Misc,
Protostellar Disk Formation Enabled by Removal of Small Dust Grains
(Feb 8, 2016)
Abstract
It has been shown that a realistic level of magnetization of dense molecular
cloud cores can suppress the formation of a rotationally supported disk (RSD)
through catastrophic magnetic braking in the axisymmetric ideal MHD limit. In
this study, we present conditions for the formation of RSDs through non-ideal
MHD effects computed self-consistently from an equilibrium chemical network. We
find that removing from the standard MRN distribution the large population of
very small grains (VSGs) of \~10 \$\AA\$ to few 100 \$\AA\$ that dominate the
coupling of the bulk neutral matter to the magnetic field increases the
ambipolar diffusivity by \~1--2 orders of magnitude at densities below 10\$^10\$
cm\$^-3\$. The enhanced ambipolar diffusion (AD) in the envelope reduces the
amount of magnetic flux dragged by the collapse into the circumstellar
disk-forming region. Therefore, magnetic braking is weakened and more angular
momentum can be retained. With continuous high angular momentum inflow, RSDs of
tens of AU are able to form, survive, and even grow in size, depending on other
parameters including cosmic-ray ionization rate, magnetic field strength, and
rotation speed. Some disks become self-gravitating and evolve into rings in our
2D (axisymmetric) simulations, which have the potential to fragment into
(close) multiple systems in 3D. We conclude that disk formation in magnetized
cores is highly sensitive to chemistry, especially to grain sizes. A moderate
grain coagulation/growth to remove the large population of VSGs, either in the
prestellar phase or during free-fall collapse, can greatly promote AD and help
formation of tens of AU RSDs.
