2024A&A...688A.201M

Context. Methanol is an abundant and widespread molecule in the interstellar medium. The column density of its ¹⁸O isotopolog, CH₃¹⁸OH, is in some star-forming regions so high that the search for CH₃¹⁷OH is promising. But only very few transition frequencies of CH₃¹⁷ OH with a microwave accuracy have been published prior to our investigation.
Aims. We want to extend the very limited rotational line list of CH₃¹⁷OH to be able to search for this isotopolog in the interstellar medium.
Methods. We recorded the rotational spectrum of CH₃¹⁷OH between 38 and 1095 GHz employing a methanol sample enriched in ¹⁷O to 20%. A torsion-rotation Hamiltonian model based on the rho-axis method was employed to fit the data, as in our previous studies. We searched for rotational transitions of CH₃¹⁷OH in the imaging spectral line survey ReMoCA obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming region Sgr B2(N). The observed spectra were modeled under the assumption of local thermodynamic equilibrium (LTE).
Results. The assignments cover 0 ≤ J ≤ 45, K_a ≤ 16, and mainly the v_t = 0 and 1 torsional states. The Hamiltonian model describes our data well. The model was applied to derive a line list for radio-astronomical observations. We report a tentative detection of CH₃¹⁷OH along with secure detections of the more abundant isotopologs of methanol toward Sgr B2(N2b). The derived column densities yield isotopic ratios ¹²C/¹³C = 25, ¹⁶O/¹⁸O = 240, and ¹⁸O/¹⁷O = 3.3, which are consistent with values found earlier for other molecules in Sgr B2.
Conclusions. The agreement between the ¹⁸O/¹⁷O isotopic ratio that we obtained for methanol and the ¹⁸O/¹⁷O ratios reported in the past for other molecules in Sgr B2(N) strongly supports our tentative interstellar identification of CH₃¹⁷OH. The accuracy of the derived line list is sufficient for further radio astronomical searches for this methanol isotopolog toward other star-forming regions.