SOLIS. XII. SVS13-A Class I chemical complexity as revealed by S-bearing species

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 × 10⁻⁶ (CS), 7 × 10⁻⁹-1 × 10⁻⁷ (SO), 1-10 × 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 × 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.

The reduced datacubes are also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/654/A52