2002A&A...381L..33H

The NS/CS ratio can be used to test if shocks or thermal evaporation remove grain ices during massive star formation. The two scenarios lead to differences in the subsequent chemistry: in particular, timescales are shorter if shocks are present. We have measured NS/CS ratios in six hot core sources through observation of high excitation NS, N³⁴S, C³⁴S and C³³S with the JCMT. The NS/CS ratios we find are low, 0.02-0.05, and surprisingly consistent between sources. Comparing with the models of Viti et al. (2001A&A...370.1017V), these values rule out the standard assumption of instantaneous thermal evaporation, and favour a scenario in which hot cores are shocked at an early stage with all grain mantles evaporating instantaneously.