SOLIS

Abstract

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 × 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 (Eup) 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 nH2 ≥ 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.