2022A&A...662A.103B

Context. Deuteration is a precious tool for investigating the origin and formation routes of interstellar complex organic molecules in the different stages of the star formation process. Methyl cyanide (CH₃CN) is one of the most abundant interstellar complex organic molecules (iCOMs); it is of particular interest because it is among the very few iCOMs detected not only around protostars but also in protoplanetary disks. However, its formation pathways are not well known and only a few measurements of its deuterated isotopologue (CH₂DCN) have been made to date.
Aims. We studied the line emission from CH₃CN and its deuterated isotopologue CH₂DCN towards the prototypical Class I object SVS13-A, where the deuteration of a large number of species has already been reported. Our goal is to measure the CH₃CN deuteration in a Class I protostar, for the first time, in order to constrain the CH₃CN formation pathways and the chemical evolution from the early prestellar core and Class 0 to the evolved Class I stages.
Methods. We imaged CH₂DCN towards SVS13-A using the IRAM NOEMA interferometer at 3mm in the context of the Large Program SOLIS (with a spatial resolution of 1''.8 x 1''.2). The NOEMA images were complemented by the CH₃CN and CH₂DCN spectra collected by the IRAM-30m Large Program ASAI, which provided an unbiased spectral survey at 3 mm, 2 mm, and 1.3 mm. The observed line emission was analysed using local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) approaches.
Results. The NOEMA/SOLIS images of CH₂DCN show that this species emits in an unresolved area centred towards the SVS13-A continuum emission peak, suggesting that methyl cyanide and its isotopologues are associated with the hot corino of SVS13-A, previously imaged via other iCOMs. In addition, we detected 41 and 11 ASAI transitions of CH₃CN and CH₂DCN, respectively, which cover upper level energies (E_up) from 13 to 442 K and from 18 K to 200 K. The non-LTE LVG analysis of the CH₃CN lines points to a kinetic temperature of (140 ± 20) K, a gas density n_H2 >= 10⁷ cm^–3, and an emitting size of ∼0''.3, in agreement with the hypothesis that CH₃CN lines are emitted in the SVS13-A hot corino. The derived [CH₂DCN]/[CH₃CN] ratio is ∼9%. This value is consistent with those measured towards prestellar cores and a factor 2-3 higher than those measured in Class 0 protostars.
Conclusions. Contrarily to what expected for other molecular species, the CH₃CN deuteration does not show a decrease in SVS13-A with respect to measurements in younger prestellar cores and Class 0 protostars. Finally, we discuss why our new results suggest that CH₃CN was likely synthesised via gas-phase reactions and frozen onto the dust grain mantles during the cold prestellar phase.