Seeds of Life in Space (SOLIS) I. Carbon-chain growth in the Solar-type protocluster OMC2-FIR4

The interstellar delivery of carbon atoms locked into molecules might be one of the key ingredients for the emergence of life. Cyanopolyynes are carbon chains delimited at their two extremities by an atom of hydrogen and a cyano group, meaning that they could be excellent reservoirs of carbon. The simplest member, HC3N, is ubiquitous in the galactic interstellar medium and found also in external galaxies. Thus, understanding the growth of cyanopolyynes in regions forming stars similar to our Sun, and what affects them, is particularly relevant. In the framework of the IRAM/NOEMA Large Program SOLIS (Seeds Of Life In Space), we have obtained a map of two cyanopolyynes, HC3N and HC5N, in the protocluster OMC-2 FIR4. Because our Sun is thought to be born in a rich cluster, OMC-2 FIR4 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 similar to 1-30, with the smallest values (<= 10) in FIR5 and in the eastern region of FIR4. The ratios <= 10 can be reproduced by chemical models only if: (1) the cosmic-ray ionisation rate zeta is similar to 4 x 10(-14) s(-1); (2) the gaseous elemental ratio C/O is close to unity; and (3) oxygen and carbon are largely depleted. The large zeta 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. We suggest that these sources could lie east of FIR4 and FIR5. A temperature gradient across FIR4, with T decreasing from east to west by about 10 K, could also explain the observed change in the HC3N/HC5N line ratio, without the need of a cosmic ray ionisation rate gradient. However, 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.