Wolf B. Dapp, Shantanu Basu, Matthew W. Kunz
Context: Ideal MHD simulations have revealed catastrophic magnetic braking
(MB) in the protostellar phase, which prevents the formation of a centrifugal
disk around a nascent protostar. Aims: We determine if non-ideal MHD, including
the effects of ambipolar diffusion and Ohmic dissipation determined from a
detailed chemical network model, allows for disk formation at the earliest
stages of star formation (SF). Methods: We employ the axisymmetric thin-disk
approximation in order to resolve a dynamic range of 9 orders of magnitude in
length and 16 in density, while also calculating partial ionization using up to
19 species in a detailed chemical equilibrium model. MB is applied using a
steady-state approximation, and a barotropic relation is used to capture the
thermal evolution. Results: We resolve the formation of the first and second
cores, with expansion waves at the periphery of each, a magnetic diffusion
shock, and prestellar infall profiles at larger radii. Power-law profiles in
each region can be understood analytically. After the formation of the second
core, centrifugal support rises rapidly and a low-mass disk of radius ~10 R_Sun
is formed, when the second core has mass ~0.001 M_Sun. The mass-to-flux ratio
is ~10,000 times the critical value in the central region. Conclusions: A
centrifugal disk can indeed form in the earliest stage of SF, due to a shut-off
of MB caused by magnetic field dissipation in the first core region. There is
enough angular momentum loss to allow the second collapse to occur directly,
and a low-mass stellar core to form with a surrounding disk. The disk mass and
size will depend upon how the angular momentum transport mechanisms within the
disk can keep up with mass infall onto the disk. We estimate that the disk will
remain <~10 AU, undetectable even by ALMA, in the early Class 0 phase.
View original:
http://arxiv.org/abs/1112.3801
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