Thermal and chemical evolution of interstellar clouds: complex molecule formation

We study the chemical evolution of an interstellar cloud at constant pressure, in which the kinetic temperature varies with time, t, or is assumed to be constant. At $$t\approx 10^{6}$$ yr, the variable temperature model is found to yield similar fractional abundances to those found at ‘early times’ $$(t\approx 10^{5}\,\text{yr})$$ in the isothermal model. Adsorption of molecules on to grains does not alter this conclusion. We suggest that the duration of the peak abundances of complex hydrocarbons and cyanopolyynes is much more realistic in the case of the variable temperature model than when a constant temperature is assumed. Comparison is made with the fractional abundances observed in the dark cloud TMC-1.