arXiv 2006.097412020-06-17astro-ph.GA

无星和星前核中晶粒尺寸分布和与尺寸相关的晶粒加热对分子丰度的影响

Effect of grain size distribution and size-dependent grain heating on molecular abundances in starless and pre-stellar cores

O. Sipilä, B. Zhao, P. Caselli

备注：: Accepted by A&A; 16 pages incl. appendices; abstract abridged to meet ArXiV requirements
发表信息：: A&A 640, A94 (2020)
DOI：: 10.1051/0004-6361/202038353

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摘要：

我们提出了一个新的气体-颗粒化学模型来限制颗粒尺寸分布对无星核和星前核中分子丰度的影响。我们同时引入宇宙射线（CR）诱导的解吸效率和晶粒平衡温度的晶粒尺寸的依赖性。我们对一组谷物种群的冰丰度进行了明确的跟踪。我们发现，依赖于大小的CR解吸效率影响冰丰度在一个高度非平凡的方式，取决于分子。起源于气相的物种遵循一个简单的模式，即最小颗粒上的冰丰度最高（分布中最丰富）。一些分子，如HCN，在整个时间演化过程中反而集中在大晶粒上，而其他分子（如$\rm N_2$）最初集中在大晶粒上，但由于解吸和氢化之间的晶粒尺寸依赖性竞争，在后期集中在小晶粒上。大多数水冰位于高密度和中等密度的小颗粒上（$n({\rm H_2}) \gtrsim 10^6 \, \rm cm^{-3}$），其中相对于总水冰储层，水冰分数可以低至大颗粒（> 0.1 $μ$ m）上的$\sim 10^{-3}$。允许晶粒的平衡温度随晶粒尺寸的变化引起强烈的变化，在相对冰丰度在低密度条件下，星际辐射场，特别是其紫外线分量不衰减。我们的研究不仅意味着在无星核阶段之前冰的初始形成，而且还意味着在进入原恒星阶段的颗粒群上的相对冰丰度。特别是，如果最小的颗粒可以失去他们的地幔，由于颗粒颗粒碰撞的核心是崩溃，冰的组成在开始的原恒星阶段可能是非常不同的，在崩溃前的阶段。

英文摘要：

We present a new gas-grain chemical model to constrain the effect of grain size distribution on molecular abundances in starless and pre-stellar cores. We introduce grain-size dependence simultaneously for cosmic-ray (CR)-induced desorption efficiency and for grain equilibrium temperatures. We keep explicit track of ice abundances on a set of grain populations. We find that the size-dependent CR desorption efficiency affects ice abundances in a highly non-trivial way that depends on the molecule. Species that originate in the gas phase follow a simple pattern where the ice abundance is highest on the smallest grains (the most abundant in the distribution). Some molecules, such as HCN, are instead concentrated on large grains throughout the time evolution, while others (like $\rm N_2$) are initially concentrated on large grains, but at late times on small grains, due to grain-size-dependent competition between desorption and hydrogenation. Most of the water ice is on small grains at high medium density ($n({\rm H_2}) \gtrsim 10^6 \, \rm cm^{-3}$), where the water ice fraction, with respect to total water ice reservoir, can be as low as $\sim 10^{-3}$ on large (> 0.1 $μ$m) grains. Allowing the grain equilibrium temperature to vary with grain size induces strong variations in relative ice abundances in low-density conditions where the interstellar radiation field and in particular its ultraviolet component are not attenuated. Our study implies consequences not only for the initial formation of ices preceding the starless core stage, but also for the relative ice abundances on the grain populations going into the protostellar stage. In particular, if the smallest grains can lose their mantles due to grain-grain collisions as the core is collapsing, the ice composition in the beginning of the protostellar stage could be very different to that in the pre-collapse phase.

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