 0.06dex or less over many elements. The five associations show log {epsilon}(Li) consistent with their age and share a solar chemical composition for all elements with the exception of Ba. We find that all the heavy elements (Y, Zr, La, Ce, Nd, Sm and Eu) exhibit solar ratios, i.e. [X/Fe]=~0, while Ba is overabundant by about 0.2-0.3dex. The origin of the overabundance of Ba is a puzzle. Within the formulation of the s-process, it is difficult to create a higher Ba abundance without a similar increase in the s-process contributions to other heavy elements (La-Sm). Given that Ba is represented by strong lines of Ba II and La-Sm are represented by rather weak ionized lines, the suggestion, as previously made by other studies, is that the Ba abundance may be systematically overestimated by standard methods of abundance analysis perhaps because the upper reaches of the stellar atmospheres are poorly represented by standard model atmospheres. A novel attempt to analyse the Ba I line at 5535{AA} gives a solar Ba abundance for stars with effective temperatures hotter than about 5800K but increasingly subsolar Ba abundances for cooler stars with apparent Ba deficiencies of 0.5dex at 5100K. This trend with temperature may signal a serious non-local thermodynamical equilibrium effect on the Ba I line.
- ID:
- ivo://CDS.VizieR/J/AJ/131/1766
- Title:
- Abundances and equivalent widths in NGC 2808
- Short Name:
- J/AJ/131/1766
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the abundance analysis of stars from the tip of the red giant branch (RGB) to below the RGB bump in the globular cluster NGC 2808 based on high-resolution echelle spectra. We derive abundances of Al, {alpha}-process elements (SiI, CaI, TiI, and TiII), and Fe-group elements (ScII, VI, CrI, CrII, MnI, CoI, and NiI). Apart from Mg being somewhat reduced, likely because it has been depleted at the expense of Al in the MgAl cycle, the other {alpha}-element ratios show the overabundance typical of halo stars of similar metallicity.
- ID:
- ivo://CDS.VizieR/J/AJ/129/433
- Title:
- Abundances and equivalent widths of 14 Cepheids
- Short Name:
- J/AJ/129/433
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of a detailed multiphase spectroscopic analysis of 14 classical Cepheids with pulsation periods longer than 10 days. For each star, we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. We show that the elemental abundance results for these Cepheids are consistent for all pulsational phases.
- ID:
- ivo://CDS.VizieR/J/AJ/120/1841
- Title:
- Abundances and Kinematics of Halo and Disk Stars
- Short Name:
- J/AJ/120/1841
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The table gives the equivalent widths of the 168 stars used in the abundance analyses. For stars that may have multiple observations the mean value of the measurements is given for each line. Only lines used in the final abundance analysis are provided.
- ID:
- ivo://CDS.VizieR/J/ApJ/743/140
- Title:
- Abundances (Be,{alpha}) in metal-poor stars
- Short Name:
- J/ApJ/743/140
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The light elements, Li, Be, and B, provide tracers for many aspects of astronomy including stellar structure, Galactic evolution, and cosmology. We have made observations of Be in 117 metal-poor stars ranging in metallicity from [Fe/H]=-0.5 to -3.5 with Keck I/HIRES. Our spectra are high resolution (~42000) and high signal to noise (the median is 106 per pixel). We have determined the stellar parameters spectroscopically from lines of FeI, FeII, TiI, and TiII. The abundances of Be and O were derived by spectrum synthesis techniques, while abundances of Fe, Ti, and Mg were found from many spectral line measurements. We have kinematic information on 114 stars in our sample and they divide equally into dissipative and accretive stars.
- ID:
- ivo://CDS.VizieR/J/AJ/152/176
- Title:
- Abundances for all seven stars in Latham 1
- Short Name:
- J/AJ/152/176
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present elemental abundances for all seven stars in Moving Group W11450 (Latham 1) to determine if they may be chemically related. These stars appear to be both spatially and kinematically related, but no spectroscopic abundance analysis exists in literature. Abundances for eight elements were derived via equivalent width analyses of high-resolution (R~60000), high-signal-to-noise ratio (<S/N> ~100) spectra obtained with the Otto Struve 2.1m telescope and the Sandiford Echelle Spectrograph at McDonald Observatory. The large star-to-star scatter in metallicity, -0.55{<=}[Fe/H]{<=}0.06dex ({sigma}=0.25), implies these stars were not produced from the same chemically homogeneous molecular cloud, and are therefore not part of a remnant or open cluster as previously proposed. Prior to this analysis, it was suggested that two stars in the group, W11449 and W11450, are possible wide binaries. The candidate wide binary pair show similar chemical abundance patterns with not only iron but with other elements analyzed in this study, suggesting the proposed connection between these two stars may be real.
