  I report on close encounters of stars to the Sun found in the first Gaia data
release (GDR1). Combining Gaia astrometry with radial velocities of around 320
000 stars drawn from various catalogues, I integrate orbits in a Galactic
potential to identify those stars which pass within a few parsecs. Such
encounters could influence the solar system, for example through gravitational
perturbations of the Oort cloud. 16 stars are found to come within 2 pc
(although a few of these have dubious data). This is fewer than were found in a
similar study based on Hipparcos data, even though the present study has many
more candidates. This is partly because I reject stars with large radial
velocity uncertainties (>10 km/s), and partly because of missing stars in GDR1
(especially at the bright end). The closest encounter found is Gl 710, a K
dwarf long-known to come close to the Sun in about 1.3 Myr. The Gaia astrometry
predict a much closer passage than pre-Gaia estimates, however: just 16 000 AU
(90% confidence interval: 10 000 to 21 000 AU), which will bring this star well
within the Oort cloud. Using a simple model for the spatial, velocity, and
luminosity distributions of stars, together with an approximation of the
observational selection function, I model the incompleteness of this Gaia-based
search as a function of the time and distance of closest approach. Applying
this to a subset of the observed encounters (excluding duplicates and stars
with implausibly large velocities), I estimate the rate of stellar encounters
within 5 pc averaged over the past and future 5 Myr to be 545 +/- 59 per Myr.
Assuming a quadratic scaling of the rate within some encounter distance (which
my model predicts), this corresponds to 87 +/- 9 per My within 2 pc. A more
accurate analysis and assessment will be possible with future Gaia data
releases.
