The Discovery of 25 um Interstellar Methanol

TL;DR

This paper reports the first detection of interstellar methanol at 25 um using high-resolution mid-infrared spectroscopy, revealing molecular details in a protostellar environment and providing tools for future JWST observations.

Contribution

First astrophysical detection of methanol in the 25 um torsional band, with an updated line list to aid future JWST/MIRI studies.

Findings

Detected over seventy CH3OH absorption lines between 20-28 um.

Derived temperature of 180 K and column density of 2 x 10^17 cm^-2.

Identified multiple velocity components indicating complex protostellar structures.

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.