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We present 2.5-square-degree C$_{2}$H N=1-0 and N$_2$H$^+$ J=1-0 maps of the$\rho$ Ophiuchi molecular cloud complex. These are the first large-scale mapsof the $\rho$ Ophiuchi molecular cloud complex with these two tracers. TheC$_{2}$H emission is spatially more extended than the N$_2$H$^+$ emission. Onefaint N$_2$H$^+$ clump Oph-M and one C$_{2}$H ring Oph-RingSW are identifiedfor 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$_{2}$Hand N$_2$H$^+$ abundances with 1-D chemical models which show a clear declineof [C$_2$H]/[N$_2$H$^+$] with chemical age. Such an evolutionary trend islittle 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 abundanceratio to drop at least one order of magnitude becomes less than the dynamicaltime (e.g., turbulence crossing time $\rm \sim$10$^5$ years). The observed[C$_2$H]/[N$_2$H$^+$] difference between L1688 and L1689 can be explained byL1688 having chemically younger gas in relatively less dense regions. Theobserved [C$_{2}$H]/[N$_{2}$H$^{+}$] values are the results of time evolution,accelerated at higher densities. For the relative low density regions in L1688where only C$_2$H emission was detected, the gas should be chemically younger.

astrophysical journal/the astrophysical journal

Large-Scale Spectroscopic Mapping of the $\rho$ Ophiuchi Molecular Cloud  Complex I. The C$_{2}$H to N$_2$H$^+$ Ratio as a Signpost of Cloud  Characteristics