Velocity-resolved [O I] 63 μm Emission in the HD 50138 Circumstellar Disk

HH 50138 is one of the brightest B[e] stars at a distance of ∼ 380 pc with strong infrared excess. The star was observed in [O I] 63 μ m and [C II] 158 μ m with high velocity resolution with upGREAT on SOFIA. The velocity resolved [O I] emission provides evidence for a large gas-disk, ∼ 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 ± 0.2 km~s −1 .