6 Conclusions

As found previously (Hatchell et al. 1998a), the fractional abundances of various sulphur-bearing species depend rather sensitively on depletions, hot core "age'', and grain thermal history. The work presented here shows that the abundance ratios of HCO/H₂CO and NS/CS seem particularly well suited to infer whether a shock has passed through a hot core. However, they are probably useful in this way only if freeze-out during the pre-stellar phase is not total. It should be noted that we have made some rather strong assumptions about the role of surface reactions in the chemistry of the hot cores. We have assumed that hydrogenation of species to form saturated hydrocarbons dominates on grain surfaces. In fact, the use of an approach like that of Caselli et al. (1998) might well lead us to conclude that the grain surface reactions lead to far richer surface compositions than we have supposed. We have also assumed that the shock is not powerful enough to destroy the refractory material. The release of the latter, however, would not change the conclusion of this study as the silicon released should not significantly affect the NS/CS nor the HCO/H₂CO ratios. At a fractional abundance of $\sim$10^-10, HCO is probably too weak to be detectable other than in the (3 mm) ground state transitions, even in Orion and Sgr-B2 (Turner 1989) but measurements of the NS to CS abundance ratios in hot cores should be made. NS is not a commonly observed species but detections of NS emission towards a number of hot cores were reported by Hatchell et al. (1998b): their observations of NS and CS give ratios close to $\sim$0.1. The hot cores in their sample are believed to be all at the same evolutionary stage as the authors did not find a large variation in their chemistry. Also, McGonagle & Irvine (1997) reported an extensive survey for nitrogen sulfide in Giant Molecular Clouds (GMC). They found that its fractional abundance is of the order of few 10^-10. In general, this is consistent with the estimates of our models. Their estimate for the column density of NS is, however, too uncertain to substantiate our conclusions that the CS/NS ratio could be a good indicator of shocks in hot cores: their observations were performed with spatial resolution of 86, 45 and 35 arcsec for observations of NS  $J=3/2 \Rightarrow 1/2$, NS  $J=5/2 \Rightarrow 3/2$ and NS  $J=7/2 \Rightarrow 5/2$ respectively, and they conclude that much higher spatial resolution observations of NS are indeed needed. Comparisons should be made with high resolution (interferometric) observations for a large sample of hot cores where a resolution of about 1 arcsec (necessary even for the nearest hot core) can be achieved. Indeed, the low rotational and excitation temperatures deduced from their observations suggest that NS emission arises from extended material of relatively modest density ( $< \sim 10^5$ cm^-3).

Acknowledgements

SV acknowledges the financial support of PPARC. DAW is grateful to PPARC for the award of a Senior Fellowship. PC acknowledges support from the ASI-grants ARS-98-116 and ARS-78-1999, and from the MURST research program "Dust and Molecules in Astrophysical Environments''. The authors thank A. G. Gibb and M. G. Hoare for useful discussions.