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 , HC _ { 3 } N , 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 , HC _ { 3 } N and HC _ { 5 } N 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 HC _ { 3 } N/HC _ { 5 } N abundance ratio across the source in the range \sim 1 - 30 , with the smallest values ( \leq 10 ) in FIR5 and in the Eastern region of FIR4 .
The ratios \leq 10 can be reproduced by chemical models only if : ( 1 ) the cosmic-ray ionisation rate \zeta is \sim 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 ( \geq 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 HC _ { 3 } N/HC _ { 5 } N 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 .