Opening a New Mid-Infrared Window: The First 25 um Interstellar Methanol Detection

Methanol ($C{H}_{3}OH)$ is a cornerstone molecule in astrochemistry, both as an abundant constituent of interstellar ices and as a gateway to more complex organics relevant to prebiotic chemistry. 

A new study reports the first astrophysical detection of methanol in its torsional band near 25 micrometers, identified in high resolution mid-infrared spectra toward the massive protostar system NGC 7538 IRS 1 using the EXES spectrograph on the airborne SOFIA observatory. 

The team fits more than seventy gas-phase $C{H}_{3}OH$ absorption lines between 20 and 28 um and derives a temperature of about 180 K and a total column density near  $2\times {10}^{17}{\text{ cm}}^{-2}$   values consistent with sub-mm inventories when beam effects are considered.

The work matters beyond a single source. The authors release an updated 25 um $C{H}_{3}OH$ line list, derived by combining laboratory spectroscopy and modeling, and argue that this band offers a practical path to identify methanol with JWST's (James Web Telescope) MIRI instrument where other $C{H}_{3}OH$ bands are blended or suppressed by silicate absorption (Rieke et al., 2015; Wright et al., 2023). The analysis also provides new kinematic clues about IRS 1, consistent with multiple embedded protostars with edge-on disks (Moscadelli & Goddi, 2014; Beuther et al., 2017).

Key takeaways

• First interstellar detection of \(CH_3OH\) 25 um torsional band using SOFIA/EXES, with >70 absorption lines identified.
  
  
• Derived gas properties toward NGC 7538 IRS 1: T ~ 180 K, N($C{H}_{3}OH$ ) ~ $2\times {10}^{17}{\text{ cm}}^{-2}$ ; E- over A-type ratio ~ 1.43, hinting at non-equilibrium processing.
  
  
• A complementary analysis of $C_2{H}_2$  shows similar velocities but higher temperatures (~300 K), pointing to stratified layers in the same component.
  
  
• Unresolved second velocity component appears in both species, consistent with two edge-on disks and a multi-protostar system.
  
  
• The 25 um band is likely the only practical mid-IR route for $C{H}_{3}OH$ with JWST/MIRI because the 9-10 um region is blended with S(3) H2 transition and is suppressed by silicates (Gordon et al., 2022).
  
  
• Updated $C{H}_{3}OH$ line list near 25 um will aid searches in MIRI spectra and enable better modeling of disk chemistry.

How the detection works

EXES is a cross-dispersed echelle spectrograph capable of high spectral resolution in the mid-IR when flown on SOFIA (Stratospheric Observatory for Infrared Astronomy high altitude airliner) above most atmospheric water vapor (Richter et al., 2018; Young et al., 2012). 

At R ~ 60,000 the authors could isolate individual $C{H}_{3}OH$ transitions in the 20-28 um window and fit Gaussian profiles to derive optical depths, line widths, and LSR velocities. 

They then constructed rotation diagrams, assuming LTE populations and using updated partition functions and Einstein A coefficients, to estimate total column densities and excitation temperatures. The analysis was performed separately for A- and E-type methanol, which are distinct symmetry species with forbidden interconversion in the gas phase.

For IRS 1 the team finds T ~ 183 +/- 14 K and N ~ $8.1x{10}^{16}c{m}^{-2}$ for A-type, and T ~ 186 +/- 15 K and N ~ $1.15x{10}^{17}c{m}^{-2}$ for E-type, yielding an E/A abundance ratio near 1.43 +/- 0.23. The velocities cluster around $-58km{s}^{-1}$, matching the systemic speed of the source and earlier sub-mm $C{H}_{3}OH$ measurements after accounting for beam sizes (Bisschop et al., 2007; van der Tak et al., 2000). Curve-of-growth checks indicate the lines are optically thin in aggregate, supporting the rotation diagram approach.

Why 25 um is special

JWST/MIRI delivers R ~ 1500-4000 in its medium resolution integral-field spectroscopy mode, which is generally insufficient to separate many individual mid-IR molecular lines in crowded regions. 

For $C{H}_{3}OH$ the 9-10 um feature is dominated by the nu8 band but overlaps with H2 S(3) transition at 9.665 um and sits in a spectral region depressed by silicate absorption in protostellar environments, making detections ambiguous (Gordon et al., 2022; Wright et al., 2023). 

The 25 um torsional band, by contrast, emerges in a cleaner part of the spectrum and appears rich in discrete transitions even at moderate resolution. This makes it a practical target for $C{H}_{3}OH$ searches in protoplanetary disks and embedded protostars with MIRI, especially when combined with templates and the updated line list published by the authors.

Figure 1. 25.3 µm map of the NGC 7538 region from SOFIA/Faint Object infraRed Camera (FORCAST; Herter et al. 2013) archival data (Cycle 1 Program 0034, PI A. G. G. M. Tielens). Our target and brightest source, IRS 1, is at the centre with offset 0′′, 0′′ corresponding to α(J2000) = 23:13:45.37, δ(J2000) = +61:28:10.5. The positions of IRS 1 and the dimmer sources (IRS 2, IRS 3, IRS 1E, and IRS 1SE) are given by Sandell et al. (2020). In solid green is the 8.7′′×3.2′′ EXES slit from this work’s 23.9 µm setting (Table A1), and in dashed blue the 14′′ and 18′′ JCMT beams (van der Tak et al. 2000; Bisschop et al. 2007) whose CH3OH results we will use for a comparison (Section 5.1). au scale uses a distance of 2.65 kpc (Moscadelli et al. 2009). Note the logarithmic scale for the flux, which exaggerates the brightness of the non-IRS 1 sources. Credit: Nickerson et al.

What the figures show

Figure 1 overlays the EXES slit on a 25.3 um SOFIA/FORCAST map of the NGC 7538 region. IRS 1 sits at the center, with the slit enclosing the three protostars that dominate the local mid-IR continuum. The figure also shows the much larger JCMT beams used for earlier sub-mm $C{H}_{3}OH$ measurements, illustrating how emission studies average over a wider area than the pencil-beam absorption probed here. This spatial context is important when comparing column densities across wavelengths (Sandell et al., 2020).

Figure 2. Several CH3OH lines in normalized flux as observed by EXES towards NGC 7538 IRS 1. Transition labels are indicated, all of which are 2ν12 and A-type, except Q14R1, which is E-type. Note the two blended lines. There are no telluric lines present in this plot. Other features are instrumental noise. Credit: Nickerson et al.

Figure 2 presents representative $C{H}_{3}OH$ lines in normalized EXES spectra along with transition labels. 

Credit: Nickerson et al.

Figure 3 shows Gaussian fits for $C{H}_{3}OH$ and $C_2{H}_2$ lines and marks the systemic velocities of IRS 1 and two embedded protostars identified via maser kinematics. Subtle asymmetries in deeper lines point to an unresolved second velocity component, suggesting two edge-on disks in the beam consistent with prior radio maser and mm interferometric studies (Moscadelli & Goddi, 2014; Beuther et al., 2017). 

Credit: Nickerson et al.

Figure 4 shows rotation diagrams for A- and E-type CH3OH and for ortho and para states of the two $C_2{H}_2$ bands. The fits yield temperatures near 180 K for $C{H}_{3}OH$ and near 300 K for $C_2{H}_2$, pointing to chemical and thermal stratification in the same component.

Why this matters

Mid-IR absorption spectroscopy probes warm molecular gas along lines of sight to bright continuum sources, often reaching radii of 1-10 au in disks where planets assemble.

Methanol's presence and abundance there constrain how much complex organic material is inherited from cold cloud ices versus synthesized in situ. The team's $C{H}_{3}OH$ abundance estimate of order $4x{10}^{-6}$ relative to $H_2$ for the warm component is consistent with efficient ice-phase formation with subsequent thermal release in the inner disk or envelope layers. 

The measured E/A ratio above unity may indicate post-formation processing, possibly related to shocks or thermal history that perturbs the nuclear spin symmetry populations beyond the 0.69-1.0 range expected from simple formation temperature arguments (Friberg et al., 1988; Zhao et al., 2023).

Comparing $C_2{H}_2$ bands at 7.6 and 13.5 um reveals differing column densities for the same lower states, a pattern seen in other massive protostars and explained by continuum filling and spatial gradients across wavelengths in a disk geometry (Martins et al 2004). 

The kinematic consistency between $C{H}_{3}OH$ and $C_2{H}_2$ supports a common origin, while their temperature difference implies vertical layering: methanol tracing cooler outer layers and acetylene tracing warmer inner layers of an edge-on disk. Together with the unresolved line asymmetries, the picture aligns with a multiple-protostar system with at least two edge-on disks within the EXES slit.

Limitations and next steps

The EXES resolution could not fully separate the two velocity components, and the continuum origin in massive protostars remains debated. Higher spectral resolution in the same band or complementary kinematic tracers could refine the disk interpretation. 

On JWST, MIRI's moderate resolution will blend many individual lines, but the updated line list and characteristic band structure at 25 um should still enable robust template-based detections. Future work should search for this band across a broader sample of disks and embedded protostars and explore the E/A ratio as a probe of thermal and shock histories.

Conclusion

This paper opens a practical mid-IR window onto methanol in warm protostellar environments. By detecting the 25 um torsional band in absorption with SOFIA/EXES, the authors demonstrate that CH3OH can be measured close to the central sources where planet-forming material circulates. The updated line list makes the band accessible to JWST/MIRI, enabling systematic searches for methanol and more detailed modeling of chemical inheritance and disk vertical structure. For readers working on disk chemistry or JWST observations, this is a strong invitation to add the 25 um torsional band to your toolkit and to cross-check methanol inventories inferred from the sub-mm.