2018ApJ...859..136F

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 ∼40 K, a H₂ density of ∼3 x 10⁵ cm^–3, and a c-C₃H₂ abundance relative to H₂ 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 ∼1000 times larger than the interstellar one, and by a flux of ionizing particles ∼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.