2025A&A...699A.359B

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.