2026A&A...705A.132W

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.