THE EFFECTS OF GRAIN SIZE AND GRAIN GROWTH ON THE CHEMICAL EVOLUTION OF COLD DENSE CLOUDS
THE EFFECTS OF GRAIN SIZE AND GRAIN GROWTH ON THE CHEMICAL EVOLUTION OF COLD DENSE CLOUDS
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DOI：
10.1088/0004-637x/732/2/73
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发表时间：
2011-05
期刊：
影响因子：
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通讯作者：
K. Acharyya;G. Hassel;E. Herbst
中科院分区：
文献类型：
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作者：
K. Acharyya;G. Hassel;E. Herbst
为了研究颗粒大小分布和颗粒生长对分子丰度的影响，我们用气粒数值模拟程序研究了冷密星际云化学演化过程中分子的形成。基于较早的模型，使用了三种初始尺寸分布。为了合并不同的颗粒大小，我们将所用尺寸的分布划分为五个对数等距范围，在每个范围内进行积分以求出其总颗粒数密度，并将该数密度分配给该范围内的平均尺寸。我们利用表面反应、吸附和解吸的速率系数作为颗粒大小的函数。然后，我们跟踪了五个平均尺寸的颗粒表面布居的化学演化以及气相化学。我们发现，无论有无颗粒生长，分布的总有效颗粒表面积是确定表面丰度的重要参数。对气相丰度的影响也可能是相当大的。随着时间的推移，颗粒的生长增加了气相中产生的分子(如CO)的耗尽速度。与含有固定半径为0.1μm的“经典”颗粒的模型相比，在气体-颗粒模型中使用颗粒的尺寸分布并不能改善计算和观测丰度的一致性，这一结果有助于验证冷云模型的经典颗粒近似的质量。此外，它还为今后的气粒模型提供了重要依据。
We investigate the formation of molecules during the chemical evolution of a cold dense interstellar cloud using a gas–grain numerical code in order to study the effects of grain-size distribution and grain growth on molecular abundances. Three initial size distributions have been used, based on earlier models. To incorporate different granular sizes, we divided the distribution of sizes utilized into five logarithmically equally spaced ranges, integrated over each range to find its total granular number density, and assigned that number density to an average size in that range. We utilized rate coefficients for surface reactions, accretion, and desorption as functions of grain size. We then followed the chemical evolution of the surface populations of the five average-sized grains along with the gas-phase chemistry. We find that the total effective granular surface area of a distribution is an important parameter in the determination of surface abundances, with and without grain growth. The effect on gas-phase abundances can also be sizable. Grain growth with time increases the rate of depletion of molecules, such as CO, produced in the gas phase. Use of a size distribution for grains in gas–grain models does not improve the agreement of calculated and observed abundances, in the gas or on grains, as compared with models containing “classical” grains of a fixed radius of 0.1 μm. This result helps to verify the quality of the classical grain approximation for cold cloud models. Further, it provides an important basis for future gas–grain models.