L183（= L134N）的化学建模：邻位/对位H2比值的估计
Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio
摘要：
上下文在一些星前核中观察到的高氘化程度取决于H3+分馏过程中的邻-对H2比率。目标。我们想限制L183星前核的邻位/对位H2比率。这对于正确描述诸如L183的贫化岩心中的氘化放大现象以及将总（邻+对）H2 D+丰度与唯一的邻-H2 D+柱密度测量相关联是强制性的。方法.为了限制这个ortho/para H2的比例，并推导出它的配置文件，我们利用N2 D + /N2 H+的比例和ortho-H2 D+的观测进行整个星前核。我们使用两个简单的化学模型，仅限于一个几乎完全耗尽的核心描述。本文提出了新的解离复合和三氢阳离子-二氢反应速率（包括所有同位素），并将其纳入我们的模型。结果我们估计的H2 D + ortho/para的比率在L183云，并限制H2 ortho/para的比率：我们表明，它是由至少一个数量级的变化在整个星前核仍然非常高（~0.1）在大多数的云。我们的时间依赖模型表明，星前核的年龄可能超过1.5-2 × 10^5年，但可能不会老得多。我们还表明，它最近才达到目前的密度，它的收缩从一个均匀的密度云可以被约束。结论.如果不考虑分子氢和三氢阳离子同位素的整个邻/对位家族，就不能正确理解氘代化学，因为它们的关系在全球方案中至关重要。跟踪ortho/para H2比也应该给有用的约束星前核的动态演化。
英文摘要：
Context. The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H2 ratio through the H3+ fractionation. Aims. We want to constrain the ortho/para H2 ratio across the L183 prestellar core. This is mandatory to correctly describe the deuter- ation amplification phenomenon in depleted cores such as L183 and to relate the total (ortho+para) H2D+ abundance to the sole ortho-H2D+ column density measurement. Methods. To constrain this ortho/para H2 ratio and derive its profile, we make use of the N2D+ /N2H+ ratio and of the ortho-H2D+ observations performed across the prestellar core. We use two simple chemical models limited to an almost totally depleted core description. New dissociative recombination and trihydrogen cation-dihydrogen reaction rates (including all isotopologues) are presented in this paper and included in our models. Results. We estimate the H2D+ ortho/para ratio in the L183 cloud, and constrain the H2 ortho/para ratio : we show that it is varying across the prestellar core by at least an order of magnitude being still very high (~0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 x 10^5 years but that it may not be much older. We also show that it has reached its present density only recently and that its contraction from a uniform density cloud can be constrained. Conclusions. A proper understanding of deuteration chemistry cannot be attained without taking into account the whole ortho/para family of molecular hydrogen and trihydrogen cation isotopologues as their relations are of utmost importance in the global scheme. Tracing the ortho/para H2 ratio should also give useful constraints on the dynamical evolution of prestellar cores.
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