Galactic runaway O and Be stars found using Gaia DR3

Abstract

[Context] A relevant fraction of massive stars are runaway stars. These stars move with a significant peculiar velocity with respect to their environment.

[Aims] We aim to discover and characterize the population of massive and early-type runaway stars in the GOSC and BeSS catalogs using Gaia DR3 astrometric data.

[Methods] We present a two-dimensional method in the velocity space to discover runaway stars as those that deviate significantly from the velocity distribution of field stars. Field stars are considered to follow the Galactic rotation curve.

[Results] We found 106 O runaway stars, 42 of which were not previously identified as runaways. We found 69 Be runaway stars, 47 of which were not previously identified as runaways. The dispersion of runaway stars is a few times higher in Z and b than that of field stars. This is explained by the ejections they underwent when they became runaways. The percentage of runaways is 25.4% for O-type stars, and it is 5.2% for Be-type stars. In addition, we conducted simulations in three dimensions for our catalogs. They revealed that these percentages could increase to ∼30% and ∼6.7%, respectively. Our runaway stars include seven X-ray binaries and one gamma-ray binary. Moreover, we obtain velocity dispersions of ∼5 km s−1 perpendicular to the Galactic plane for O- and Be-type field stars. These values increase in the Galactic plane to ∼7 km s−1 for O-type stars due to uncertainties and to ∼9 km s−1 for Be-type stars due to Galactic velocity diffusion.

[Conclusions] The excellent Gaia DR3 astrometric data have allowed us to identify a significant number of O-type and Be-type runaways in the GOSC and BeSS catalogs. The higher percentages and higher velocities found for O-type compared to Be-type runaways underline that the dynamical ejection scenario is more likely than the binary supernova scenario. Our results open the door to identifying new high-energy systems among our runaways by conducting detailed studies.