$ρ$蛇夫座分子云复合体的大尺度光谱制图I。C $_{2}$ H与N $_2$ H $^+$比值作为云特征的标志
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
摘要：
我们提出了2.5平方度C $_{2}$ HN =1-0和N $_2$ H $^+$ J=1-0的$ρ$蛇夫座分子云复合体的地图。这是使用这两种示踪剂绘制的第一张$ρ$蛇夫座分子云复合体的大尺度地图。C $_{2}$ H发射在空间上比N $_2$ H $^+$发射更扩展。首次鉴定出一个微弱的N-$_2$-H-$^+$团簇Oph-M和一个C-$_{2}$-H环Oph-RingSW。观察到的C $_{2}$ H与N $_{2}$ H $^{+}$丰度比（[C $_{2}$ H]/[N $_{2}$ H $^{+}$ ]）在5和110之间变化。我们用1-D化学模型模拟了C $_{2}$ H和N $_2$ H $^+$丰度，该模型显示[C $_2$ H]/[N $_2$ H $^+$ ]随化学年龄明显下降。当温度低于40 K时，这种演化趋势几乎不受温度的影响.然而，在高密度（n $_H$ $>$ 10 $^5$ cm $^{-3}$）下，丰度比下降至少一个数量级所花费的时间变得小于动态时间（例如，湍流穿越时间$\rm \sim$ 10 $^5$年）。观察到的L1688和L1689之间的[C $_2$ H]/[N $_2$ H $^+$ ]差异可以通过L1688在相对较不致密的区域中具有化学上较年轻的气体来解释。观察到的[C $_{2}$ H]/[N $_{2}$ H $^{+}$ ]值是在较高密度下加速的时间演化的结果。对于L1688中仅检测到C $_2$ H发射的相对低密度区域，气体应该是化学上较年轻的。
英文摘要：
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 $\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.
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