| Type |
Journal Article |
| Names |
R. Santos-Lima, E. M. de Gouveia Dal Pino, A. Lazarian |
| Publication |
The Astrophysical Journal |
| Volume |
747 |
| Issue |
1 |
| Pages |
21 |
| Date |
March 1, 2012 |
| URL |
http://adsabs.harvard.edu/abs/2012ApJ...747...21S |
| Library Catalog |
NASA ADS |
| Abstract |
The formation of protostellar disks out of molecular cloud cores is
still not fully understood. Under ideal MHD conditions, the removal of
angular momentum from the disk progenitor by the typically embedded
magnetic field may prevent the formation of a rotationally supported
disk during the main protostellar accretion phase of low-mass stars.
This has been known as the magnetic braking problem and the most
investigated mechanism to alleviate this problem and help remove the
excess of magnetic flux during the star formation process, the so-called
ambipolar diffusion (AD), has been shown to be not sufficient to weaken
the magnetic braking at least at this stage of the disk formation. In
this work, motivated by recent progress in the understanding of magnetic
reconnection in turbulent environments, we appeal to the diffusion of
magnetic field mediated by magnetic reconnection as an alternative
mechanism for removing magnetic flux. We investigate numerically this
mechanism during the later phases of the protostellar disk formation and
show its high efficiency. By means of fully three-dimensional MHD
simulations, we show that the diffusivity arising from turbulent
magnetic reconnection is able to transport magnetic flux to the
outskirts of the disk progenitor at timescales compatible with the
collapse, allowing the formation of a rotationally supported disk around
the protostar of dimensions ~100 AU, with a nearly Keplerian profile in
the early accretion phase. Since MHD turbulence is expected to be
present in protostellar disks, this is a natural mechanism for removing
magnetic flux excess and allowing the formation of these disks. This
mechanism dismisses the necessity of postulating a hypothetical increase
of the ohmic resistivity as discussed in the literature. Together with
our earlier work which showed that magnetic flux removal from molecular
cloud cores is very efficient, this work calls for reconsidering the
relative role of AD in the processes of star and planet formation. |
| Tags |
ISM: magnetic fields, Magnetohydrodynamics: MHD, STARS: FORMATION, accretion, accretion disks, diffusion, turbulence |