Laboratory measurements and astronomical search for the HSO radical

Abstract

Context: 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.

Aims: 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.

Methods: 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.

Results: 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.

Conclusions: 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.

Keywords: ISM: molecules; line: identification; molecular data; molecular processes; radio lines: ISM.

Fig. 1

The N = 10_6,4(5) - 9_6,3(4) transition at ~397.2 GHz. In black, the signal when a mixture of H₂S and O₂ is employed in the discharge for producing HSO. In red, the recording when the radical is produced by using H₂O only and sulfur adsorbed on cell walls.

Fig. 2

The N = 17_1,16 - 16_1,15 transition at ~683.8 GHz. In black, the signal obtained by HSO produced by discharging (current = 55 mA) 43 mTorr of H₂O only and employing a modulation depth of 1.6 MHz. In red we show the signal when a magnetic field of 126 G has been applied to the previous conditions.

Fig. 3

Observed data of Orion KL, Sgr B2, and B1-b taken with different instruments (histogram black spectrum) together with the synthetic spectra obtained using the column densities given as upper limits in Table 2 (thin red curve). The synthetic total model for the 30m data of Sgr B2(N) (see Belloche et al. 2013) and Orion KL (see Cernicharo et al. 2016) is overlaid in green. A v_LSR of +9.0 km s⁻¹, +64.0 km s⁻¹, and +6.7 km s⁻¹ is assumed for Orion KL, Sgr B2, and B1-b, respectively.