2025MNRAS.544.4043G

Astrochemical observations have revealed a surprisingly high level of chemical complexity, including long carbon chains, in the earliest stages of Sun-like star formation. The origin of these species and whether they undergo further growth, possibly contributing to the molecular complexity of planetary systems, remain open questions. We present recent observations performed using the 100-m Green Bank Telescope of the prestellar core L1544, and the protostellar system IRAS 16293-2422. In L1544, we detected several complex carbon-bearing species, including C$_2$S, C$_3$S, C$_3$N, c-C$_3$H, C$_4$H, and C$_6$H, complementing previously reported emission of cyanopolyynes. In IRAS 16293-2422, we detected c-C$_3$H and, for the first time, HC$_7$N. Thanks to the high spectral resolution, we refine the rest frequencies of several c-C$_3$H and C$_6$H transitions. We perform radiative transfer analysis, highlighting a chemical difference between the two sources: IRAS 16293-2422 shows column densities 10 to 100 times lower than L1544. We perform astrochemical modelling, employing an up-to-date chemical network with revised reaction rates. Models reproduce the general trends, with cyanopolyyne and polyynyl radical abundances decreasing as molecular size increases, but underestimate the abundances of cyanopolyynes longer than HC$_5$N by up to two orders of magnitude. Current models, which include the dominant neutral-neutral formation routes, cannot account for this discrepancy, suggesting that the chemical network is incomplete. We propose that additional ion-molecule reactions are crucial for the formation of these species. Developing a more comprehensive chemical network for long carbon chains is essential for accurately interpreting present and future observations.