# 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

## TL;DR

This study provides large-scale spectroscopic maps of the $
ho$ Ophiuchi cloud, revealing how the C$_{2}$H to N$_2$H$^+$ ratio varies with chemical age and density, offering insights into cloud evolution.

## Contribution

First large-scale maps of $
ho$ Ophiuchi with C$_{2}$H and N$_2$H$^+$ tracers, analyzing their abundance ratio as an indicator of cloud chemical evolution.

## Key findings

- C$_{2}$H emission is more extended than N$_2$H$^+$.
- The abundance ratio [C$_{2}$H]/[N$_{2}$H$^+$] varies from 5 to 110.
- Higher density regions show faster chemical evolution.

## Abstract

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 maps of the $\rho$ 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 $\rm \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.