Very Low-Mass Stars and Brown Dwarfs in Upper Scorpius using Gaia DR1:
  Mass Function, Disks and Kinematics
  Our understanding of the brown dwarf population in star forming regions is
dependent on knowing distances and proper motions, and therefore will be
improved through the Gaia space mission. In this paper, we select new samples
of very low mass objects (VLMOs) in Upper Scorpius using UKIDSS colors and
optimised proper motions calculated using Gaia DR1. The scatter in proper
motions from VLMOs in Upper Scorpius is now (for the first time) dominated by
the kinematic spread of the region itself, not by the positional uncertainties.
With age and mass estimates updated using Gaia parallaxes for early type stars
in the same region, we determine masses for all VLMOs. Our final most complete
sample includes 453 VLMOs of which $\sim$125 are expected to be brown dwarfs.
The cleanest sample is comprised of 131 VLMOs, with $\sim$105 brown dwarfs. We
also compile a joint sample from the literature which includes 415 VLMOs, out
of which 152 are likely brown dwarfs. The disc fraction among low-mass brown
dwarfs ($M<0.05 M_{\odot}$) is substantially higher than in more massive
objects, indicating that discs around low-mass brown dwarfs survive longer than
in low-mass stars overall. The mass function for $0.01<M<0.1$ $M_{\odot}$ is
consistent with the Kroupa IMF. We investigate the possibility that some
'proper motion outliers' have undergone a dynamical ejection early in their
evolution. Our analysis shows that the color-magnitude cuts used when selecting
samples introduce strong bias into the population statistics due to varying
level of contamination and completeness.
