High Resolution SOFIA/EXES Spectroscopy of CH4 and SO2 toward Massive Young Stellar Objects

The ro-vibrational transitions of molecules in the near to mid-infrared are excellent tracers of the composition, dynamics, and excitation of the inner regions of Young Stellar Objects (YSOs). They sample a wide range of excitations in a short wavelength range, they can be seen in absorption against strong hot dust continuum sources, and they trace molecules without permanent dipole moment not observable at radio wavelengths. In particular, at high infrared spectral resolution, spatial scales smaller than those imaged by millimeter wave interferometers can be studied dynamically.We present high resolution (R=λ/Δλ∼50,000-100,000 6-12 km/s) infrared (7-8 μm) spectra of massive YSOs observed with the Echelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory For Infrared Astronomy (SOFIA). Absorption lines of gas phase methane (CH₄) are detected in our Cycle 2 observations. CH₄ is thought to be a starting point of the formation of carbon chain molecules. Abundances are derived in the different dynamical regions along the sight-line towards the central star by comparing the line profiles to those of CO and other species observed at ground based facilities such as EXES' sister instrument TEXES at IRTF and Gemini. A search is also conducted for sulfur-dioxide, using data from our ongoing Cycle 4 program. SO₂ was previously detected towards these massive YSOs with the space-based ISO/SWS instrument (Keane et al. 2001, A&A 376, L5) at much lower spectral resolution (R∼2,000). At high spectral resolution we should be able to pin-point the dynamical location of this SO₂ gas. Up to 98% of the sulfur in dense clouds and protostellar envelopes is presently missing, and we are searching for that with the EXES/SOFIA observations.