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arXiv 1708.03432astro-ph.GA
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Sgr B2视线内漫射和平移云的分子化学

The Molecular Chemistry of Diffuse and Translucent Clouds in the Line-of-Sight to Sgr B2

Joanna Corby, Brett McGuire, Eric Herbst, Anthony Remijan

发表信息：
A&A 610, A10 (2018)
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摘要：

1-50 GHz GBT PRIMOS数据包含在位于银河系中心、Bar和Sgr B2（N）视线内的螺旋臂中的弥漫和半透明云中观察到的约50条分子吸收线。我们测量的列密度和估计丰度，相对于H2，11个分子和额外的同位素。我们使用c-C3 H2的光学薄跃迁的吸收来估计N（H2），并认为这种方法比更常用的方法更可取。我们讨论了小分子的运动学结构和丰度模式，包括含硫物质CS，SO，CCS，H2 CS和HCS+;含氧分子OH，SiO和H2 CO;和简单的烃分子c-C3 H2，l-C3 H和l-C3 H+。我们讨论了所观察到的化学结构的气体和尘埃中的ISM的影响。突出的成果包括以下方面。首先，尽管盘中气体的氢分子分数为0.65，但在银河棒外缘和银河中心或附近的云的氢分子分数分别为0.85和>0.9。其次，我们观察到同位素比值随银心距的变化趋势;当碳和硅的稀有同位素在银心距较低时表现出增强的趋势时，硫则没有随银心距变化的趋势; c-C3 H2/c-H13 CCCH的比值可以很好地估计12 C：13 C的比值，而H2 CO/H2^13CO则表现出分馏。第三，我们首次报道了l-C3 H+在弥漫云中的存在。最后，我们认为，CS有一个增强的丰度内密度较高的团块的材料在磁盘中，因此可能是诊断云的条件。如果这一点成立，那么银河系盘中的弥散云在一个星团结构中包含了多个嵌入的高密度，并且密度轮廓不是A_V的简单函数。

英文摘要：

The 1-50 GHz GBT PRIMOS data contains ~50 molecular absorption lines observed in diffuse and translucent clouds located in the Galactic Center, Bar, and spiral arms in the line-of-sight to Sgr B2(N). We measure the column densities and estimate abundances, relative to H2, of 11 molecules and additional isotopologues. We use absorption by optically thin transitions of c-C3H2 to estimate the N(H2), and argue that this method is preferable to more commonly used methods. We discuss the kinematic structure and abundance patterns of small molecules including the sulfur-bearing species CS, SO, CCS, H2CS, and HCS+; oxygen-bearing molecules OH, SiO, and H2CO; and simple hydrocarbon molecules c-C3H2, l-C3H, and l-C3H+. We discuss the implications of the observed chemistry for the structure of the gas and dust in the ISM. Highlighted results include the following. First, whereas gas in the disk has a molecular hydrogen fraction of 0.65, clouds on the outer edge of the Galactic Bar and in or near the Galactic Center have molecular fractions of 0.85 and >0.9, respectively. Second, we observe trends in isotope ratios with Galactocentric distance; while carbon and silicon show enhancement of the rare isotopes at low Galactocentric distances, sulfur exhibits no trend with Galactocentric distance; the ratio of c-C3H2/c-H13CCCH provides a good estimate of the 12C:13C ratio, whereas H2CO/H2^13CO exhibits fractionation. Third, we report the presence of l-C3H+ in diffuse clouds for the first time. Finally, we suggest that CS has an enhanced abundance within higher density clumps of material in the disk, and therefore may be diagnostic of cloud conditions. If this holds, the diffuse clouds in the Galactic disk contain multiple embedded hyperdensities in a clumpy structure, and the density profile is not a simple function of A_V.

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