Surveying the inner structure of massive young stellar objects using L-band spectroscopy^★

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300-RA Leiden, The Netherlands
e-mail: barr@strw.leidenuniv.nl
² Astronomy Department, University of Maryland, College Park, MD 20742, USA
³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ Stony Brook University, 100 Nicolls Rd, Stony Brook, NY 11794, USA
⁵ SRA, SOFIA, NASA Ames Research Center, MS 232-11, Moffett Field, CA 94035, USA

Received: 24 December 2021
Accepted: 15 August 2022

Abstract

We present results from a high spectral resolution (6 km s-1) survey of five massive protostars in the wavelength range of 2.95 and 3.25 µm, conducted with iSHELL at the InfraRed Telescope Facility (IRTF). Our targets are Mon R2 IRS 2, Mon R2 IRS 3, AFGL 2136, Orion BN and S140 IRS 1. Two of our five targets (Mon R2 IRS 3 and AFGL 2136) show transitions from organic species, with MonR2 IRS 3 showing HCN lines in emission, and AFGL 2136 showing HCN and C₂H2 lines in absorption. The velocity of the emission lines of HCN of MonR2 IRS 3A are consistent with CO emission features in lines up to J = 26, as both are red-shifted with respect to the systemic velocity. Carbon monoxide lines also show blue-shifted absorption. This P-Cygni line profile, commonly observed towards massive young stellar objects, is likely due to an expanding shell, which is supported by sub-millimetre velocity maps of HCN. Alternatively HCN emission may arise from the upper layers of a disk photosphere, as has been suggested for the massive protostar AFGL 2591. Absorption lines in AFGL 2136 may either originate in foreground cloud or in the disk photosphere. For a foreground cloud, the data require that the foreground gas only covers the source partially (0.3) at 13 µm. In contrast, absorption lines at 3 and 7 µm require a covering factor of >0.9. Analysing the 13 µm HCN absorption lines in terms of absorption by gas in the photosphere of a disk, results in physical conditions that are consistent over all three vibrational modes. C₂H₂ absorption lines reveal an increasing temperature and abundance with decreasing wavelength, indicative of a radial abundance gradient. We conclude that the disk model is the best interpretation of the absorption lines of AFGL 2136.