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

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.