Monoceros OB4: a new association in Gaia DR2
  We use Gaia DR2 data to survey the classic Monoceros OB1 region and look for
the existence of a dispersed young population, co-moving with the cloud
complex. An analysis of the distribution of proper motions reveals a 20-30 Myr
association of young stars, about 300-400 pc away from the far side of the Mon
OB1 complex, along the same general line-of-sight. We characterize the new
association, Monoceros OB4, and estimate it contains between 1400 and 2500
stars, assuming a standard IMF, putting it on par in size with NGC\,2264. We
find from the internal proper motions that Mon OB4 is unbound and expanding.
Our results seem to unveil a larger and more complex Monoceros star formation
region, suggesting an elongated arrangement that seems to be at least 300 x 60
pc.
