In section 7. Stellar structure and evolution
Estimating stellar ages and metallicities from parallaxes and
broadband photometry: successes and shortcomings
A comprehensive study of the Milky Way requires detailed observations of many millions of individual stars. Photometry provides an efficient and economical method to estimate stellar parameters. The addition of distance information, such as that now available from Gaia DR2 for more than a billion stars, could allow the derivation of reliable ages from photometry for huge samples of stars in the Milky Way.
The authors present a new Bayesian method, which generates 2D probability maps of a star's age and metallicity from photometry and parallax using isochrones, and finds well-constrained ages and metallicities for turn-off and subgiant stars with colors including a U-band and a parallax with uncertainties less than -20%. The results show good agreement for the Gaia benchmark stars, particularly for turn-off and young stars. However, the analysis of the the old open cluster NGC 188 reveals significant limitations of the stellar isochrones, where the ages and metallicities derived vary significantly with evolutionary stage, and for different filter combinations. Improvements are needed in the modeling of giant branch stars, and in the creation and calibration of synthetic near-UV photometry to improve these issues. This work serves as a warning against indiscriminately applying isochrone fitting, and the necessity of quantitatively taking into account the uncertainties in the stellar models. Improving the stellar models will result in significant advancements in our ability to study the Galaxy.