S. L. Nickerson, N. Rangwala, K. Sung, X. Huang, E. Montiel, C. DeWitt, V. J. M. Le Gouellec, S. W. J. Colgan, J. Dittmann, J. P. Fonfría, G. M. Harper, K. E. Kraemer, J. Li, C. A. Nixon, M. F. Rashman, C. Sousa-Silva, A. G. G. M. Tielens, W. D. Vacca

Abstract
We present the first astrophysical detection of methanol (CH₃OH) in the torsional band near 25 μm. Using high-resolution mid-infrared (MIR) spectroscopy, we identified over 70 gas-phase CH₃OH absorption lines between 20 and 28 μm toward the massive protostar NGC 7538 IRS 1 with the Stratospheric Observatory for Infrared Astronomy/Echelon-Cross-Echelle Spectrograph. We derive a temperature of 180 K and a total column density of 2 × 10¹⁷ cm^‑2, comparable to submillimeter measurements. Complementary analysis of acetylene (C₂H₂) absorption lines is also included. Both CH₃OH and C₂H₂ reveal an unresolved second velocity component. These MIR absorption lines likely probe the molecular material in two edge-on disks, supporting the scenario that NGC 7538 IRS 1 consists of multiple protostars. We provide an updated line list for the torsional band of CH₃OH, which was generated from lab work and model calculations. This discovery and the updated line list will enable the search for CH₃OH in JWST/MIRI spectra.

Keywords
astrochemistry / protostars / stars fundamental parameters / techniques spectroscopic / astrochemistry / ism general

The Astrophysical Journal
Volume 989, Number L51, Page 16
2025 August

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