CoCCoA: Complex Chemistry in hot Cores with ALMA

Selected oxygen-bearing species

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: ychen@strw.leidenuniv.nl
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
⁵ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Center for Star and Planet Formation, Niels Bohr Institute & Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Florence, Italy
¹⁰ Université Grenoble Alpes, Centre national de la recherche scientifique (CNRS), Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 414 Rue de la Piscine, 38400 Saint-Martind’Hères, France
¹¹ Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver BC V6T 1Z1, Canada
¹² Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹³ Department of Chemistry, University of Virginia, Charlottesville, 409 McCormick Road, PO Box 400319, VA 22904, USA
¹⁴ Department of Astronomy, University of Virginia, Charlottesville, 530 McCormick Road, PO Box 400325, VA 22904, USA
¹⁵ Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C 8000, Denmark
¹⁶ Astrochemistry Laboratory, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁷ Department of Physics, Catholic University of America, 200 Hannan Hall, Washington, DC 20064, USA
¹⁸ Physics Institute, Space Research and Planetary Sciences, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
¹⁹ Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA

Received: 20 March 2023
Accepted: 1 August 2023

Abstract

Context. Complex organic molecules (COMs), especially the oxygen-bearing species, have been observed to be abundant in the gas phase toward low-mass and high-mass protostars. Deep line surveys have been carried out only for a limited number of well-known star-forming regions using the Atacama Large Millimeter/submillimeter Array (ALMA), which has unprecedented resolution and sensitivity, and statistical studies on oxygen-bearing COMs (O-COMs) in a large sample of high-mass protostars using ALMA are still lacking.

Aims. We aim to determine the column density ratios of six O-COMs with respect to methanol (CH₃OH) in a sample of 14 high-mass protostellar sources in order to investigate their origin through ice and/or gas-phase chemistry. The selected species are: acetalde-hyde (CH₃CHO), ethanol (C₂H₅OH), dimethyl ether (DME; CH₃OCH₃), methyl formate (MF; CH₃OCHO), glycolaldehyde (GA; CH₂OHCHO), and ethylene glycol (EG; (CH₂OH)₂).

Methods. We fit the spectra of 14 high-mass sources observed as part of the Complex Chemistry in hot Cores with ALMA (CoCCoA) survey and derived the column densities and excitation temperatures of the six selected O-COMs. We used the minor isotopologue of methanol CH₃¹⁸OH to infer the column density of the main isotopologue CH₃OH, of which the lines are generally optically thick. We compared our O-COM ratios with those of five low-mass protostars studied with ALMA from the available literature as well as with the results from experiments and simulations.

Results. Although the CoCCoA sources have different morphologies and brightness in their continuum and methanol emission, the O-COM ratios with respect to methanol have very similar values in the high-mass and low-mass samples. The DME and MF have the highest and most constant ratios within one order of magnitude, while the other four species have lower ratios and exhibit larger scatter by one to two orders of magnitude. The ratio between DME and MF is close to one, which agrees well with previous observational findings. Current simulations and experiments can reproduce most observational trends with a few exceptions. For example, they tend to overestimate the abundance of ethanol and GA with respect to methanol.

Conclusions. The constant column density ratios of the selected O-COMs among the low- and high-mass sources suggest that these species are formed in similar environments during star formation, probably on icy dust grains in the pre-stellar stages. Where deviations are found, hypotheses exist to explain the differences between observations, simulations, and experiments, such as the involvement of gas-phase chemistry and different emitting areas of molecules.