Carbon-Chain Species in Warm-up Models

Abstract

In previous warm-up chemical models of the low-mass star-forming region L1527, we investigated the evolution of carbon-chain unsaturated hydrocarbon species when the envelope temperature is slightly elevated to $T\approx 30$ K. These models demonstrated that enhanced abundances of such species can be explained by gas-phase ion-molecule chemistry following the partial sublimation of methane from grain surfaces. We also concluded that the abundances of hydrocarbon radicals such as the C$_{\rm n}$H family should be further enhanced as the temperatures increase to higher values, but this conclusion stood in contrast with the lack of unambiguous detection of these species toward hot core and corino sources. Meanwhile, observational surveys have identified C$_2$H, C$_4$H, CH$_3$CCH, and CH$_3$OH toward hot corinos (especially IRAS 16293-2422) as well as towards L1527, with lower abundances for the carbon chain radicals and higher abundances for the other two species toward the hot corinos. In addition, the {\it Herschel Space Telescope} has detected the bare linear chain C$_3$ in 50 K material surrounding young high-mass stellar objects. To understand these new results, we revisit previous warm-up models with an augmented gas-grain network that incorporated reactions from a gas-phase network that was constructed for use with increased temperature up to 800 K. Some of the newly adopted reactions between carbon-chain species and abundant H$_2$ possess chemical activation energy barriers. The revised model results now better reproduce the observed abundances of unsaturated carbon chains under hot-corino (100 K) conditions and make predictions for the abundances of bare carbon chains in the 50 K regions observed by Herschel HIFI.