On the nature of the enigmatic object IRAS 19312+1950: A rare phase of massive star formation?

Abstract

IRAS 19312+1950 is a peculiar object that has eluded firm characterization since its discovery, with combined maser properties similar to an evolved star and a young stellar object (YSO). To help determine its true nature, we obtained infrared spectra of IRAS 19312+1950 in the range 5-550 $μ$m using the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a compact, slightly asymmetric continuum source at 170 $μ$m, indicative of a large, dusty circumstellar envelope. The far-IR CO emission line spectrum reveals two gas temperature components: $\approx0.22M_{\odot}$ of material at $280\pm18$ K, and $\approx1.6M_{\odot}$ of material at $157\pm3$ K. The OI 63 $μ$m line is detected on-source but no significant emission from atomic ions was found. The HIFI observations display shocked, high-velocity gas with outflow speeds up to 90 km s$^{-1}$ along the line of sight. From Spitzer spectroscopy, we identify ice absorption bands due to H$_2$O at 5.8 $μ$m and CO$_2$ at 15 $μ$m. The spectral energy distribution is consistent with a massive, luminous ($\sim2\times10^4L_{\odot}$) central source surrounded by a dense, warm circumstellar disk and envelope of total mass $\sim500$-$700M_{\odot}$, with large bipolar outflow cavities. The combination of distinctive far-IR spectral features suggest that IRAS 19312+1950 should be classified as an accreting high-mass YSO rather than an evolved star. In light of this reclassification, IRAS 19312+1950 becomes only the 5th high-mass protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH maser line ratios may not be reliable observational discriminators between evolved stars and YSOs.