The Discovery of 25 μm Interstellar Methanol

We present the first astrophysical detection of methanol (CH 3 OH) in the torsional band near 25 μ m. Using high-resolution mid-infrared (MIR) spectroscopy, we identified over 70 gas-phase CH 3 OH absorption lines between 20 and 28 μ m 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 × 10 17 cm −2 , comparable to submillimeter measurements. Complementary analysis of acetylene (C 2 H 2 ) absorption lines is also included. Both CH 3 OH and C 2 H 2 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 CH 3 OH, which was generated from lab work and model calculations. This discovery and the updated line list will enable the search for CH 3 OH in JWST/MIRI spectra.