Globules and pillars in Cygnus X IV. Velocity-resolved [OI] 63 mu map of a peculiar proplyd-like object

Abstract

We investigated an isolated, globule-shaped object (0.37x0.11 pc), located near the centre of the Cygnus OB2 cluster and named proplyd #7 in optical observations. The source can be a massive star (with or without disc) with a HII region or a G-type T Tauri star with a photo-evaporating disc, embedded in a molecular envelope. We obtained a map of the OI line at 63 micron with 6" angular resolution and employed archival data of the CII 158 micron line (14" resolution), using the upGREAT heterodyne receiver aboard SOFIA. We also collected IRAM 30m CO data at 1mm (11" resolution). All the lines were detected across the whole object. The peak integrated OI emission of ~5 K km/s is located ~10" west of an embedded YSO. The OI and CII data near the source show bulk emission at ~11 km/s and a line wing at ~13 km/s, while the 12CO 2-1 data reveal additional blue-shifted high-velocity emission. The KOSMA-tau PDR model can explain the emissions in the tail with a low external UV field (<350 Go, mostly consistent with our UV field estimates), but not at the location of the YSO. There, the high line intensities and increased line widths for all lines and a possible bipolar CO outflow suggest the presence of a protostellar disc. However, the existence of a thermal HII region, revealed by combining existing and new radio continuum data, points towards a massive star - and not a T Tauri-type one. We derived molecular and atomic gas masses of ~20 Msun and a few Msun, respectively. The photo-evaporation (only considering external illumination) lifetime of 1.6x10^5 yrs is shorter than the free-fall lifetime of 5.2x10^5 yrs; thus, we find that proplyd #7 might not have had the time to produce many more stars.