Molecular abundance ratios as a tracer of accelerated collapse in   regions of high mass star formation?
  Recent observations suggest that the behaviour of tracer species such as N_2H+ and CS is significantly different in regions of high and low mass star formation. In the latter, N_2H+ is a good tracer of mass, while CS is not. Observations show the reverse to be true in high-mass star formation regions. We use a computational chemical model to show that the abundances of these and other species may be significantly altered by a period of accelerated collapse in high mass star forming regions. We suggest these results provide a potential explanation of the observations, and make predictions for the behaviour of other species. 
C. J. Lintott, S.Viti, J. M. C. Rawlings, D. A. Williams, T. W.   Hartquist, P. Caselli, I. Zinchenko and P. Myers