Investigation of the rotational spectrum of CH₃17OH and its tentative detection toward Sagittarius B2(N)

Context. Methanol is an abundant and widespread molecule in the interstellar medium. The column density of its 18 O isotopolog, CH 3 18 OH, is in some star-forming regions so high that the search for CH 3 17 OH is promising. But only very few transition frequencies of CH 3 17 OH with a microwave accuracy have been published prior to our investigation. Aims. We want to extend the very limited rotational line list of CH 3 17 OH to be able to search for this isotopolog in the interstellar medium. Methods. We recorded the rotational spectrum of CH 3 17 OH between 38 and 1095 GHz employing a methanol sample enriched in 17 O 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 CH 3 17 OH 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 CH 3 17 OH along with secure detections of the more abundant isotopologs of methanol toward Sgr B2(N2b). The derived column densities yield isotopic ratios 12 C/ 13 C = 25, 16 O/ 18 O = 240, and 18 O/ 17 O = 3.3, which are consistent with values found earlier for other molecules in Sgr B2. Conclusions. The agreement between the 18 O/ 17 O isotopic ratio that we obtained for methanol and the 18 O/ 17 O ratios reported in the past for other molecules in Sgr B2(N) strongly supports our tentative interstellar identification of CH 3 17 OH. The accuracy of the derived line list is sufficient for further radio astronomical searches for this methanol isotopolog toward other star-forming regions.