Large-Scale Spectroscopic Mapping of the $\ddot{I}$ Ophiuchi Molecular Cloud Complex I. The C${}_2$H to N${}_2$H${}^{+}$ Ratio as a Signpost of Cloud Characteristics

Abstract

We present 2.5-square-degree C${}_2$H N=1-0 and N${}_2$H${}^{+}$ J=1-0 maps of the $\ddot{I}$ Ophiuchi molecular cloud complex. These are the first large-scale maps of the $\ddot{I}$ Ophiuchi molecular cloud complex with these two tracers. The C${}_2$H emission is spatially more extended than the N${}_2$H${}^{+}$ emission. One faint N${}_2$H${}^{+}$ clump Oph-M and one C${}_2$H ring Oph-RingSW are identified for the first time. The observed C${}_2$H to N${}_2$H${}^{+}$ abundance ratio ([C${}_2$H]/[N${}_2$H${}^{+}$]) varies between 5 and 110. We modeled the C${}_2$H and N${}_2$H${}^{+}$ abundances with 1-D chemical models which show a clear decline of [C${}_2$H]/[N${}_2$H${}^{+}$] with chemical age. Such an evolutionary trend is little affected by temperatures when they are below 40 K. At high density (n${}_H$ $>$ 10${}^5$ 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 $\sim$10${}^5$ years). The observed [C${}_2$H]/[N${}_2$H${}^{+}$] difference between L1688 and L1689 can be explained by L1688 having chemically younger gas in relatively less dense regions. The observed [C${}_2$H]/[N${}_2$H${}^{+}$] values are the results of time evolution, accelerated at higher densities. For the relative low density regions in L1688 where only C${}_2$H emission was detected, the gas should be chemically younger.

Accepted by ApJ, 45 pages, 10 figures