Observation of CH

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.