Evolution of Chemistry in the envelope of HOt corinoS (ECHOS): I. Extremely young sulphur chemistry in the isolated Class 0 object B 335

Abstract

Context. Within the project Evolution of Chemistry in the envelope of HOt corinoS (ECHOS), we present a study of sulphur chemistry in the envelope of the Class 0 source B 335 through observations in the spectral range λ = 7, 3, and 2 mm. Aims. Our goal is to characterise the sulphur chemistry in this isolated protostellar source and compare it with other Class 0 objects to determine the environmental and evolutionary effects on the sulphur chemistry in these young sources. Methods. We have modelled observations and computed column densities assuming local thermodynamic equilibrium and large velocity gradient approximation. We have also used the code Nautilus to study the time evolution of sulphur species, as well as of several sulphur molecular ratios. Results. We have detected 20 sulphur species in B 335 with a total gas-phase S abundance similar to that found in the envelopes of other Class 0 objects, but with significant differences in the abundances between sulphur carbon chains and sulphur molecules containing oxygen and nitrogen. Our results highlight the nature of B 335 as a source especially rich in sulphur carbon chains unlike other Class 0 sources. The low presence or absence of some molecules, such as SO and SO+, suggests a chemistry not particularly influenced by shocks. We, however, detect a large presence of HCS+ that, together with the low rotational temperatures obtained for all the S species (<15 K), reveals the moderate or low density of the envelope of B 335. Model results also show the large influence of the cosmic ray ionisation rate and density variations on the abundances of some S species (e.g. SO, SO2, CCS, and CCCS) with differences of up to ~4 orders of magnitude. We also find that observations are better reproduced by models with a sulphur depletion factor of 10 with respect to the sulphur cosmic elemental abundance. Conclusions. The comparison between our model and observational results for B 335 reveals an age of 104 < t < 105 yr, which high-lights the particularly early evolutionary stage of this source. B 335 presents a different chemistry compared to other young protostars that have formed in dense molecular clouds, which could be the result of accretion of surrounding material from the diffuse cloud onto the protostellar envelope of B 335. In addition, the theoretical analysis and comparison with observations of the SO2/C2S, SO/CS, and HCS+/CS ratios within a sample of prestellar cores and Class 0 objects show that they could be used as good chemical evolutionary indicators of the prestellar to protostellar transition.