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Description
Cyanopolyynes are carbon chains delimited at their two extremities by an atom of hydrogen and a cyano group, hence they could be excellent reservoirs of carbon. The simplest member, HC3N, is ubiquitous in the galactic interstellar medium, and it is detected also in external galaxies. Because of their potential to form (macro-)molecules of biogenic importance, understanding the growth of cyanopolyynes in regions forming stars similar to our Sun (and what affects it) is particularly relevant for pre-biotic astrochemistry. In the framework of the IRAM/NOEMA Large Program SOLIS (Seeds Of Life In Space), we have mapped the two simplest cyanopolyynes, HC3N and HC5N, in the protocluster OMC-2 FIR4, which is one of the closest and best known representatives of the environment in which the Sun may have been born. We find a HC3N/HC5N abundance ratio across the source in the range 1-30. The ratios ≤10 can be reproduced by chemical models only if the cosmic-ray ionisation rate ζ is 4 × 10-14 s-1. This large ζ is comparable to that measured in FIR4 by previous works and was interpreted as due to a flux of energetic (≥10 MeV) particles from embedded sources. A temperature gradient across FIR4 could also explain the observed change in the HC3N/HC5N line ratio, but even in this case a high constant cosmic-ray ionisation rate (of the order of 10-14 s-1) is necessary to reproduce the observations. Therefore, we find that energetic particle irradiation promotes the production of carbon chains. As irradiation was also present during the early phases of our Solar System, such energetic processes could have also promoted the production of important carbon reservoirs in the Solar Nebula. Such reservoirs could then be delivered to the early Earth to foster pre-biotic chemistry evolution.
