Ortho-H2 and the age of prestellar cores
Ortho-H2 and the age of prestellar cores
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DOI：
10.1051/0004-6361/201117161
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发表时间：
2013-03
影响因子：
6.5
通讯作者：
L. Pagani;P. Lesaffre;M. Jorfi;P. Honvault;T. González-Lezana;A. Faure
中科院分区：
文献类型：
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作者：
L. Pagani;P. Lesaffre;M. Jorfi;P. Honvault;T. González-Lezana;A. Faure
恒星前核是由冷气和尘埃物质在暗云中收缩而形成的，然后它们坍塌形成原恒星。有几种同时存在的理论来描述这种收缩，但它们目前很难区分。一个主要的区别是形成前恒星核心所涉及的时间尺度：一些理论主张通过例如快速湍流衰变来接近自由落体速度，而另一些理论可以容纳更长的周期来让气体通过例如两极扩散来积累。要区分这些理论之间的区别，测量恒星前核心的年龄可能会有很大帮助。然而，目前并不存在可靠的时钟。我们提出了一个简单的化学钟，它基于邻位氢原子丰度对重氢的调节，氢原子的丰度从3的邻位-对数统计比慢慢衰减到或小于0.001。我们使用一个完全耦合到流体动力学模型的化学网络，该模型跟踪云的收缩，从均匀密度开始，并达到典型的前恒星核心的密度分布。我们计算了密度分布上的N2D+/N2H+比值。邻位-H2的消失与收缩持续时间有关，随着邻位-H2丰度的降低，N2D+/N2H+比值增大。通过调整收缩时间，我们可以得到不同的氚分布，我们可以将其与观测结果进行比较。我们的模型可以测试快速收缩(从104到106 cm−3 in∼0.5My)和慢收缩(从104到106 cm−3 in∼5My)。我们测试了模型对各种初始条件的敏感性。慢收缩文件对这些变化几乎不敏感，而快收缩文件显示出显著的变化。我们发现，在所有情况下，无论是来自快速崩塌还是来自缓慢崩塌，氚的轮廓都是清晰可辨的。我们还研究了对-d2H+/邻位-h2d+比值，发现它的变化不是单调的，因此它不区分模型。将这个模型应用于L183(=L134N)，我们发现，对于几个百万耳的演化时间尺度，N2D+/N2H+比值将高于1，而与其他参数无关，如宇宙线电离率或颗粒尺寸(在合理范围内)。只在快速收缩时才能获得与观测结果很好的拟合(≤0.7My来自收缩开始，≤4My来自分子云的诞生，这是基于当收缩开始时需要保持高的邻位-H2丰度-Oro-H2/Paro-H2≥0.2-以匹配观察结果)。因此，这个化学时钟排除了L183和稳态化学模型中缓慢收缩的可能性，因为这里显然没有达到稳态。这个时钟应该应用于其他岩心，以帮助区分大样本情况下的慢收缩和快收缩理论。
Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based on the regulation of the deuteration by the abundance of ortho‐H2 that slowly decays away from the ortho-para statistical ratio of 3 down to or less than 0.001. We use a chemical network fully coupled to a hydrodynamical model that follows the contraction of a cloud, starting from uniform density, and reaches a density profile typical of a prestellar core. We compute the N2D + /N2H + ratioalong the density profile. The disappearance of ortho-H2 is tied tothe duration of the contraction and the N2D + /N2H + ratio increases in the wake of the ortho-H2 abundance decrease. By adjusting the time of contraction, we obtain different deuteration profiles that we can compare to the observations. Our model can test fast contractions (from 10 4 to 10 6 cm −3 in ∼0.5 My) and slow contractions (from 10 4 to 10 6 cm −3 in ∼5 My). We have tested the sensitivity of the models to various initial conditions. The slowcontraction deuteration profile is approximately insensitive to these variations, while the fast-contraction deuteration profile shows significant variations. We found that, in all cases, the deuteration profile remains clearly distinguishable whether it comes from the fast collapse or the slow collapse. We also study the para-D2H + /ortho-H2D + ratio and find that its variation is not monotonic, so it does not discriminate between models. Applying this model to L183 (=L134N), we find that the N2D + /N2H + ratio would be higher than unity for evolutionary timescales of a few megayears independently of other parameters, such as cosmic ray ionization rate or grain size (within reasonable ranges). A good fit to the observations is only obtained for fast contraction (≤0.7 My from the beginning of the contraction and ≤4 My from the birth of the molecular cloud based on the need to keep a high ortho-H2 abundance when the contraction starts ‐ ortho-H2/para-H2 ≥ 0.2 ‐ to match the observations). This chemical clock therefore rules out slow contraction in L183 and steady-state chemical models, since steady state is clearly not reached here. This clock should be applied to other cores to help distinguish slow and fast contraction theories over a large sample of cases.