GaiaFGK benchmark stars: Metallicity

Abstract

Context. To calibrate automatic pipelines that determine atmospheric parameters of stars, one needs a sample of stars, or "benchmark stars", with well-defined parameters to be used as a reference. Aims. We provide detailed documentation of the iron abundance determination of the 34 FGK-type benchmark stars that are selected to be the pillars for calibration of the one billion Gaia stars. They cover a wide range of temperatures, surface gravities, and metallicities. Methods. Up to seven different methods were used to analyze an observed spectral library of high resolutions and high signal-to-noise ratios. The metallicity was determined by assuming a value of effective temperature and surface gravity obtained from fundamental relations; that is, these parameters were known a priori and independently from the spectra. Results. We present a set of metallicity values obtained in a homogeneous way for our sample of benchmark stars. In addition to this value, we provide detailed documentation of the associated uncertainties. Finally, we report a value of the metallicity of the cool giant ψ Phe for the first time.

Non‐LTE iron abundances in cool stars: The role of hydrogen collisions

“…Three benchmark stars of different metallicities and stellar parameters were considered in this study, namely the Sun, the metal-rich dwarf α Centauri A and the metal-poor halo subgiant HD 140283. Their atmospheric parameters were adopted from the study of the Gaia benchmark stars by Jofré et al (2014), where the effective temperatures T eff and surface gravities log g were determined homogeneously and independently from spectroscopic models, while the stellar metallicities were determined spectroscopically from Fe i lines by fixing T eff and log g to the previous values and applying a line-by-line non-LTE correction to each line. The parameters are listed in Table 1. 1D MARCS (Gustafsson et al 2008) atmospheric models were interpolated to the atmospheric parameters (in T eff , log g, and [Fe/H]) of each star using the code interpol marcs.f written by Thomas Masseron 1 , except for the solar case where the reference solar MARCS model was used.…”

THE CANNON: A DATA-DRIVEN APPROACH TO STELLAR LABEL DETERMINATION

“…In many cases, the model spectra do not account for all relevant molecular opacities, for convection, stellar winds, and the chromosphere. As a consequence, it happens that different research groups obtain discrepant results for same stars, resulting from analysis across different wavelength regions and different input assumptions and methods used (e.g., Allende Prieto et al 1999;Hinkel et al 2014;Jofré et al 2014). Even when the input assumptions are held fixed, differences in the employed analysis methods lead to substantial differences in assigned labels (e.g., Smiljanic et al 2014).…”

“…Stated minimal signalto-noise ratio (hereafter S/N) requirements to obtain robust labels in this way are S N 100 per resolution element, especially if the labels are to include individual element abundances. Often, a post-calibration procedure is applied to bring the stellar labels derived by such a fitting pipeline in accord with external information of higher fidelity: for example with stellar labels from benchmark stars studied at high resolution or well characterized open and globular cluster stars (e.g., Kordopatis et al 2013;Mészáros et al 2013;Jofré et al 2014). These calibration stars are also used by surveys to provide a reasonable estimate of their label accuracy.…”

Gaia benchmark stars and their twins in the Gaia‐ESO Survey

“…Since the benchmark stars are very bright objects, they have been extensively studied in the past by several different authors. However, as discussed in Jofré et al (2014), the metallicity values of benchmark stars found in the literature are highly inhomogeneous. This is because the different authors use different methods, input data and scales for the effective temperature, surface gravity or the solar abundances.…”

“…Works in the literature are normally focused on one type of star only, i.e., giants or dwarfs. It is thus important to re-determine metallicity (and elemental abundances) in an homogeneous fashion, which was presented in Jofré et al (2014). For that, we collected high resolution and high signal-to-noise archive spectra from UVES, HARPS, NARVAL and ESPaDOnS, and compiled a dedicated library of high-resolution and high signal-to-noise spectra such that every spectrum would have the same format and resolution.…”