Velocity resolved [O I] 63 $\mu$m emission in the HD 50138 circumstellar disk

TL;DR

This study uses high-resolution far-infrared observations to reveal a large, warm gas disk around the B[e] star HD 50138, providing new insights into its circumstellar environment and gas composition.

Contribution

First unambiguous detection of a large warm gas disk around HD 50138 using velocity-resolved [O I] emission, confirming the disk's size and properties.

Findings

Detected a ~760 au gas disk around HD 50138.

Confirmed the star's heliocentric velocity as 40.8 km/s.

Found the disk to be carbon-poor with low CO abundance.

Abstract

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$^{-1}$.