arXiv 1312.09712013-12astro-ph.SR

稠密云核中氘分馏时间尺度的参数空间探索

The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration

Shuo Kong, Paola Caselli, Jonathan C. Tan, Valentine Wakelam, Olli Sipilä

备注：: 23 pages, 18 figures, accepted by ApJ
发表信息：: 2015 ApJ 804 98K
DOI：: 10.1088/0004-637X/804/2/98

摘要：

氘分数[N $_2$ D $^+$ ]/[N $_2$ H $^+$ ]可以提供有关致密冷气体结构年龄的信息，这对于与云核形成和演化的动力学模型进行比较很重要。在这里，我们介绍了一个完整的化学网络与物种含有多达三个原子，与氧化学，其中涉及H $_3$ O $^+$和它的氘代形式的反应已被添加，显着提高了与全面的化学网络的一致性例外。氘化学和H $_2$和H $_3^+$同位素的自旋状态包括在这个主要的气相化学模型。我们研究了氘化学对模型参数的依赖性：密度（$n_{\rm H}$），温度，宇宙线电离率和重元素的气相耗尽因子（$f_{\rm D}$）。我们还探讨了随时间变化的冻结出的气相物种和动态演化的密度在各种速率相对于自由落体崩溃的影响。对于一个广泛的模型参数，时间尺度达到大值的$D_{\rm frac}^{\rm N_2H^+} \gtrsim 0.1$，在一些低质量和高质量的无星核心，是相对较长的本地自由落体的时间尺度相比。这些结论不受引入时间依赖性冻结和考虑具有演变密度的模型的影响，除非初始$f_{\rm D} \gtrsim$ 10。对于基准模型参数，实现$D_{\rm frac}^{\rm N_2H^+} \gtrsim 0.1$需要塌缩进行的速度至少比自由落体塌缩的速度慢几倍，这可能表明磁场在支持无星核心中的动态重要作用，从而调节恒星形成。

英文摘要：

The deuterium fraction [N$_2$D$^+$]/[N$_2$H$^+$], may provide information about the ages of dense, cold gas structures, important to compare with dynamical models of cloud core formation and evolution. Here we introduce a complete chemical network with species containing up to three atoms, with the exception of the Oxygen chemistry, where reactions involving H$_3$O$^+$ and its deuterated forms have been added, significantly improving the consistency with comprehensive chemical networks. Deuterium chemistry and spin states of H$_2$ and H$_3^+$ isotopologues are included in this primarily gas-phase chemical model. We investigate dependence of deuterium chemistry on model parameters: density ($n_{\rm H}$), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements ($f_{\rm D}$). We also explore the effects of time-dependent freeze-out of gas-phase species and dynamical evolution of density at various rates relative to free-fall collapse. For a broad range of model parameters, the timescales to reach large values of $D_{\rm frac}^{\rm N_2H^+} \gtrsim 0.1$, observed in some low- and high-mass starless cores, are relatively long compared to the local free-fall timescale. These conclusions are unaffected by introducing time-dependent freeze-out and considering models with evolving density, unless the initial $f_{\rm D} \gtrsim$ 10. For fiducial model parameters, achieving $D_{\rm frac}^{\rm N_2H^+} \gtrsim 0.1$ requires collapse to be proceeding at rates at least several times slower than that of free-fall collapse, perhaps indicating a dynamically important role for magnetic fields in the support of starless cores and thus the regulation of star formation.

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