Cyanopolyyne Chemistry in TMC-1

Details Cyanopolyyne Chemistry in TMC-1 Cyanopolyyne Chemistry in TMC-1

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996ap%26ss.240...13w&link_type=abstract

Astrophysics and Space Science, Volume 240, Issue 1, pp.13-37

Astronomy and Astrophysics

Astrophysics

5

Scientific paper

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 C_n H^ + xrightarrow{N}C_n N^ + xrightarrow{{H_2 }}HC_n N^ + xrightarrow{{H_2 }}H_2 C_n N^ + xrightarrow{{e^ - }}HC_n N wheren=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), HCNxrightarrow{{C_2 H}}HC_3 Nxrightarrow{{C_2 H}}HC_5 Nxrightarrow{{C_2 H}}HC_7 Nxrightarrow{{C_2 H}}HC_9 N and (b) C_n H_2 + CN to HC_{n + 1} N + H,{text{ }}n = 4,6, and 8 depending on the reaction network used. The results indicate that for route (a) large abundances ofC 2 H (fractional abundances of ˜10-7), and for route (b) large abundances ofC 2 H 2 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 att≳5×105 yrs (i.e. photochemical timescale). The effect of other physical parameters, such as the cosmic-ray ionization 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.

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