Our Galaxy's youngest disc

We investigate the structure of our Galaxy's young stellar disc by fitting the distribution functions (DFs) of a new family to 5D Gaia data for a sample of 47 000 OB stars. Tests of the fitting procedure show that the young disc's DF would be strongly constrained by Gaia data if the distribution of Galactic dust were accurately known. The DF that best fits the real data accurately predicts the kinematics of stars at their observed locations, but it predicts the spatial distribution of stars poorly, almost certainly on account of errors in the best-available dust map. We argue that dust models could be greatly improved by modifying the dust model until the spatial distribution of stars predicted by a DF agreed with the data. The surface density of OB stars is predicted to peak at R similar or equal to 5.5 kpc, slightly outside the reported peak in the surface density of molecular gas; we suggest that the latter radius may have been underestimated through the use of poor kinematic distances. The velocity distributions predicted by the best-fitting DF for stars with measured line-of-sight velocities v(parallel to) reveal that the outer disc is disturbed at the level of 10 km s(-1) in agreement with earlier studies, and that the measured values of v(parallel to) have significant contributions from the orbital velocities of binaries. Hence the outer disc is colder than it is sometimes reported to be.