Large-scale Spectroscopic Mapping of the ρ Ophiuchi Molecular Cloud Complex. I. The C2H-to-N2H+ Ratio as a Signpost of Cloud Characteristics
Large-scale Spectroscopic Mapping of the ρ Ophiuchi Molecular Cloud Complex. I. The C2H-to-N2H+ Ratio as a Signpost of Cloud Characteristics
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DOI：
10.3847/1538-4357/aa5c33
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发表时间：
2017-02
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影响因子：
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通讯作者：
Z. Pan;Di Li;Q. Chang;L. Qian;E. Bergin;Junzhi Wang
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作者：
Z. Pan;Di Li;Q. Chang;L. Qian;E. Bergin;Junzhi Wang
我们给出了2.5平方度C2H N = 1-0和N2H+ J = 1-0的蛇夫座ρ分子云复合体图。这是用这两种示踪剂绘制的蛇夫座ρ分子云复合体的第一张大尺度图。C2H发射在空间上比N2H+发射更广泛。一个微弱的N2H+团，ophh - m和一个C2H环，ophh - ringsw，是第一次被发现。观测到的C2H- N2H+丰度比（[C2H]/[N2H+]）在5 ~ 110之间变化。我们用1D化学模型模拟了C2H和N2H+的丰度，结果表明[C2H]/[N2H+]随着化学年龄的增长而明显下降。当温度低于40 K时，这种进化趋势几乎不受温度的影响。然而，在高密度（nh> 105 cm−3）下，丰度比至少下降一个数量级所需的时间小于动态时间（例如，湍流穿越时间为~ 105年）。L1688和L1689之间观测到的[C2H]/[N2H+]的差异可以用L1688在相对密度较小的区域拥有化学上较年轻的气体来解释。观测到的[C2H]/[N2H+]值是时间演化的结果，在更高的密度下加速。对于L1688中密度相对较低的区域，只检测到C2H发射，气体的化学年龄应该更小。
We present 2.5-square-degree C2H N = 1–0 and N2H+ 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 C2H emission is spatially more extended than the N2H+ emission. One faint N2H+ clump, Oph-M, and one C2H ring, Oph-RingSW, are identified for the first time. The observed C2H-to-N2H+ abundance ratio ([C2H]/[N2H+]) varies between 5 and 110. We modeled the C2H and N2H+ abundances with 1D chemical models, which show a clear decline of [C2H]/[N2H+] with chemical age. Such an evolutionary trend is little affected by temperatures when they are below 40 K. At high density (nH > 105 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 of ∼105 yr). The observed [C2H]/[N2H+] difference between L1688 and L1689 can be explained by L1688 having chemically younger gas in relatively less dense regions. The observed [C2H]/[N2H+] values are the results of time evolution, accelerated at higher densities. For the relatively low density regions in L1688 where only C2H emission was detected, the gas should be chemically younger.