An empirical metallicity tracer in CEMP and C-normal stars

Abstract

Context. Deriving the metallicity, [Fe/H], in low-resolution spectra of carbon-enhanced metal-poor (CEMP) stars is a tedious task that, owing to the large number of line blends, often leads to uncertainties on [Fe/H] exceeding 0.25 dex. The CEMP stars increase in number with decreasing [Fe/H] and some of these are known to be bona fide second generation halo stars. Hence, knowing their [Fe/H] is important for tracing the formation and chemical evolution of the Galaxy.
Aims: Here, we aim to improve the [Fe/H] measurements in low-resolution spectra by avoiding issues related to blends. In turn, we improve our chemical tagging in such spectra at low metallicities.
Methods: We developed an empirical way of deriving [Fe/H] in CEMP (and C-normal) stars that relates the equivalent width (EW) of strong lines, which remain detectable in lower resolution, metal-poor spectra, such as X-shooter spectra to [Fe/H].
Results: The best [Fe/H] tracers are found to be Cr I and Ni I, which both show strong transitions in spectral regions that are free of molecular bands (between ~5200-6800Å, a region accessible to most surveys). We derive different relations for dwarfs and giants. The relations are valid in the ranges ~ - 3 < [Fe/H] < -0.5 and 10 < EW < 800 m Å (Cr) or [Fe/H] > -3.2 and EW > 5 m Å (Ni), depending on the trace element and line as well as the stellar evolutionary stage.
Conclusions: The empirical relations are valid for both CEMP and C-normal stars and have been proven to be accurate tracers in a sample of ~400 stars (mainly giants). The metallicities are accurate to within ± ~0.2 dex depending on the sample and resolution, and the empirical relations are robust to within 0.05-0.1 dex. Our relations will improve the metallicity determination in future surveys, which will encounter a large number of CEMP stars, and will greatly speed up the process of determining [Fe/H] as the EWs only need to be measured in two or three lines in relatively clean regions compared to dealing with numerous blended Fe lines.

Based on observations obtained at ESO Paranal Observatory, programmes 084.D-0117(A), 085.D-0041(A), and 090.D-0321(A), see also acknowledgements.