The Connection between Warm Carbon-chain Chemistry and Interstellar Irradiation of Star-forming Cores

Abstract

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 1/10 of such irradiation. In the full irradiation model, the abundances of carbon chains were found to be higher by a factor of a few to a 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.

Chemical exploration of Galactic cold cores

“…If this trend is genuine, following the interpretation of the ratio of carbon chain to methanol by Lattanzi et al (2020), 1390 would be the most highly irradiated of our three targets. This is in line with the fact that it lies at the border of the λ Ori HII region, but contradicts the conclusion derived from the comparison of c-C 3 H 2 and C 4 H abundances in the model by Kalvāns (2021) and in our data. This discrepancy may be a sign of the model limitations in the prestellar phase.…”

“…Although the interest in carbon chains has somewhat shifted to protostellar objects following the discovery of warm carbon-chain chemistry (WCCC) in the envelope of a young stellar object (Sakai et al 2007), the understanding of carbon-chain chemistry in starless and prestellar cores remains of high interest for several reasons. The presence of carbon chains near protostars may partly originate in the prestellar chemistry (Aikawa et al 2020;Kalvāns 2021), but more importantly for our study, the current understanding is that the chemical differentiation observed from the comparison of carbon chains and oxygen-rich species such as methanol is caused by different physical (e.g. gas temperature) and environmental conditions.…”

“…gas temperature) and environmental conditions. Recent observational (Higuchi et al 2018;Lattanzi et al 2020) and modelling (Aikawa et al 2020;Kalvāns 2021) studies showed that the irradiation by interstellar photons can play a major role because in their absence, carbon atoms are locked in CO, which leads to oxygen-bearing species.…”

“…To our knowledge, no comprehensive chemodynamical models dedicated to prestellar core evolution have been published so far. A few studies (Aikawa et al 2020;Kalvāns 2021) included the prestellar phase in their calculations, but only with the explicit goal of providing realistic conditions for the protostellar phase that was the focus of the studies. We attempt a comparison of these recent modelling works with our data below, however, while keeping this limitation in mind.…”

“…Assuming that the three sources are young, the increase in the abundance of C 3 H 2 and C 4 H from 869 to 4149 may reflect an increase in age. However, it would remain possible, for example, that source 1390 is the oldest of the three sources and has already reached the abundance decrease due to freeze-out after a few 10 5 yr. Kalvāns (2021) computed the time evolution of chemical abundances during the collapse of a 1 M dense core for most of the species detected here. They provide the time evolution for cores with strong or weak shielding from the external interstellar radiation field (ISRF) and cosmic rays.…”