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-C 3 H 2 , 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-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-30 m data, the c-C 3 H 2 gas has an average temperature of ∼40 K, a H 2 density of ∼3 × 10 5 cm −3 , and a c-C 3 H 2 abundance relative to H 2 of (7 ± 1) × 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 ± 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-C 3 H 2 abundance with the predictions from an astrochemical PDR model indicates that OMC-2 FIR 4 is irradiated by an FUV field ∼1000 times larger than the interstellar one, and by a flux of ionizing particles ∼4000 times larger than the canonical value of 1 × 10 −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, source inside the OMC-2 FIR 4 region emits energetic (≥10 MeV) particles.