SOLIS II. 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, so that they might 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 it, 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 OMC2-FIR4. Because our Sun is thought to be born in a rich
cluster, OMC2-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 ~ 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 ~ $4
\times 10^{-14}$ s$^{-1}$; (2) the gaseous elemental ratio C/O is close to
unity; (3) O and C 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 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.
