The local Milky Way unveiled by its OB stars and associations

Abstract: From star formation to stellar multiplicity, O- and B-type stars constitute vital tools across many areas of astronomy. Being short-lived, they remain close to their birth environment, and can therefore serve as tracers of the position and motion of their birthplaces, such as the Galactic spiral arms. Furthermore, they effectively act as both creators and destructors of stellar systems, and therefore life. Through their feedback processes, they enrich the interstellar medium in new chemical elements subsequently used to form new stars and planets, but their powerful core-collapse supernova explosions are able to deplete the atmosphere of Earth-like planets in their vicinity, thereby engendering cataclysmic events. They also constitute the brightest members of OB associations, which are young, low-density and gravitationally unbound stellar groups. As a transition between star-forming regions and the Galactic field population of stars, OB associations are crucial to study early stellar evolution and Galactic structure. 

Gaia data offers an unprecedented opportunity to provide more extensive and accurate 3D maps of the spatial distribution of OB stars than ever before. During the first part of this talk, I will present my catalogue of 24,706 O- and B-type stars (effective temperature greater than 10,000 K, hereafter OB stars) within 1 kpc of the Sun, by applying evolutionary and atmospheric model fits to observed SEDs compiled from astro-photometric survey data, and assessing its quality by comparing it with other Gaia-based catalogues of OB(A) stars. 

 
During the second part of the talk, I will discuss the various applications enabled but this highly-reliable and complete catalogue of OB stars. This includes the calculation of several quantities such as the scale height of the OB stars, as well as the local and Galactic star formation and core-collapse supernova rates, with statistics compatible with a past mass extinction event on Earth caused by a nearby supernova. Moreover, the local surface density formation rate derived from these OB stars, compared with a recent catalogue of star clusters, shows that star formation primarily occurs in compact clusters, in agreement with the clustered scenario of star formation. From this census were identified runaway stars as well, with estimated runaway fractions of 20% and 7% for the O- and B-type stars, respectively. 

Finally, I have applied the clustering algorithm HDBSCAN on the 3D position and the proper motions of these OB stars. In doing so I have identified 57 new OB associations within 1 kpc, substantially increasing the census in this volume, whose validity I have assessed by comparing their members with individual OB associations alongside modern catalogues of young stellar groups and open clusters. I have characterized these OB associations physically and kinematically, and found most of them are expanding in at least one direction. By comparing their location with surrounding superclouds such as the Radcliffe Wave, I show that these OB associations serve as tracers of the star formation history of the local Milky Way, encouraging to extend this census to larger distances where the position and motion of the Galactic spiral arms can be traced.