2017ApJ...836..194P

We present 2.5-square-degree C₂H N = 1-0 and N₂H⁺ J = 1-0 maps of the ρ Ophiuchi molecular cloud complex. These are the first large-scale maps of the ρ Ophiuchi molecular cloud complex with these two tracers. The C₂H emission is spatially more extended than the N₂H⁺ emission. One faint N₂H⁺ clump, Oph-M, and one C₂H ring, Oph-RingSW, are identified for the first time. The observed C₂H-to-N₂H⁺ abundance ratio ([C₂H]/[N₂H⁺]) varies between 5 and 110. We modeled the C₂H and N₂H⁺ abundances with 1D chemical models, which show a clear decline of [C₂H]/[N₂H⁺] with chemical age. Such an evolutionary trend is little affected by temperatures when they are below 40 K. At high density (n_H > 10⁵ cm^–3), however, the time it takes for the abundance ratio to drop at least one order of magnitude becomes less than the dynamical time (e.g., turbulence crossing time of ∼10⁵ yr). The observed [C₂H]/[N₂H⁺] difference between L1688 and L1689 can be explained by L1688 having chemically younger gas in relatively less dense regions. The observed [C₂H]/[N₂H⁺] values are the results of time evolution, accelerated at higher densities. For the relatively low density regions in L1688 where only C₂H emission was detected, the gas should be chemically younger.