A comprehensive study of the gas-phase formation network of HC₅N: theory, experiments, observations and models

Cyanopolyynes are among the largest and most commonly observed interstellar Complex Organic Molecules in star-forming regions. They are believed to form primarily in the gas-phase, but their formation routes are not well understood. We present a comprehensive study of the gas-phase formation network of cyanobutadiyne, HC₅N, based on new theoretical calculations, kinetics experiments, astronomical observations, and astrochemical modeling. We performed new quantum mechanics calculations for six neutral-neutral reactions in order to derive reliable rate coefficients and product branching fractions. We also present new CRESU data on the rate coefficients of three of these reactions (C₃N + C₂H₂, C₂H + HC₃N, CN + C₄H₂) obtained at temperatures as low as 24 K. In practice, six out of nine reactions currently used in astrochemical models have been updated in our reviewed network. We also report the tentative detection of the ¹³C isotopologues of HC₅N in the L1544 prestellar core. We derived a lower limit of ¹²C/¹³C > 75 for the HC₅N isotopologues, which does not allow to bring new constraints to the HC5N chemistry. Finally, we verified the impact of the revised reactions by running the GRETOBAPE astrochemical model. We found good agreement between the HC₅N predicted and observed abundances in cold (∼10 K) objects, demonstrating that HC₅N is mainly formed by neutral-neutral reactions in these environments. In warm molecular shocks, instead, the predicted abundances are a factor of ten lower with respect to observed ones. In this environment possessing an higher gas ionization fraction, we speculate that the contribution of ion-neutral reactions could be significant.