The Discovery of 25 um Interstellar Methanol
Abstract
We present the first astrophysical detection of methanol (CH3OH) in the torsional band near 25 um. Using high resolution mid-infrared (MIR) spectroscopy, we identified over seventy gas-phase CH3OH absorption lines between 20 and 28 um towards the massive protostar NGC 7538 IRS 1 with SOFIA/EXES. We derive a temperature of 180 K and a total column density of 2 x 10^17 cm-2, comparable to sub-mm 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.
Cite
@article{arxiv.2508.00059,
title = {The Discovery of 25 um Interstellar Methanol},
author = {Sarah L. Nickerson and Naseem Rangwala and Keeyoon Sung and Xinchuan Huang and Edward J. Montiel and Curtis DeWitt and Valentin J. M. Le Gouellec and Sean W. J. Colgan and Jason Dittmann and Jose Pablo Fonfría and Graham M. Harper and Kathleen E. Kraemer and Jialu Li and Conor A. Nixon and Maisie Frances Rashman and Clara Sousa-Silva and Alexander G. G. M. Tielens and William D. Vacca},
journal= {arXiv preprint arXiv:2508.00059},
year = {2025}
}
Comments
Accepted for publication in The Astrophysical Journal Letters (ApJL); 21 pages, 6 Figures