Observation of CH$_{3}$$^{17}$OH and CH$_{3}$$^{18}$OH in Orion KL: A New Tool to Study Star-Formation History

Abstract

Methanol is a seed species of complex organic molecules that is of fundamental importance in astrochemistry. Although various isotopologues of CH$_3$OH have been detected in the interstellar medium (ISM), CH$_{3}$$^{17}$OH is only tentatively detected in Sgr~B2. To confirm the presence of CH$_{3}$$^{17}$OH in the ISM and to investigate its abundance, we search for its emission lines in the Orion~KL region. We have obtained image cubes covering the frequency ranges 236.40~GHz-236.65~GHz and 231.68~GHz-231.88~GHz using ALMA archival data observed toward the Orion~KL region. The column densities of CH$_3$$^{17}$OH and CH$_3$$^{18}$OH are estimated under the assumption of local thermodynamic equilibrium condition with fixed excitation temperatures at the two CH$_3$$^{18}$OH peaks, MeOH1 and MeOH2,. We have identified six emission lines of CH$_{3}$$^{17}$OH in MeOH1 and MeOH2 and confirmed that the line profiles and spatial distributions are consistent with those of CH$_3$$^{18}$OH. The abundance ratios of CH$_3$$^{18}$OH/CH$_3$$^{17}$OH are evaluated to be $\sim 3.4-3.5$ and are similar to the canonical value of $^{18}$O/$^{17}$O $\sim 3-4$ derived from CO observations in the Orion~KL region. We have compared the results with the previous study of CH$_3$OH and evaluated CH$_3$$^{16}$OH/CH$_3$$^{17}$OH ratios to be $\sim 2300-2500$ at a resolution of $\sim 4$~arcsec. The ratios are close to the $^{16}$O/$^{17}$O ratio in the local ISM. This result indicates that the CH$_3$OH isotopologues can serve as new tracers of oxygen isotope ratios in star-forming regions because the opacity of CH$_3$OH 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.