title: Laboratory measurements and astronomical search for the HSO radical.
abstract: Despite the fact that many sulfur-bearing molecules, ranging from simple diatomic species up to astronomical complex molecules, have been detected in the interstellar medium, the sulfur chemistry in space is largely unknown and a depletion in the abundance of S-containing species has been observed in the cold, dense interstellar medium (ISM). The chemical form of the missing sulfur has yet to be identified.
  
  
  
  For these reasons, in view of the fact that there is a large abundance of triatomic species harbouring sulfur, oxygen, and hydrogen, we decided to investigate the HSO radical in the laboratory to try its astronomical detection.
  
  
  
  High-resolution measurements of the rotational spectrum of the HSO radical were carried out within a frequency range well up into the THz region. Subsequently, a rigorous search for HSO in the two most studied high-mass star-forming regions, Orion KL and Sagittarius (Sgr) B2, and in the cold dark cloud Barnard 1 (B1-b) was performed.
  
  
  
  The frequency coverage and the spectral resolution of our measurements allowed us to improve and extend the existing dataset of spectroscopic parameters, thus enabling accurate frequency predictions up to the THz range. These were used to derive the synthetic spectrum of HSO, by means of the MADEX code, according to the physical parameters of the astronomical source under consideration. For all sources investigated, the lack of HSO lines above the confusion limit of the data is evident.
  
  
  
  The derived upper limit to the abundance of HSO clearly indicates that this molecule does not achieve significant abundances in either the gas phase or in the ice mantles of dust grains.
authors.affiliation: Dipartimento di Chimica "Giacomo Ciamician", Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy.
authors.forename: Gabriele
authors.initials: G
authors.lastname: Cazzoli
authors.affiliation: Dipartimento di Chimica "Giacomo Ciamician", Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy; Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching, Germany.
authors.forename: Valerio
authors.initials: V
authors.lastname: Lattanzi
authors.affiliation: Institut für Physikalische Chemie, Universität Mainz, D-55099 Mainz, Germany.
authors.forename: Till
authors.initials: T
authors.lastname: Kirsch
authors.affiliation: Institut für Physikalische Chemie, Universität Mainz, D-55099 Mainz, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Gauss
authors.affiliation: Grupo de Astrofísica Molecular. Instituto de CC. de Materiales de Madrid (ICMM-CSIC). Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain.
authors.forename: Belén
authors.initials: B
authors.lastname: Tercero
authors.affiliation: Grupo de Astrofísica Molecular. Instituto de CC. de Materiales de Madrid (ICMM-CSIC). Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain.
authors.forename: José
authors.initials: J
authors.lastname: Cernicharo
authors.affiliation: Dipartimento di Chimica "Giacomo Ciamician", Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy.
authors.forename: Cristina
authors.initials: C
authors.lastname: Puzzarini
keywords: ISM: molecules; line: identification; molecular data; molecular processes; radio lines: ISM