Observation of CH₃¹⁷OH and CH₃¹⁸OH in Orion KL: A new tool to study star formation history

¹ College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan
² RIKEN Pioneering Research Institute, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan
³ Max Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 July 2025
Accepted: 9 October 2025

Abstract

Aims. Methanol is a seed species of complex organic molecules that is of fundamental importance in astrochemistry. Although various isotopologues of CH₃OH have been detected in the interstellar medium (ISM), CH₃¹⁷OH is only tentatively detected in Sgr B2. To confirm the presence of CH₃¹⁷OH in the ISM and to investigate its abundance, we searched for its emission lines in the Orion KL region.

Methods. We obtained image cubes covering the frequency ranges 236.40–236.65 GHz and 231.68–231.88 GHz with a resolution of ~2 arcsec using ALMA archival data observed toward the Orion KL region. The spectra detected at the two CH₃¹⁸OH peaks, MeOH1 and MeOH2, were compared to the spectrum and frequencies of CH₃¹⁷OH measured in the laboratory. The column densities of CH₃¹⁷OH and CH₃¹⁸OH were estimated under the assumption of local thermodynamic equilibrium conditions with fixed excitation temperatures.

Results. We have identified six emission lines of CH₃¹⁷OH in MeOH1 and MeOH2 and confirmed that the line profiles and spatial distributions are consistent with those of CH₃¹⁸OH. The abundance ratios of CH₃¹⁸OH/CH₃¹⁷OH are evaluated to be ~3.4–3.5 and are similar to the canonical value of ¹⁸O/¹⁷O ~3–4 derived from CO observations in the Orion KL region. We have compared the results with the previous study of CH₃OH and evaluated CH₃¹⁶OH/CH₃¹⁷OH ratios to be ~2300–2500 at a resolution of ~4 arcsec. The ratios are close to the ¹⁶O/¹⁷O ratio in the local ISM.

Conclusions. This result indicates that the CH₃OH isotopologues can serve as new tracers of oxygen isotope ratios in star-forming regions because the opacity of CH₃OH can be evaluated using transition lines spanning a wide range of line intensities. Moreover, this method enables us to study the star formation history of our Galaxy with the aid of the Galactic chemical evolution models.