A standard stellar library for evolutionary synthesis. III. Metallicity calibration

TL;DR

This paper extends the BaSeL stellar library to include non-solar metallicities and discusses the challenges in calibrating stellar colours with metallicity, proposing two new calibration versions for different applications.

Contribution

It introduces two new metallicity calibration versions of the BaSeL library, addressing calibration challenges and providing tools for both individual star and stellar population studies.

Findings

Calibration issues due to convection and observational data gaps.

Two calibration versions: WLBC 99 for stars, PADOVA 2000 for clusters.

Highlighting the need for better observational data.

Abstract

We extend the colour calibration of the widely used BaSeL standard stellar library (Lejeune, Cuisinier, & Buser 1997, 1998) to non-solar metallicities, down to [Fe/H] ~ -2.0 dex. Surprisingly, we find that at the present epoch it is virtually impossible to establish a unique calibration of UBVRIJHKL colours in terms of stellar metallicity [Fe/H] which is consistent simultaneously with both colour-temperature relations and colour-absolute magnitude diagrams (CMDs) based on observed globular cluster photometry data and on published, currently popular standard stellar evolutionary tracks and isochrones. The problem appears to be related to the long-standing incompleteness in our understanding of convection in late-type stellar evolution, but is also due to a serious lack of relevant observational calibration data that would help resolve, or at least further significant progress towards resolving this issue. In view of the most important applications of the BaSeL library, we here propose two different metallicity calibration versions: (1) the "WLBC 99" library, which consistently matches empirical colour-temperature relations and which, therefore, should make an ideal tool for the study of individual stars; and (2), the "PADOVA 2000" library, which provides isochrones from the Padova 2000 grid (Girardi et al., 2000) that successfully reproduce Galactic globular-cluster colour-absolute magnitude diagrams and which thus should prove particularly useful for studies of collective phenomena in stellar populations in clusters and galaxies.