Letter to the Editor

The ALMA-PILS survey: First detection of nitrous acid (HONO) in the interstellar medium

¹ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
e-mail: audrey.coutens@u-bordeaux.fr
² Center for Space and Habitability (CSH), University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³ Institut des Sciences Moléculaires (ISM), CNRS, Université Bordeaux, 351 cours de la Libération, 33400 Talence, France
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, 41296 Gothenburg, Sweden
⁵ Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany

Received: 10 January 2019
Accepted: 7 March 2019

Abstract

Nitrogen oxides are thought to play a significant role as a nitrogen reservoir and to potentially participate in the formation of more complex species. Until now, only NO, N₂O, and HNO have been detected in the interstellar medium. We report the first interstellar detection of nitrous acid (HONO). Twelve lines were identified towards component B of the low-mass protostellar binary IRAS 16293–2422 with the Atacama Large Millimeter/submillimeter Array, at the position where NO and N₂O have previously been seen. A local thermodynamic equilibrium model was used to derive the column density (∼9 × 10¹⁴ cm⁻² in a 0 _.″5 beam) and excitation temperature (∼100 K) of this molecule. HNO, NO₂, NO⁺, and HNO₃ were also searched for in the data, but not detected. We simulated the HONO formation using an updated version of the chemical code Nautilus and compared the results with the observations. The chemical model is able to reproduce satisfactorily the HONO, N₂O, and NO₂ abundances, but not the NO, HNO, and NH₂OH abundances. This could be due to some thermal desorption mechanisms being destructive and therefore limiting the amount of HNO and NH₂OH present in the gas phase. Other options are UV photodestruction of these species in ices or missing reactions potentially relevant at protostellar temperatures.