N2H+ and C18O depletion in a cold dark cloud
N2H+ and C18O depletion in a cold dark cloud
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DOI：
10.1086/340950
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发表时间：
2002-05-10
影响因子：
4.9
通讯作者：
Lada, CJ
中科院分区：
文献类型：
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作者：
Bergin, EA;Alves, J;Lada, CJ
我们提出了敏感的，高角分辨率的分子线观测（CO）-O-18和N2 H+对暗球B68。我们直接将这些数据与Alves，Lada和Lada的近红外消光测量进行比较，以获得星前暗云中N2 H+耗尽的第一个证据，并推断N-2。我们还发现广泛的（CO）-O-18耗尽整个中央凝聚核心的B68云。具体来说，我们发现N2 H+发射峰值在壳部分周围的尘埃消光峰。此外，N2 H+在大得多的（CO）-O-18耗尽空穴内达到峰值，并且具有较小的耗尽区，证实了理论预测。这些数据进行了分析，通过直接耦合的时间依赖性的化学模型的辐射传输代码。这一分析强调了光解离在云边缘的重要性，并表明CO丰度从边缘到中心下降了2个数量级。相比之下，N2 H+的丰度至少下降至少2倍。事实上，N2 H+和N-2完全可能在B68核心的中心区域完全不存在。N2 H+及其母体分子N-2的耗尽，开启了一种可能性，即在星星形成之前，致密核心的中心可能会逃避使用分子发射探测核心的传统方法的检测。在这些条件下，H2 D+可能是恒星形成核心最内部区域唯一可行的分子探针。
We present sensitive, high angular resolution molecular line observations of (CO)-O-18 and N2H+ toward the dark globule B68. We directly compare these data with the near-infrared extinction measurements of Alves, Lada, & Lada to derive the first evidence for the depletion of N2H+, and by inference N-2, in a prestellar dark cloud. We also find widespread (CO)-O-18 depletion throughout the centrally condensed core of the B68 cloud. Specifically, we find the N2H+ emission to peak in a shell partially surrounding the peak of dust extinction. Moreover, N2H+ peaks inside the much larger (CO)-O-18 depletion hole and has a smaller depletion zone, confirming theoretical predictions. These data are analyzed through a direct coupling of time-dependent chemical models to a radiation transfer code. This analysis highlights the importance of photodissociation at cloud edges and suggests that the CO abundance declines by 2 orders of magnitude from edge to center. In contrast, N2H+ declines in abundance, at minimum, by at least a factor of 2. Indeed, it is entirely possible that both N2H+ and N-2 are completely absent from the central regions of the B68 core. The depletion of N2H+, and its parent molecule N-2, opens up the possibility that the centers of dense cores, prior to the formation of a star, may evade detection by conventional methods of probing cores using molecular emission. Under these conditions, H2D+ may be the sole viable molecular probe of the innermost regions of star-forming cores.