Cyanopolyyne chemistry in TMC-1

Abstract

Using pseudo-time-dependent models and three different reaction networks, a detailed study of the dominant reaction pathways for the formation of cyanopolyynes and their abundances in TMC-1 is presented. The analysis of the chemical reactions show that for the formation of cyanopolyynes there are two major chemical regimes. First, early times of less than ∼ 104 yrs when ion-molecule reactions are dominant, the main chemical route for the formation of larger cyanopolyynes is CnH+ →N CnN+ →H2 HCnN+ →H2 H2CnN+ →e- HCnN where n = 5, 7, and 9. Second, at times greater than 104 yrs, when neutral-neutral reactions become dominant, two major reaction routes for the formation of cyanopolyynes are (a), HCN →C2H HC3N →C2H HC5N →C2H HC7N →C2H HC9N and (b) CnH2 + CN → HCn+1N + H, n = 4,6, and 8 depending on the reaction network used. The results indicate that for route (a) large abundances of C2H (fractional abundances of ∼ 10-7), and for route (b) large abundances of C2H2 are required in order to reproduce the observed abundances of cyanopolyynes. The calculated abundances of cyanopolyynes show great sensitivity to the value of extinction particularly at t ≳ 5 x 105 yrs (i.e. photochemical timescale). The effect of other physical parameters, such as the cosmic-ray ionization rate, and the cosmic-ray induced photoreactions, the radiation field intensity and the total density on abundances are also examined. In general, the model calculations show that the observed abundances of cyanopolyynes can be achieved by pseudo-time-dependent models at late times of several million years.