Revised spectroscopic parameters of SH${}^{+}$ from ALMA and IRAM 30 m observations

Abstract

Hydrides represent the first steps of interstellar chemistry. Sulfanylium (SH${}^{+}$), in particular, is a key tracer of energetic processes. We used ALMA and the IRAM 30 m telescope to search for the lowest frequency rotational lines of SH${}^{+}$ toward the Orion Bar, the prototypical photo-dissociation region illuminated by a strong UV radiation field. On the basis of previous $Herschel$/HIFI observations of SH${}^{+}$, we expected to detect emission of the two SH${}^{+}$ hyperfine structure (HFS) components of the $N_J=1_0-0_1$ fine structure (FS) component near 346 GHz. While we did not observe any lines at the frequencies predicted from laboratory data, we detected two emission lines, each $\sim$15 MHz above the SH${}^{+}$ predictions and with relative intensities and HFS splitting expected for SH${}^{+}$. The rest frequencies of the two newly detected lines are more compatible with the remainder of the SH${}^{+}$ laboratory data than the single line measured in the laboratory near 346 GHz and previously attributed to SH${}^{+}$. Therefore, we assign these new features to the two SH${}^{+}$ HFS components of the $N_J=1_0-0_1$ FS component and re-determine its spectroscopic parameters, which will be useful for future observations of SH${}^{+}$, in particular if its lowest frequency FS components are studied. Our observations demonstrate the suitability of these lines for SH${}^{+}$ searches at frequencies easily accessible from the ground.

arXiv comment5 pages, 1 figure, Astron. Astrophys., accepted

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• ExoMol Line List XXI: Nitric Oxide (NO)
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• Spatially resolved images of reactive ions in the Orion Bar
• Formation of interstellar SH${}^{+}$ from vibrationally excited H${}_2$: Quantum study of S${}^{+}$ + H${}_2$ $\rightleftarrows$ SH${}^{+}$ + H reactions and inelastic collisions
• Herschel/HIFI Spectral Mapping of C${}^{+}$, CH${}^{+}$, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH${}^{+}$ Formation
• Detection of CH+, SH+, and their 13C- and 34S- isotopologues toward PKS1830-211
• High-speed molecular cloudlets around the Galactic Center supermassive black hole
• Hyperfine excitation of SH${}^{+}$ by H
• Electron-impact excitation of diatomic hydride cations II: OH${}^{+}$ and SH${}^{+}$