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 $\mu$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: $\approx 0.22M_{\odot }$ of material at $280\pm 18$ K, and $\approx 1.6M_{\odot }$ of material at $157\pm 3$ 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 ($\sim 2\times 10^4{L}_{\odot }$) central source surrounded by a dense, warm circumstellar disk and envelope of total mass $\sim 500$-$700M_{\odot }$, 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.

16 pages. Accepted for publication in ApJ

References in corpus (19)

• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• Theory of Star Formation
• The UKIRT Wide Field Camera ZYJHK Photometric System: Calibration from 2MASS
• A Class I and Class II Methanol Maser Survey of Extended Green Objects (EGOs) from the GLIMPSE Survey
• Intrinsically Red Sources observed by Spitzer in the Galactic Mid-Plane
• The dynamics of collapsing cores and star formation
• The wind of W Hya as seen by Herschel - I. The CO envelope
• A Survey of OH Masers Towards High Mass Protostellar Objects
• Multidimensional chemical modelling, II. Irradiated outflow walls
• WFCAM, Spitzer-IRAC and SCUBA observations of the massive star forming region DR21/W75: II. Stellar content and star formation
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Spitzer-IRAC GLIMPSE of high mass protostellar objects II - SED modelling of a bonafide sample
• The cooling of atomic and molecular gas in DR21
• Characterizing the Youngest Herschel-detected Protostars I. Envelope Structure Revealed by CARMA Dust Continuum Observations
• OH maser disc and outflow in the Orion-BN/KL region
• The warm CO gas along the UV-heated outflow cavity walls: a possible interpretation for the Herschel/PACS CO spectra of embedded YSOs
• Near-infrared polarimetric study of the bipolar nebula IRAS 19312+1950
• Intrinsic short time scale variability of W3(OH) maser
• Observations of the 6 Centimeter Lines of OH in Evolved (OH/IR) Stars

Cited by in corpus (9)

• First detection of methanol towards a post-AGB object, HD101584
• Structure of the Source I disk in Orion-KL
• Molecules in the peculiar age-defying source IRAS 19312+1950
• First detection of CS masers around a high-mass young stellar object, W51 e2e
• Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes
• One of Everything: The Breakthrough Listen Exotica Catalog
• Extended Ionized Fe Objects in the UWIFE Survey
• Is the nitrogen-rich source PN K4-47 a true planetary nebula?
• A New Catalog of Asymptotic Giant Branch Stars in Our Galaxy