Abstract
Context. The degree of coupling between the gas and the magnetic field during the collapse of a core and the subsequent formation of a disk depends on the assumed dust size distribution. Aims. We study the impact of grain–grain coagulation on the evolution of magnetohydrodynamic (MHD) resistivities during the collapse of a prestellar core. Methods. We use a 1D model to follow the evolution of the dust size distribution, out-of-equilibrium ionisation state, and gas chemistry during the collapse of a prestellar core. To compute the grain–grain collisional rate, we consider models for both random and systematic, size-dependent, velocities. We include grain growth through grain–grain coagulation and ice accretion, but ignore grain fragmentation. Results. Starting with a Mathis-Rumpl-Nordsieck (MRN) size distribution (Mathis et al. 1977, ApJ, 217, 425), we find that coagulation in grain–grain collisions generated by hydrodynamical turbulence is not efficient at removing the smallest grains and, as a consequence, does not have a large effect on the evolution of the Hall and ambipolar diffusion MHD resistivities, which still drop significantly during the collapse like in models without coagulation. The inclusion of systematic velocities, possibly induced by the presence of ambipolar diffusion, increases the coagulation rate between small and large grains, removing small grains earlier in the collapse and therefore limiting the drop in the Hall and ambipolar diffusion resistivities. At intermediate densities (nH ~ 108 cm−3), the Hall and ambipolar diffusion resistivities are found to be higher by 1 to 2 orders of magnitude in models with coagulation than in models where coagulation is ignored, and also higher than in a toy model without coagulation where all grains smaller than 0.1 μm would have been removed in the parent cloud before the collapse. Conclusions. When grain drift velocities induced by ambipolar diffusion are included, dust coagulation happening during the collapse of a prestellar core starting from an initial MRN dust size distribution appears to be efficient enough to increase the MHD resistivities to the values necessary to strongly modify the magnetically regulated formation of a planet-forming disk. A consistent treatment of the competition between fragmentation and coagulation is, however, necessary before reaching firm conclusions.
Highlights
While it is well established that stars form through the collapse of prestellar cores, understanding the exact outcome of this process remains a challenge (Li et al 2014)
We present our results for the evolution of the grain size distribution, plasma ionisation, and MHD conductivities and resistivities during the collapse of a spherical core for different scenarios of dust dynamics and evolution
We have used the Dust Dynamics and Processing code (DUSTDaP) code, a numerical tool that was designed to follow the evolution of the dust size distribution and its feedback on the evolution of the ionisation, chemical content and dynamics of interstellar shocks (Guillet et al 2011)
Summary
While it is well established that stars form through the collapse of prestellar cores, understanding the exact outcome of this process remains a challenge (Li et al 2014). Many studies have found that the properties of the centrifugally supported disks that form around protostars sensitively depend on the intensity of the magnetic field and even possibly on its orientation (Allen et al 2003; Mellon & Li 2008; Hennebelle & Fromang 2008; Joos et al 2012; Li et al 2013; Gray et al 2018) This is due to the magnetic braking that can efficiently transport angular momentum from the inner part of the collapsing cloud to the surrounding envelope. This process largely dominates over the other non-ideal MHD processes in the interstellar medium (ISM) such as the Hall effect and the Ohmic resistivity, which are due to the imperfect coupling between the magnetic field and the ions and the electrons, respectively
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