2013A&A...554A..87P

We study the chemistry of small hydrocarbons in the photon-dominated regions (PDRs) associated with the ultra-compact HII region (UCHII) Mon R2. Our goal is to determine the variations in the abundance of small hydrocarbons in a high-UV irradiated PDR and investigate the chemistry of these species. We present an observational study of the small hydrocarbons CH, CCH, and c-C₃H₂ in Mon R2 that combines spectral mapping data obtained with the IRAM-30 m telescope and the Herschel space observatory. We determine the column densities of these species, and compare their spatial distributions with that of polycyclic aromatic hydrocarbon (PAH), which trace the PDR. We compare the observational results with different chemical models to explore the relative importance of gas-phase, grain-surface, and time-dependent chemistry in these environments. The emission of the small hydrocarbons show different spatial patterns. The CCH emission is extended, while CH and c-C₃H₂ are concentrated towards the more illuminated layers of the PDR. The ratio of the column densities of c-C₃H₂ and CCH shows spatial variations up to a factor of a few, increasing from N(c-C₃H₂)/N(CCH)≃0.004 in the envelope to a maximum of ≃0.015-0.029 towards the 8µm emission peak. Comparing these results with other galactic PDRs, we find that the abundance of CCH is quite constant over a wide range of G₀, whereas the abundance of c-C₃H₂ is higher in low-UV PDRs, with the N(c-C₃H₂)/N(CCH) ratio ranging ≃0.008-0.08 from high to low UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account relatively well for the abundances of CH and CCH in the most exposed layers of the PDR, but falls short by a factor of 10 of reproducing c-C₃H₂. In the low-density molecular envelope, time-dependent effects and grain surface chemistry play dominant roles in determining the hydrocarbon abundances. Our study shows that the small hydrocarbons CCH and c-C₃H₂ present a complex chemistry in which UV photons, grain-surface chemistry, and time dependent effects contribute to determining their abundances. Each of these effects may be dominant depending on the local physical conditions, and the superposition of different regions along the line of sight leads to the variety of measured abundances.