PRODIGE – envelope to disk with NOEMA

V. Low ¹²C/¹³C ratios for CH₃OH and CH₃CN in hot corinos

¹ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
² Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Taiwan Astronomical Research Alliance (TARA), Taiwan
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan
⁶ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁷ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406, Saint-Martin d’Hères, France
⁸ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
¹⁰ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France

^★ Corresponding author: lbusch@mpe.mpg.de

Received: 11 February 2025
Accepted: 27 May 2025

Abstract

Context. The ¹²C/¹³C isotope ratio has been derived towards numerous cold clouds (~20-50 K) and a couple of protoplanetary disks and exoplanet atmospheres. However, direct measurements of this ratio in the warm gas (>100 K) around young low-mass protostars remain scarce but are required to study its evolution during star and planet formation.

Aims. We aim to derive ¹²C/¹³C ratios from the isotopologues of the complex organic molecules (COMs) CH₃OH and CH₃CN in the warm gas towards seven Class 0/I protostellar systems to improve our understanding of the evolution of the ¹²C/¹³C ratios during star and planet formation.

Methods. We used the data that were taken as part of the PROtostars & DIsks: Global Evolution (PRODIGE) large program with the Northern Extended Millimetre Array (NOEMA) at 1 mm. The ¹³C isotopologue of CH₃OH was detected towards seven sources of the sample, those of CH₃CN were detected towards six sources. The emission spectra were analysed by deriving synthetic spectra and population diagrams assuming conditions of local thermodynamic equilibrium.

Results. The emission of CH₃OH and CH₃CN is spatially unresolved in the PRODIGE data with a resolution of ~1″(~300 au) for the seven targeted systems. Rotational temperatures derived from both COMs exceed 100 K, telling us that they trace the gas of the hot corino, where CH₃CN probes hotter regions than CH₃OH on average (290 K versus 180 K). The column density ratios between the ¹²C and ¹³C isotopologues range from 4 to 30, which is lower by factors of a few up to an order of magnitude than the expected isotope ratio of the local interstellar medium of ~68. We conducted astrochemical models to understand the origins of the observed low ratios. We studied potential precursor molecules of CH₃ OH and CH₃ CN since the model does not include COMs, assuming that the ratio is transferred in reactions from the precursors to the COMs. The model predicts ¹²C/¹³C ratios close to the ISM value for CO and H₂CO, precursors of CH₃OH, in contrast to our observational results. For the potential precursors of CH₃CN (CN, HCN, and HNC), the model predicts low ¹²C/¹³C ratios close to the protostar (<300 au). Hence, they may also be expected for CH₃CN.

Conclusions. Our results show that an enrichment in ¹³C in COMs at the earliest protostellar stages is likely inherited from the precursor species of the COMs, whose ¹²C/¹³C ratios are set during the prestellar stage via isotopic exchange reactions. This also implies that low ¹²C/¹³C ratios observed at later evolutionary stages such as protoplanetary disks and exoplanetary atmospheres could at least partially be inherited. A definitive conclusion on ¹²C/¹³C ratios in protostellar environments requires observations at higher angular and spectral resolution that simultaneously cover a broad bandwidth, to tackle current observational limitations, and additional modelling efforts.