Stellar ages and masses in the solar neighbourhood: Bayesian analysis using spectroscopy and Gaia DR1 parallaxes

We present a Bayesian implementation of isochrone fitting in deriving stellar ages and masses, incorporating absolute K magnitude ($\rm M_K$) derived from 2MASS photometry and Gaia DR1 parallax and differentiation between initial bulk metallicity and present day surface metallicity, with allowance for incorporating further constraints (e.g., asteroseismology) when available. As a test, we re-computed stellar ages and masses of $\sim 4000$ stars in the solar neighbourhood from six well-studied literature samples using both Hipparcos and TGAS parallaxes. Our ages are found to be compatible with literature values but with reduced uncertainties in general. The inclusion of parallax-based $\rm M_K$ serves as an additional constraint on the derived quantities, especially when systematic errors in stellar parameters are underestimated. We reconstructed the age-metallicity relationship in the solar neighbourhood by re-analysing the Geneva-Copenhagen Survey with the inclusion of TGAS-parallaxes and initial bulk metallicity sampling. We found a flat trend for disk stars with ages $<$11\,Gyr but with smaller scatter at all ages compared to literature.