  Some observations of warm carbon chain chemistry (WCCC) cores indicate that
they are often located near the edges of molecular clouds. This finding may
suggest that WCCC is promoted in star-forming cores exposed to radiation from
the interstellar medium. We aim to investigate the chemistry of carbon chains
in such a core. A chemical simulation of a gas parcel in a low-mass
star-forming core with a full level of irradiation by interstellar photons and
cosmic rays was compared to a simulation of a core receiving only one-tenth of
such irradiation. In the full irradiation model, the abundances of carbon
chains were found to be higher by a factor of few to few hundred, compared to
the model with low irradiation. Higher carbon-chain abundances in the
prestellar stage and, presumably, in the extended circumstellar envelope, arise
because of irradiation of gas and dust by interstellar photons and cosmic rays.
A full standard rate of cosmic-ray induced ionization is essential for a high
carbon-chain abundance peak to occur in the circumstellar envelope, which is
heated by the protostar (the "true" WCCC phenomenon). The full irradiation
model has lower abundances of complex organic molecules than the
low-irradiation model. We conclude that WCCC can be caused by exposure of a
star-forming core to interstellar radiation, or even just to cosmic rays. The
Appendix describes an updated accurate approach for calculating the rate of
cosmic-ray induced desorption.
