H-MM1的干涉图。I. NH3耗竭的直接观察
An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion
摘要：
氨的光谱线NH $_3$是探测稠密分子云核物理条件的有用探针。除了在光谱学上的优势外，氨还被认为是抗冻结在颗粒表面上的，这应该使其成为研究冷的致密核心内部的优越工具。在这里，我们提出了高分辨率的NH $_3$观测甚大阵列（VLA）和绿岸望远镜（GBT）对星前核心。这些观测结果显示了一个外部区域，其部分NH $_3$丰度为X（NH $_3$）=（1.975 $\pm$ 0.005）$\times 10^{-8}$（$\pm 10\%$系统性），但它也揭示了，毕竟，X（NH $_3$）在H $_2$柱密度为$\approx 2.6 \times 10^{22}$ cm $^{-2}$以上时开始减少。我们推导了一个密度模型，发现丰度分数的断点出现在密度n（H $_2$）$\sim 2\times10^5$ cm $^{-3}$处，超过这个点，丰度分数随密度的增加而减小，遵循幂律$n^{-1.1}$。这种幂律行为很好地再现了化学模型，其中吸附到颗粒上占主导地位的去除氨和相关物种的气体在高密度。我们认为，断点密度的变化从核心到核心取决于温度和颗粒的属性，但无论如何，耗尽功率法可能是接近$n^{-1}$由于吸积的主导地位，在中央部分的无星核心。
英文摘要：
Spectral lines of ammonia, NH$_3$, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH$_3$ observations with the Very Large Array (VLA) and Green Bank Telescope (GBT) towards a prestellar core. These observations show an outer region with a fractional NH$_3$ abundance of X(NH$_3$) = (1.975$\pm$0.005)$\times 10^{-8}$ ($\pm 10\%$ systematic), but it also reveals that after all, the X(NH$_3$) starts to decrease above a H$_2$ column density of $\approx 2.6 \times 10^{22}$ cm$^{-2}$. We derive a density model for the core and find that the break-point in the fractional abundance occurs at the density n(H$_2$) $\sim 2\times10^5$ cm$^{-3}$, and beyond this point the fractional abundance decreases with increasing density, following the power law $n^{-1.1}$. This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to $n^{-1}$ owing to the dominance of accretion in the central parts of starless cores.
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