Cyanopolyynes as indicators of late-time chemistry and depletion in star-forming regions

Quantitative results for the time-dependent behaviour of the fractional abundance of HC3N in dense cores are given. In addition to the standard ‘early-time’ HC3N maximum found for many situations in which elements other than hydrogen are initially in atomic form, a secondary maximum occurs for a variety of conditions after depletion has become significant. The secondary maximum can be sufficiently large and occur for a sufficiently wide range of physical conditions that the assertion that the presence of high fractional abundances of cyanopolyynes implies chemical and dynamical youth for the cyanopolyyne emission peak in TMC-1 and other star-forming regions may not be secure.