2014A&A...569L...5M

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 30m 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=1₀-0₁ fine structure (FS) component near 346GHz. While we did not observe any lines at the frequencies predicted from laboratory data, we detected two emission lines, each ∼15MHz 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 346GHz and previously attributed to SH⁺. Therefore, we assign these new features to the two SH⁺ HFS components of the N_J=1₀-0₁ 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.