Theoretical studies of interstellar molecular shocks. VI - The formation of molecules containing two or three carbon atoms

The formation of 'heavy' carbon-bearing molecules in MHD shocks propagating in interstellar clouds is discussed. It is shown that substantial column densities of species such as C2, C2H, C2H2, C3, C3H and C3H2 can be produced in media where the transverse magnetic field strength is sufficiently large and the ultraviolet radiation field is sufficiently attenuated to give rise to extended acceleration zones ('magnetic precursors'). Ambipolar diffusion drives key endothermic ion-neutral reactions, in particular C3H(+) (H2, H) C3H2(+) and C3H2(+) (H2, H) C3H3(+), which are now believed to produce cyclic C3H2(+) and C3H3(+). Cyclic C3H2 may then form through dissociative recombination of C3H3(+). It is suggested that the broad emission lines of cyclic C3H2, observed by Matthews and Irvine (1985) towards W49, may be formed in a MHD shock.