Dissecting the Gaia HR diagram within 200 pc

TL;DR

This study uses Gaia DR2 data to analyze the Hertzsprung-Russell diagram within 200 parsecs, improving binary star modeling with a new tool and fitting star formation history and binary fraction through MCMC methods.

Contribution

Introduces BinaPSE, a new tool for modeling interacting binaries, and applies it within TRILEGAL to fit Gaia HRD data, enhancing understanding of binary populations and star formation history.

Findings

Binary fraction is about 0.4 on the lower main sequence.

Models predict hot subdwarfs within a factor of 2 of observations.

Star formation rate peaks slightly above 2 Gyr and declines at older ages.

Abstract

We analyse the high-quality Hertzsprung-Russell diagram (HRD) derived from Gaia data release 2 for the Solar Neighbourhood. We start building an almost-complete sample within 200 pc and for |b|>25 deg, so as to limit the impact of known errors and artefacts in the Gaia catalog. Particular effort is then put into improving the modelling of population of binaries, which produce two marked features in the HRD: the sequence of near-equal mass binaries along the lower main sequence, and the isolated group of hot subdwarfs. We describe a new tool, BinaPSE, to follow the evolution of interacting binaries in a way that improves the consistency with PARSEC evolutionary tracks for single stars. BinaPSE is implemented into the TRILEGAL code for the generation of "partial models" for both single and binary stellar populations, taking into account the presence of resolved and unresolved binaries. We then fit the Gaia HRD via MCMC methods that search for the star formation history (SFH) and initial binary fraction (by mass) that maximise the likelihood. The main results are (i) the binary fraction derived from the lower main sequence is close to 0.4, while twice larger values are favoured when the upper part of the HRD is fitted; (ii) present models predict the observed numbers of hot subdwarfs to within a factor of 2; (iii) irrespective of the prescription for the binaries, the star formation rate peaks at values 1.5e-4 Msun/yr at ages slightly above 2 Gyr, and then decreases to 0.8e-4 Msun/yr at very old ages.