2021A&A...654A..52C

Context. Recent results in astrochemistry have revealed that some molecules, such as interstellar complex organic species and deuterated species, can serve as valuable tools in the investigation of star-forming regions. Sulphuretted species can also be used to follow the chemical evolution of the early stages of a Sun-like star formation process.
Aims. The goal is to obtain a census of S-bearing species using interferometric images towards SVS13-A, a Class I object associated with a hot corino that is rich in interstellar complex organic molecules.
Methods. To this end, we used the NGC 1333 SVS13-A data at 3 mm and 1.4 mm obtained with the IRAM-NOEMA interferometer in the framework of the SOLIS (Seeds of Life in Space) Large Program. The line emission of S-bearing species was imaged and analyzed using local thermodynamic equilibrium (LTE) and large velocity gradient (LVG) approaches.
Results. We imaged the spatial distribution on ≤300 au scale of the line emission of ³²SO, ³⁴SO, C³²S, C³⁴S, C³³S, OCS, H₂C³²S, H₂C³⁴S, and NS. The low excitation (9 K) ³²SO line traces: (i) the low-velocity SVS13-A outflow and (ii) the fast (up to 100 km s^–1 away from the systemic velocity) collimated jet driven by the nearby SVS13-B Class 0 object. Conversely, the rest of the lines are confined in the inner SVS13-A region, where complex organics were previously imaged. More specifically, the non-LTE LVG analysis of SO, SO₂, and H₂CS indicates a hot corino origin (size in the 60-120 au range). Temperatures between 50 K and 300 K, as well as volume densities larger than 10⁵ cm^–3 have been derived. The abundances of the sulphuretted are in the following ranges: 0.3-6 x 10^–6 (CS), 7 x 10^–9-1 x 10^–7 (SO), 1-10 x 10^–7 (SO₂), a few 10^–10 (H₂CS and OCS), and 10^–10-10^–9 (NS). The N(NS)/N(NS⁺) ratio is larger than 10, supporting the assessment that the NS⁺ ion is mainly formed in the extended envelope.
Conclusions. The [H₂CS]/[H₂CO] ratio, once measured at high-spatial resolutions, increases with time (from Class 0 to Class II objects) by more than one order of magnitude (from ≤10^–2 to a few 10^–1). This suggests that [S]/[O] changes along the process of Sun-like star formation. Finally, the estimate of the [S]/[H] budget in SVS13-A is 2-17% of the Solar System value (1.8 x 10^–5), which is consistent with what was previously measured towards Class 0 objects (1-8%). This finding supports the notion that the enrichment of the sulphuretted species with respect to dark clouds remains constant from the Class 0 to the Class I stages of low-mass star formation. The present findings stress the importance of investigating the chemistry of star-forming regions using large observational surveys as well as sampling regions on the scale of the Solar System.