SOLIS IV. Hydrocarbons in the OMC-2 FIR 4 region, a probe of energetic
  particle irradiation of the region
  We report new interferometric images of cyclopropenylidene, c-C$_3$H$_2$,
towards the young protocluster OMC-2 FIR\,4. The observations were performed at
82 and 85 GHz with the NOrthern Extended Millimeter Array (NOEMA) as part of
the project Seeds Of Life In Space (SOLIS). In addition, IRAM-30m data
observations were used to investigate the physical structure of OMC-2 FIR\,4.
We find that the c-C$_3$H$_2$ gas emits from the same region where previous
SOLIS observations showed bright HC$_5$N emission. From a non-LTE analysis of
the IRAM-30m data, the c-C$_3$H$_2$ gas has an average temperature of
$\sim$40K, a H$_2$ density of $\sim$3$\times$10$^{5}$~cm$^{-3}$, and a
c-C$_3$H$_2$ abundance relative to H$_2$ of ($7\pm1$)$\times$10$^{-12}$. In
addition, the NOEMA observations provide no sign of significant c-C$_3$H$_2$
excitation temperature gradients across the region (about 3-4 beams), with
T$_{ex}$ in the range 8$\pm$3 up to 16$\pm$7K. We thus infer that our
observations are inconsistent with a physical interaction of the OMC-2 FIR\,4
envelope with the outflow arising from OMC-2 FIR\,3, as claimed by previous
studies. The comparison of the measured c-C$_3$H$_2$ abundance with the
predictions from an astrochemical PDR model indicates that OMC-2 FIR\,4 is
irradiated by a FUV field $\sim$1000 times larger than the interstellar one,
and by a flux of ionising particles $\sim$4000 times larger than the canonical
value of $1\times10^{-17}$~s$^{-1}$ from the Galaxy cosmic rays, which is
consistent with our previous HC$_5$N observations. This provides an important
and independent confirmation of other studies that one or more sources inside
the OMC-2 FIR\,4 region emit energetic ($\geq10$~MeV) particles.
