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 carbonrich "" cyanopolyyne peak (CP) of specic oxygen-bearing molecules, believed to be products of grain surface reactions, leads us to investigate the possibility that the gradients are produced following the removal of icy grain mantles. We identify the mantle removal mechanism as the explosive desorption of photolyzed ices, induced by MHD waves as they propagate within the cloud. By identifying the protostellar object IRAS 04381]2540 as the source of these waves, we have constructed a dynamical-chemical model for the evolution of the TMC-1 ridge. The results of detailed chemical kinetic calculations are presented. We nd that injection into the gas phase of the carbon-bearing species and C 2 H 2 ,C 2 H 4 CH 4 can transiently enhance the production of many organic molecules, particularly the cyanopolyynes and polyacetylenes, and hence that the observed gradients can indeed be produced in this model. We suggest that other mantle-driven reaction pathways, involving formation of methylated chains (e.g., by CH 3 C 3 N) gas phase methyl cation transfer, as well as formation of carbenes and suboxides by fragmentation of surface-formed cumulenone compounds, might explain the presence of many other organic molecules in TMC-1. In our view the current chemical state of TMC-1 reects 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 ..

The UMIST database for astrochemistry 1999

Abstract. We report a new version of the UMIST database for astrochemistry. The previous (1995) version has been updated and its format has been revised. The database contains the rate coecients, tempera-ture ranges and { where available { the temperature dependence of 4113 gas-phase reactions important in astrophysical environments. The data involve 396 species and 12 elements. We have also tabulated permanent electric dipole moments of the neutral species and heats of formation. A new table lists the photo process cross sections (ionisation, dissociation, fragmentation) for a few species for which these quantities have been measured. Data for Deuterium fractionation are given in a separate table. Finally, a new online Java applet for data extraction has been created and its use is explained in detail. The de-tailed new datales and associated software are availabl

Molecular Distributions in the Inner Regions of Protostellar Disks

The distributions of molecules in the inner regions of a protostellar disk are presented. These were calculated using an uncoupled chemical/dynamical model, with a numerical integration of the vertical disk structure. A comparison between models with and without the effects of X-ray ionisation is made, and molecules are identified which are good tracers of the ionisation level in this part of the disk, notably CN and C_2H. In the region considered in this paper (