Unlocking the sulphur chemistry in intermediate-mass protostars of Cygnus X
Authors: el Akel M. , Kristensen L. E. , Le Gal R. , van der Walt S. J. , Pitts R. L. , Dulieu F. 
Published: January 2022

Context. The chemistry of sulphur-bearing species in the interstellar medium remains poorly understood, but might play a key role in the chemical evolution of star-forming regions.

Aims. Coupling laboratory experiments to observations of sulphur-bearing species in different parts of star-forming regions, we aim to understand the chemical behavior of the sulphur species in cold and warm regions of protostars, and we ultimately hope to connect them.

Methods. We performed laboratory experiments in which we tested the reactivity of hydrogen sulfide (H2S) on a cold substrate with hydrogen and/or carbon monoxide (CO) under different physical conditions that allowed us to determine the products from sulphur reactions using a quadrupole mass spectrometer. The laboratory experiments were complemented by observations. We observed two luminous binary sources in the Cygnus-X star-forming complex, Cygnus X-N30 and N12, covering a frequency range of 329–361 GHz at a spatial resolution of 1′′5 with the SubMillimeter Array (SMA). This study was complemented by a 3 mm line survey of Cygnus X-N12 covering specific frequency windows in the frequency ranges 72.0–79.8 GHz at a spatial resolution of 34′′0–30′′0 and 84.2–115.5 GHz at a spatial resolution of 29′′0–21′′0, with the IRAM-30 m single-dish telescope. Column densities and excitation temperatures were derived under the local thermodynamic equilibrium approximation.

Results. We find that OCS is a direct product from H2S reacting with CO and H under cold temperatures (T < 100 K) from laboratory experiments. OCS is therefore found to be an important solid-state S-reservoir. We identify several S-species in the cold envelope of Cyg X-N12, principally organo-sulphurs (H2CS, CS, OCS, CCS, C3S, CH3SH, and HSCN). For the hot cores of Cyg X-N12 and N30, only OCS, CS and H2CS were detected. We found a difference in the S-diversity between the hot core and the cold envelope of N12, which is likely due to the sensitivity of the observations toward the hot core of N12. Moreover, based on the hot core analysis of N30, the difference in S-diversity is likely driven by chemical processes rather than the low sensitivity of the observations. Furthermore, we found that the column density ratio of NCS/NSO is also an indicator of the warm (NCS/NSO > 1), cold (NCS/NSO < 1) chemistries within the same source. The line survey and molecular abundances inferred for the sulphur species are similar for protostars N30 and N12 and depends on the protostellar component targeted (i.e., envelope or hot core) rather than on the source itself. However, the spatial distribution of emission toward Cyg X-N30 shows differences compared to N12: toward N12, all molecular emission peaks on the two continuum sources, whereas emission is spatially distributed and shows variations within molecular families (N, O, and C families) toward N30. Moreover, this spatial distribution of all the identified S-species is offset from the N30 continuum peaks. The sulphur-bearing molecules are therefore good tracers to connect the hot and cold chemistry and to provide insight into the type of object that is observed.