CoCCoA: Complex Chemistry in hot Cores with ALMA

The chemical evolution of acetone from ice to gas

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Departments of Chemistry & Astronomy, University of Virginia, Charlottesville, VA 22904, USA
³ Netherlands Organization for Applied Scientific Research (TNO), 2628 CK Delft, The Netherlands
⁴ Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
⁵ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
⁶ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

^★ Corresponding author: ychen@strw.leidenuniv.nl

Received: 11 December 2024
Accepted: 7 March 2025

Abstract

Context. Acetone (CH₃COCH₃) is one of the most abundant three-carbon oxygen-bearing complex organic molecules (O-COMs) that have been detected in space. The previous detections were made in the gas phase toward star-forming regions that are chemically rich, mostly in protostellar systems. Recently, acetone ice has also been reported as (tentatively) detected toward two low-mass protostars, allowing comparisons in acetone abundances between gas and ice. The detection of acetone ice warrants a more systematic study of its gaseous abundances which is currently lacking.

Aims. We aim to measure the gas-phase abundances of acetone in a large sample obtained from the CoCCoA program, and investigate the chemical evolution of acetone from ice to gas in protostellar systems.

Methods. We fit the ALMA spectra to determine the column density, excitation temperature, and line width of acetone in 12 high-mass protostars as part of CoCCoA. We also constrained the physical properties of propanal (C₂H₅CHO), ketene (CH₂CO), and propyne (CH₃CCH), which might be chemically linked with acetone. We discuss the possible formation pathways of acetone by making comparisons in its abundances between gas and ice and between observations and simulations.

Results. We firmly detect acetone, ketene, and propyne in the 12 high-mass protostars. The observed gas-phase abundances of acetone are surprisingly high compared to those of two-carbon O-COMs (especially aldehydes). Propanal is considered as tentatively detected due to lack of unblended lines covered in our data. The derived physical properties suggest that acetone, propanal, and ketene have the same origin from hot cores as other O-COMs, while propyne tends to trace the more extended outflows. The acetone-to-methanol ratios are higher in the solid phase than in the gas phase by one order of magnitude, which suggests gas-phase reprocessing after sublimation. There are several suggested formation pathways of acetone (in both ice and gas) from acetaldehyde, ketene, and propylene. The observed ratios between acetone and these three species are rather constant across the sample, and can be well reproduced by astrochemical simulations.

Conclusions. On the one hand, the observed high gas-phase abundances of acetone along with dimethyl ether (CH₃OCH₃) and methyl formate (CH₃OCHO) may hint at specific chemical mechanisms that favor the production of ethers, esters, and ketones over alcohols and aldehydes. On the other hand, the overall low gas-phase abundances of aldehydes may result from destruction pathways that are overlooked or underestimated in previous studies. The discussed formation pathways of acetone from acetaldehyde, ketene, and propylene seem plausible from observations and simulations, but more investigations are needed to draw more solid conclusions. We emphasize the importance of studying acetone, which is an abundant COM that deserves more attention in the future.