On the abundance gradients of organic molecules along the TMC-1 ridge

Gradients in molecular abundances along the TMC-1 ridge have been observed by several authors, most recently in a comprehensive study by Pratap et al. These can be explained by there being a difference in density, C/O ratio, or chemical evolutionary state along the ridge. The presence at the carbon- rich "cyanopolyyne peak" (CP) of specific oxygen-bearing molecules, believe d to be products of grain surface reactions, leads us to investigate the po ssibility that the gradients are produced following the removal of icy grai n mantles. We identify the mantle removal mechanism as the explosive desorp tion of photolyzed ices, induced by MHD waves as they propagate within the cloud. By identifying the protostellar object IRAS 03381+2540 as the source of these waves, we have constructed a dynamical-chemical model for the evo lution of the TMC-1. ridge. The results of detailed chemical kinetic calcul ations are presented. We find that injection into the gas phase of the carb on-bearing species C2H2, C2H4 and CH4 can transiently enhance the productio n of many organic molecules, particularly the cyanopolyynes and polyacetyle nes, and hence that the observed gradients can indeed be produced in this m odel. We suggest that other mantle-driven reaction pathways, involving form ation of methylated chains (e.g., CH3C3N) by gas phase methyl cation transf er, as well as formation of carbenes and suboxides by fragmentation of surf ace-formed cumulenone compounds, might explain the presence of many other o rganic molecules in TMC-1. In our view the current chemical state of TMC-1 reflects a transient phase that accompanies the earliest epochs of low-mass formation and that regions rich in organic molecules should be observable around known "Class 0" protostellar sources and be offset from them by abou t an ion-neutral damping length. Molecular clumps in dense cores should als o show structure on this scale-length.