On the nature of the enigmatic object IRAS 19312+1950: A rare phase of
  massive star formation?
  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 $\mu$m using
the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a
compact, slightly asymmetric continuum source at 170 $\mu$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
$\mu$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 $\mu$m and
CO$_2$ at 15 $\mu$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.
