Chemical abundances of the metal-poor horizontal-branch stars CS 22186-005 and CS 30344-033

TL;DR

This study presents the first detailed chemical abundance analysis of two very metal-poor horizontal-branch stars in the Milky Way halo, confirming their usefulness as tracers of galactic chemical evolution.

Contribution

It provides new elemental abundance measurements for two stars, including the first Ni abundance for CS 22186-005, and compares their patterns to Galactic trends.

Findings

CS 22186-005 has [Ni/Fe] = -0.21, below typical values for metal-poor stars.

The abundance patterns match Galactic trends, validating horizontal-branch stars as population tracers.

Radial velocity measurements for CS 22186-005 differ from previous data, suggesting possible variability.

Abstract

We report on a chemical-abundance analysis of two very metal-poor horizontal-branch stars in the Milky Way halo: CS 22186-005 ([Fe/H]=-2.70) and CS 30344-033 ([Fe/H]=-2.90). The analysis is based on high-resolution spectra obtained at ESO, with the spectrographs HARPS at the 3.6 m telescope, and UVES at the VLT. We adopted one-dimensional, plane-parallel model atmospheres assuming local thermodynamic equilibrium. We derived elemental abundances for 13 elements for CS 22186-005 and 14 elements for CS 30344-033. This study is the first abundance analysis of CS 30344-033. CS 22186-005 has been analyzed previously, but we report here the first measurement of nickel (Ni; Z = 28) for this star, based on twenty-two NiI lines ([Ni/Fe]=-0.21$\pm$0.02); the measurement is significantly below the mean found for most metal-poor stars. Differences of up to 0.5 dex in [Ni/Fe] ratios were determined by different authors for the same type of stars in the literature, which means that it is not yet possible to conclude that there is a real intrinsic scatter in the [Ni/Fe] ratios. For the other elements for which we obtained estimates, the abundance patterns in these two stars match the Galactic trends defined by giant and turnoff stars well. This confirms the value of horizontal-branch stars as tracers of the chemical properties of stellar populations in the Galaxy. Our radial velocities measurements for CS 22186-005 differ from previously published measurements by more than the expected statistical errors. More measurements of the radial velocity of this star are encouraged to confirm or refute its radial velocity variability.