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

TL;DR

This study refines the spectroscopic parameters of SH$^+$ using ALMA and IRAM observations, correcting previous frequency predictions and aiding future ground-based searches of this interstellar molecule.

Contribution

The paper provides revised spectroscopic parameters for SH$^+$ based on new astronomical observations, improving the accuracy of its frequency predictions.

Findings

Detected two SH$^+$ lines near 346 GHz with better accuracy.

Reassigned these lines to specific hyperfine components of SH$^+$.

Enhanced the reliability of ground-based searches for SH$^+$.

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.