Star formation history of the solar neighbourhood as told by Gaia
  The Gaia DR2 catalog is the best source of stellar astrometric and
photometric data available today. The history of the Milky Way galaxy is
written in stone in this data set. Parallaxes and photometry tell us where the
stars are today, when were they formed, and with what chemical content, i.e.
their star formation history (SFH). We develop a Bayesian hierarchical model
suited to reconstruct the SFH of a resolved stellar population. We study the
stars brighter than $G\,=\,15$ within 100 pc of the Sun in Gaia DR2 and derive
a SFH of the solar neighbourhood in agreement with previous determinations and
improving upon them because we detect chemical enrichment. Our results show a
maximum of star formation activity about 10 Gyr ago, producing large numbers of
stars with slightly below solar metallicity (Z=0.014), followed by a decrease
in star formation up to a minimum level occurring around 8 Gyr ago. After a
quiet period, star formation rises to a maximum at about 5 Gyr ago, forming
stars of solar metallicity (Z=0.017). Finally, star formation has been
decreasing until the present, forming stars of Z=0.03 at a residual level. We
test the effects introduced in the inferred SFH by ignoring the presence of
unresolved binary stars in the sample, reducing the apparent limiting
magnitude, and modifying the stellar initial mass function.
