Astrochemistry

Investigation of the Rotational Spectrum of CH317OH and its Tentative Detection Toward Sagittarius B2(N)

By Keith Cowing

Status Report

astro-ph.G

July 13, 2024

Filed under ALMA, astro-ph.GA, astrochemistry, Interstellar, Methanol, methanol isotopolog, organic chemistry, ReMoCA, Sagittarius B2 (Sgr B2), Sgr B2(N), Sgr B2(N2b)

Investigation of the Rotational Spectrum of CH317OH and its Tentative Detection Toward Sagittarius B2(N) — Selection of rotational transitions of CH₃¹⁷OH covered by the ReMoCA survey. The LTE synthetic spectrum of CH₃¹⁷OH used to derive its column density toward Sgr B2(N2b) is displayed in red and overlaid on the observed spectrum shown in black. The blue synthetic spectrum contains the contributions of all molecules identified in our survey so far, including CH₃¹⁷OH . The values written below each panel correspond from left to right to the half-power beam width, the central frequency in MHz, the width in MHz of each panel in parentheses, and the continuum level in K of the baseline-subtracted spectra in brackets. The y-axis is labeled in brightness temperature units (K). The dotted line indicates the 3σ noise level. — astro-ph.GA

Methanol is an abundant molecule in space. The column density of CH1₃⁸OH is in some star-forming regions so high that the search for CH₃¹⁷OH is promising. But only very few transition frequencies of CH₃¹⁷OH with a microwave accuracy have been published thus far. 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).

The assignments cover 0≤J≤45, Ka≤16, and mainly the vt=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.

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.

Holger S. P. Müller, Vadim V. Ilyushin, Arnaud Belloche, Frank Lewen, Stephan Schlemmer

Comments: 16 pages, including tables, figures, references and appendices. Abstract slightly shortened. Astronomy and Astrophysics, accepted for publication
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2407.01139 [astro-ph.GA] (or arXiv:2407.01139v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2407.01139
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Submission history
From: Holger Müller
[v1] Mon, 1 Jul 2024 10:06:39 UTC (875 KB)
https://arxiv.org/abs/2407.01139
Astrobiology, Astrochemistry

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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