Fundamental stellar parameters of benchmark stars from CHARA interferometry

Abstract

Context. Benchmark stars are crucial as validating standards for current as well as future large stellar surveys of the Milky Way. However, the number of suitable metal-poor benchmark stars is currently limited, owing to the difficulty in determining reliable effective temperatures (Teff) in this regime. Aims. We aim to construct a new set of metal-poor benchmark stars based on reliable interferometric effective temperature determinations and a homogeneous analysis. The aim is to reach a precision of 1% in Teff, as is crucial for sufficiently accurate determinations of the full set of fundamental parameters and abundances for the survey sources. Methods. We observed ten late-type metal-poor dwarfs and giants: HD 2665, HD 6755, HD 6833, HD 103095, HD 122563, HD 127243, HD 140283, HD 175305, HD 221170, and HD 224930. Only three of them (HD 103095, HD 122563, and HD 140283) have previously been used as benchmark stars. For the observations, we used the high-angular-resolution optical interferometric instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb-darkening models and determined effective temperatures directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities (log(g)) were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE and FIES spectrographs and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutral and singly ionised iron. Results. We inferred Teff to better than 1% for five of the stars (HD 103095, HD 122563, HD 127243, HD 140283, and HD 224930). The effective temperatures of the other five stars are reliable to between 2 and 3%; the higher uncertainty on the Teff for those stars is mainly due to their having a larger uncertainty in the bolometric fluxes. We also determined log(g) and [Fe/H] with median uncertainties of 0.03 dex and 0.09 dex, respectively. Conclusions. This study presents reliable and homogeneous fundamental stellar parameters for ten metal-poor stars that can be adopted as a new set of benchmarks. The parameters are based on our consistent approach of combining interferometric observations, 3D limb-darkening-modelling and spectroscopic observations. The next paper in this series will extend this approach to dwarfs and giants in the metal-rich regime.

The GALAH Survey: second data release

“…Their reference eff are based on angular diameter measurements (e.g. Karovicova et al 2018Karovicova et al , 2020) and when we compare with the GALAH+ DR3 results (blue error bars in upper panel of Fig. 6), we find an excellent agreement with these temperatures for most of the stars between 3500 and 6250 K. We note, however, significant differences for the two massive (∼3 M ) giant stars ξ Hya, ϵ Vir, and the subgiant ϵ For.…”

The GALAH Survey: chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and Gaia eDR3

“…While we cannot draw significant conclusions from these differences, we note that stellar surveys suffer from the ability to sufficiently benchmark iron abundances in the metal-poor regime due to still low numbers of benchmark stars. More efforts similar to those of Hawkins et al (2016) and Karovicova et al (2020) will be needed to fully validate the iron-abundances in the metal-poor regime. One very metal-poor turn-off star found as part of the GSE by Naidu et al (2020) was observed by GALAH, but without parameters reported within GALAH+ DR3 due to its high T eff , large distance, and thus low signal-to-noise GALAH+ DR3 spectrum.…”

The GALAH survey: effective temperature calibration from the InfraRed Flux Method in the Gaia system

“…Finally, we assemble a list of interferometric measurements from the recent literature: Bigot et al (2011), Boyajian et al (2012a,b), Huber et al (2012), Maestro et al (2013), White et al (2013White et al ( , 2018, Gallenne et al (2018), Baines et al (2018), Rains et al (2020), and Karovicova et al (2020). For all these stars, we adopt reddening, log (g), and [Fe/H] reported in the above papers.…”