Dependence of Gas-Phase Abundances in the Interstellar Medium on Column Density
Dependence of Gas-Phase Abundances in the Interstellar Medium on Column Density
复制标题
星际介质中气相丰度对柱密度的依赖性
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
J. Mathis
中科院分区：
文献类型：
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作者：
B. Wakker;J. Mathis
视线通过高速和中速云允许测量离子气相丰度A在非常低的值H I柱密度N（H I）。目前的观测覆盖了N（H I）的4个数量级以上。值得注意的是，对于几种离子，我们发现A与N（H I）的关系在高和低柱密度下是相同的，并且丰度在任何特定的N（H I）下具有相对低的分散度（2-3倍）。在相同的N（H I）下，Halo气体倾向于具有比盘状气体略高的A值，这表明部分分散可能归因于环境。我们注意到，Na I的离散度最大;使用Na I作为N（H I）的预测值可能导致较大的误差。低色散对星际介质物理性质的重要意义是：（1）由于成团作用，在足够长的路径长度上，N（H I）是沿视线沿着大部分H原子局域密度的合理度量;（2）颗粒的破坏主要不是发生在强冲击等灾变事件中，而是平均密度的连续函数;（3）离子附着在颗粒上和脱离颗粒的循环必须迅速，并且在各种条件下这两个速率必须大致相等;（4）在具有低平均密度的气体中，附着应该发生在更密集的浓度内。
Sight lines through high- and intermediate-velocity clouds allow measurements of ionic gas-phase abundances A at very low values of H I column density N(H I). Present observations cover over 4 orders of magnitude in N(H I). Remarkably, for several ions we find that the A versus N(H I) relation is the same at high and low column densities and that the abundances have a relatively low dispersion (factors of 2-3) at any particular N(H I). Halo gas tends to have slightly higher values of A than disk gas at the same N(H I), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for Na I; using Na I as a predictor of N(H I) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the interstellar medium are (1) because of clumping, over sufficiently long path lengths N(H I) is a reasonable measure of the local density of most of the H atoms along the sight line; (2) the destruction of grains does not mainly take place in catastrophic events such as strong shocks but is a continuous function of the mean density; (3) the cycling of the ions becoming attached to grains and being detached must be rapid, and the two rates must be roughly equal under a wide variety of conditions; and (4) in gas that has a low average density the attachment should occur within denser concentrations.