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

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: hspm@ph1.uni-koeln.de
² Instituto de Ciencias de Materiales de Madrid (CSIC), 28049 Cantoblanco, Madrid, Spain
³ IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁴ CNRS UMR 8112, LERMA, Observatoire de Paris, École Normale Supérieure and Université Pierre et Marie Curie, 75014 Paris, France
⁵ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁷ Université de Bordeaux, LAB, UMR 5804, 33270 Floirac, France

Received: 5 August 2014
Accepted: 3 September 2014

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 = 1₀−0₁ 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 ~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 = 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.