The ALMA-PILS survey: first detection of the unsaturated 3-carbon molecules Propenal (C₂H₃CHO) and Propylene (C₃H₆) towards IRAS 16293–2422 B

¹ Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
e-mail: sebastien@nbi.ku.dk
² Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400 Talence, France
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, 41296 Gothenburg, Sweden
⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁶ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁸ SKA Organisation, Jodrell Bank, Lower Withington, Macclesfield, Cheshire SK11 9FT, UK
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany

Received: 7 April 2020
Accepted: 2 July 2020

Abstract

Context. Complex organic molecules with three carbon atoms are found in the earliest stages of star formation. In particular, propenal (C₂H₃CHO) is a species of interest due to its implication in the formation of more complex species and even biotic molecules.

Aims. This study aims to search for the presence of C₂H₃CHO and other three-carbon species such as propylene (C₃H₆) in the hot corino region of the low-mass protostellar binary IRAS 16293–2422 to understand their formation pathways.

Methods. We use ALMA observations in Band 6 and 7 from various surveys to search for the presence of C₃H₆ and C₂H₃CHO towards the protostar IRAS 16293–2422 B (IRAS 16293B). The identification of the species and the estimates of the column densities and excitation temperatures are carried out by modeling the observed spectrum under the assumption of local thermodynamical equilibrium.

Results. We report the detection of both C₃H₆ and C₂H₃CHO towards IRAS 16293B, however, no unblended lines were found towards the other component of the binary system, IRAS 16293A. We derive column density upper limits for C₃H₈, HCCCHO, n-C₃H₇OH, i-C₃H₇OH, C₃O, and cis-HC(O)CHO towards IRAS 16293B. We then use a three-phase chemical model to simulate the formation of these species in a typical prestellar environment followed by its hydrodynamical collapse until the birth of the central protostar. Different formation paths, such as successive hydrogenation and radical-radical additions on grain surfaces, are tested and compared to the observational results in a number of different simulations, to assess which are the dominant formation mechanisms in the most embedded region of the protostar.

Conclusions. The simulations reproduce the abundances within one order of magnitude from those observed towards IRAS 16293B, with the best agreement found for a rate of 10⁻¹² cm³ s⁻¹ for the gas-phase reaction C₃ + O → C₂ + CO. Successive hydrogenations of C₃, HC(O)CHO, and CH₃OCHO on grain surfaces are a major and crucial formation route of complex organics molecules, whereas both successive hydrogenation pathways and radical-radical addition reactions contribute to the formation of C₂H₅CHO.