arXiv 2109.076692021-09astro-ph.GAastro-ph.EPastro-ph.SR

磁场作用下颗粒排列对颗粒生长和尘埃聚集体结构的影响

Effects of grain alignment with magnetic fields on grain growth and the structure of dust aggregates

Thiem Hoang

备注：: 21 pages, 12 figures; Revised version after the referee's report
DOI：: 10.3847/1538-4357/ac5408

摘要：

尘埃颗粒在星际介质（ISM）中漂移，并与磁场对齐。在这里，我们研究的晶粒排列和晶粒运动对晶粒生长的分子云（MC）的影响。我们首先讨论了最大惯性轴（$\hat{a}_{1}$）与其角动量（${\bf J}$）对齐的特征时标（即，{ \it内部配向}）和${\bf J}$与磁场的配向（${\bf B}$，即，{ \it外部对齐}）。我们确定的最大晶粒尺寸与有效的内部（$a_{\rm max,aJ}$）和外部对齐（$a_{\rm max,JB}$）的复合颗粒。对于$n_{\rm H}\sim 10^{3}-10^{8}\rm cm^{-3}$的MC密度，我们发现，外部对齐可以发生非常大的晶粒，但内部对齐只发生在晶粒小于$a_{\rm max,aJ}\sim 2μm$。尘埃颗粒内的铁团簇或超高温旋转的存在使$a_{\max,aJ}$增加到$\sim 10-50μm$。然后，我们研究了在气体中漂移的对准晶粒的生长。由于取向晶粒在磁场中的运动，气体吸积会增加晶粒的伸长，而不是像随机取向晶粒生长所预期的那样减少。通过颗粒碰撞的凝结也增加颗粒伸长，导致伸长随颗粒尺寸增加。对齐的颗粒凝聚形成尘埃聚集体，其中包含细长的双星，包括一对具有平行短轴的颗粒。由{ \it Rosetta}获得的67 P/Churyumov-Gerasimenko中的尘埃聚集体中的颗粒具有随颗粒半径增加的颗粒伸长，这意味着这种尘埃聚集体可能由对齐的颗粒形成。

英文摘要：

Dust grains drift through the interstellar medium (ISM) and are aligned with the magnetic field. Here we study the effect of grain alignment and grain motion on grain growth in molecular clouds (MCs). We first discuss the characteristic timescales of alignment of the grain axis of maximum inertia ($\hat{a}_{1}$) with its angular momentum (${\bf J}$) (i.e., {\it internal alignment}) and alignment of ${\bf J}$ with the magnetic field (${\bf B}$, i.e., {\it external alignment}). We determine the maximum grain size with efficient internal ($a_{\rm max,aJ}$) and external alignment ($a_{\rm max,JB}$) for composite grains. For the MC density of $n_{\rm H}\sim 10^{3}-10^{8}\rm cm^{-3}$, we find that external alignment can occur for very large grains, but internal alignment only occurs for grains smaller than $a_{\rm max,aJ}\sim 2μm$. The presence of iron clusters within dust grains or suprathermal rotation increase $a_{\max,aJ}$ to $\sim 10-50μm$. We then study the growth of aligned grains drifting through the gas. Due to the motion of aligned grains across the magnetic field, gas accretion would increase the grain elongation rather than decrease as expected from the growth of randomly oriented grains. Coagulation by grain collisions also increases grain elongation, leading to the increase of elongation with the grain size. The coagulation of aligned grains forms dust aggregates that contain elongated binaries comprising a pair of grains with parallel short axes. Grains within dust aggregates in 67P/Churyumov-Gerasimenko obtained by {\it Rosetta} have the grain elongation increasing with the grain radius, implying that such dust aggregates might form from aligned grains.

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