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

Zhichen Pan; Di Li; Qiang Chang; Lei Qian; Edwin A. Bergin; Junzhi Wang

doi:10.3847/1538-4357/aa5c33

paper

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

arXiv:1702.04857 · doi:10.3847/1538-4357/aa5c33

Abstract

We present 2.5-square-degree C $_{2}$ H N=1-0 and N $_{2}$ H $^{+}$ 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 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.

arXiv commentAccepted by ApJ, 45 pages, 10 figures

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

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Large-Scale Spectroscopic Mapping of the ρ Ophiuchi Molecular Cloud Complex I. The C2​H to N2​H+ Ratio as a Signpost of Cloud Characteristics

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Cited by in corpus (9)

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