Velocity resolved [O I] 63 $\mu$m emission in the HD 50138 circumstellar
  disk
  HH 50138 is one of the brightest B[e] stars at a distance of $\sim$ 380 pc
with strong infrared excess. The star was observed in [O I] 63 $\mu$m and [C
II] 158 $\mu$m with high velocity resolution with upGREAT on SOFIA. The
velocity resolved [O I] emission provides evidence for a large gas-disk, $\sim$
760 au in size, around HD 50138. Whereas previous interferometric observations
give strong evidence for a hot gas and dust disk in Keplerian rotation, our
bservations are the first to provide unambiguous evidence for a large warm disk
around the star. Herschel/PACS observations showed that the [C II] emission is
extended, therefore the [C II] emission most likely originates in an ionized
gas shell created by a past outflow event. We confirm the isolated nature of HD
50138. It is far from any star forming region and has low proper motion.
Neither is there any sign of a remnant cloud from which it could have formed.
The extended disk around the star appears carbon poor. It shows OH and [O I]
emission, but no CO. The CO abundance appears to be at least an order of
magnitude lower than that of OH. Furthermore $^{13}$CO is enriched by more than
a factor of five, confirming that the star is not a Herbig Be star. Finally we
note that our high spectral resolution [O I] and [C II] observations provide a
very accurate heliocentric velocity of the star, 40.8 $\pm$ 0.2 km~s$^{-1}$.
