210 research outputs found
Induced Core Formation Time in Subcritical Magnetic Clouds by Large-Scale Trans-Alfv\'enic Flows
We clarify the mechanism of accelerated core formation by large-scale
nonlinear flows in subcritical magnetic clouds by finding a semi-analytical
formula for the core formation time and describing the physical processes that
lead to them. Recent numerical simulations show that nonlinear flows induce
rapid ambipolar diffusion that leads to localized supercritical regions that
can collapse. Here, we employ non-ideal magnetohydrodynamic simulations
including ambipolar diffusion for gravitationally stratified sheets threaded by
vertical magnetic fields. One of the horizontal dimensions is eliminated,
resulting in a simpler two-dimensional simulation that can clarify the basic
process of accelerated core formation. A parameter study of simulations shows
that the core formation time is inversely proportional to the square of the
flow speed when the flow speed is greater than the Alfv\'en speed. We find a
semi-analytical formula that explains this numerical result. The formula also
predicts that the core formation time is about three times shorter than that
with no turbulence, when the turbulent speed is comparable to the Alfv\'en
speed.Comment: 22 pages, 9 figures, accepted for publication in Ap
Mass accretion rates in self-regulated disks of T Tauri stars
We have studied numerically the evolution of protostellar disks around
intermediate and upper mass T Tauri stars (0.25 M_sun < M_st < 3.0 M_sun) that
have formed self-consistently from the collapse of molecular cloud cores. In
the T Tauri phase, disks settle into a self-regulated state, with low-amplitude
nonaxisymmetric density perturbations persisting for at least several million
years. Our main finding is that the global effect of gravitational torques due
to these perturbations is to produce disk accretion rates that are of the
correct magnitude to explain observed accretion onto T Tauri stars. Our models
yield a correlation between accretion rate M_dot and stellar mass M_st that has
a best fit M_dot \propto M_st^{1.7}, in good agreement with recent
observations. We also predict a near-linear correlation between the disk
accretion rate and the disk mass.Comment: Accepted for publication in ApJ Letter
Averting the magnetic braking catastrophe on small scales: disk formation due to Ohmic dissipation
We perform axisymmetric resistive MHD calculations that demonstrate that
centrifugal disks can indeed form around Class 0 objects despite magnetic
braking. We follow the evolution of a prestellar core all the way to
near-stellar densities and stellar radii. Under flux-freezing, the core is
braked and disk formation is inhibited, while Ohmic dissipation renders
magnetic braking ineffective within the first core. In agreement with
observations that do not show evidence for large disks around Class 0 objects,
the resultant disk forms in close proximity to the second core and has a radius
of only early on. Disk formation does not require
enhanced resistivity. We speculate that the disks can grow to the sizes
observed around Class II stars over time under the influence of both Ohmic
dissipation and ambipolar diffusion, as well as internal angular momentum
redistribution.Comment: 4 pages, 3 figures, accepted by A&A Letter
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