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

Abstract

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 ($\mathbf{J}$) (i.e., {\it internal alignment}) and alignment of $\mathbf{J}$ with the magnetic field ($\mathbf{B}$, i.e., {\it external alignment}). We determine the maximum grain size with efficient internal ($a_{\max ,\mathrm{aJ}}$) and external alignment ($a_{\max ,\mathrm{JB}}$) for composite grains. For the MC density of $n_{\mathrm{H}}\sim 10^3-10^8\mathrm{c}{\mathrm{m}}^{-3}$, we find that external alignment can occur for very large grains, but internal alignment only occurs for grains smaller than $a_{\max ,\mathrm{aJ}}\sim 2\mu m$. The presence of iron clusters within dust grains or suprathermal rotation increase $a_{\max ,aJ}$ to $\sim 10-50\mu m$. 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.

21 pages, 12 figures; Revised version after the referee's report

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• Physical conditions for dust grain alignment in Class 0 protostellar cores I. Observations of dust polarization and molecular irradiation tracers
• Physical conditions for dust grain alignment in Class 0 protostellar cores II. The role of the radiation field in models aligning/disrupting dust grains
• Evidence of Grain Alignment by Magnetically Enhanced Radiative Torques from Multiwavelength Dust Polarization Modeling of HL Tau
• Synthetic Modelling of Polarized Dust Emission in Intermediate-Mass YSOs: I: Constraining the Role of Iron Inclusions and Inelastic Relaxation on Grain Alignment with ALMA Polarization
• Alignment and Rotational Disruption of Dust Grains in the Galactic Centre Revealed by Polarized Dust Emission
• The rotational disruption of porous dust aggregates from ab-initio kinematic calculations
• Far-infrared Polarization of the Supernova Remnant Cassiopeia A with SOFIA HAWC+
• Grain alignment and dust evolution physics with polarisation (GRADE-POL). I. Dust polarisation modelling for isolated starless cores
• Silicate Sundogs: Probing the Effects of Grain Directionality in Exoplanet Observations