SOLIS IV. Hydrocarbons in the OMC-2 FIR4 Region, a Probe of Energetic Particle Irradiation of the Region

We report new interferometric images of cyclopropenylidene, c-C3H2, toward 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-30 m data observations were used to investigate the physical structure of OMC-2 FIR 4. We find that the c-C3H2 gas emits from the same region where previous SOLIS observations showed bright HC5N emission. From a non-LTE analysis of the IRAM-30 m data, the c-C3H2 gas has an average temperature of similar to 40 K, a H-2 density of similar to 3 x 10(5) cm(-3), and a c-C3H2 abundance relative to H-2 of (7 +/- 1) x 10(-12). In addition, the NOEMA observations provide no sign of significant c-C3H2 excitation temperature gradients across the region (about 3-4 beams), with T-ex in the range 8 +/- 3 up to 16 +/- 7 K. 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-C3H2 abundance with the predictions from an astrochemical PDR model indicates that OMC-2 FIR 4 is irradiated by an FUV field similar to 1000 times larger than the interstellar one, and by a flux of ionizing particles similar to 4000 times larger than the canonical value of 1 x 10(-17) s(-1) from the Galaxy cosmic rays, which is consistent with our previous HC5N observations. This provides an important and independent confirmation of other studies that one, or more, source inside the OMC-2 FIR 4 region emits energetic (>= 10 MeV) particles.