Abundances of the light elements from UV (HST) and red (ESO) spectra in the very old star HD 84937

Abstract

Aims. In order to provide a better basis for the study of mechanisms of nucleosynthesis of the light elements beyond hydrogen and helium in the oldest stars, the abundances of C, O, Mg, Si, P, S, K, and Ca have been derived from UV-HST and visible-ESO high resolution spectra in the old, very metal-poor star HD 84937, at a metallicity that is 1/200 that of the Sun's. For this halo main-sequence turnoff star, the abundance determination of P and S are the first published determinations. Methods. The LTE profiles of the lines were computed and fitted to the observed spectra. Wherever possible, we compared the abundances derived from the UV spectrum to abundances derived from the visible or near-infrared spectra, and also corrected the derived abundances for non-LTE effects. Three-dimensional (3D) CO 5 BOLD model atmospheres have been used to determine the abundances of C and O from molecular CH and OH bands. Results. The abundances of these light elements relative to iron in HD 84937 are found to agree well with the abundances of these elements in classical metal-poor stars. Our HD 84937 carbon abundance determination points toward a solar (or mildly enhanced above solar) value of [C/Fe]. The modest overabundance of the α elements of even atomic number Z, typical of halo turnoff stars, is confirmed in this example. The odd-Z element P is found to be somewhat deficient in HD 84937, at [P/Fe] = −0.32, which is again consistent with the handful of existing determinations for turnoff stars of such low metallicity. We show that the abundance of oxygen, deduced from the OH band from 3D computations, is not compatible with the abundance deduced from the red oxygen triplet. This incompatibility is explained by the existence of a chromosphere heating the shallow layers of the atmosphere where the OH band, in 3D computations, is mainly formed. Conclusions. The abundance ratios are compared to the predictions of models of galactic nucleosynthesis and evolution.

Phosphorus Abundances in the Hyades and Galactic Disk

“…Targeted surveys with the Y-band P I features have confirmed an offset between chemical evolution models and abundance measurements over the -0.6 < [Fe/H] < 0.2 metallicity range (Caffau et al 2016;Maas et al 2017;Caffau et al 2019). Additional P abundances have also been measured in individual metal poor stars (Spite et al 2017) further constraining P production in the early Universe. The APOGEE survey has also measured P (Holtzman et al 2018), however the P I features in the H-band are weak, blended with telluric features, and blended with other atomic/molecular features.…”

Transition Probabilities of Co ii Weak Lines to the Ground and Low Metastable Levels

“…HD 84937 is a warm main-sequence turnoff star that has good basic observational data, such as accurate broadband colors, accurate parallax, and proper motions ( p = 13.74 ± 0.78 mas, where 1 mas = 4.85 nrad; μ α = 373.05 ± 0.91 mas yr −1 ; μ δ = −774.38 ± 0.33 mas yr −1 ; van Leeuwen 2007). These data, combined with many high-resolution spectroscopic studies (e.g., Spite et al 2017 and references therein) of HD 84937, all have led to well-determined stellar atmospheric parameters. We have employed T eff = 6300K, log g = 4.0, [Fe/H] = −2.15, 9 and v t = 1.5 km s −1 in our papers, very close to the parameters used by Spite et al Those parameters are adopted in the present work.…”

An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres

“…The line list used for the CN-band was identical to that detailed in Gallagher et al (2016b). The line list of the OH-band was adopted from part of the OH line list used by Spite et al (2017). Table 1.…”