John P. Millis, PhD for redOrbit.com – Your Universe Online
The mechanism by which stars and black holes are formed in extreme cases of high mass density has puzzled astronomers since Johannes Kepler first laid out his laws of planetary motion some 400 years ago. In the most basic sense, a large cloud of gas condenses into a rotating disk. Mass is then directed towards the center of the cloud where, due to the increasing density, a stellar body is eventually formed.
One challenge to this picture of star formation is that a mechanism is required to funnel matter inwards, since these disks are otherwise stable. “These accretion discs are extremely stable from a hydrodynamic perspective as according to Kepler’s laws of planetary motion angular momentum increases from the center towards the periphery,” explains Helmholtz-Zentrum Dresden-Rossendorf (HZDR) physicist Dr. Frank Stefani.
“In order to explain the growth rates of stars and black holes, there has to exist a mechanism, which acts to destabilize the rotating disc and which at the same time ensures mass is transported towards the center and angular momentum towards the periphery.”
For more than half a century, astronomers have suspected that magnetic fields figure prominently into this picture, supplying the turbulences that would be required to destabilize the flow and allow matter to flow to the center. Yet it took about 32 years for the magnetorotational instability (MRI) theory to be fully modeled in these systems.
Even so, such models predict the rotating field of gas must contain at least a minimum level of electrical conductivity, lest “dead zones” form within the disk. Previous work had indicated such dead zones would quell turbulent motions, slowing or stopping accretion of matter at the disk’s center.
To overcome the dead zones problem, computational models indicate a vertically oriented magnetic field would need to be unusually strong, with rotational speeds in the disk being very high. By adding circular magnetic fields to the vertical component, much lower field strengths are needed. Yet, this too has problems. In this situation, the geometry of the fields would push more to the edges of the disk, contrary to the Keplerian view.
A new study led by Stefani and his colleague Oleg Kirillov, has shown if the magnetic field at least partially arises from within the accretion disk, instead of external to it, the MRI will produce the desired Keplerian rotation profile in the disk.
“This is, in fact, a much more realistic scenario”, notes Stefani. “In the extreme case that there does not exist a vertical field, we’re looking at a problem of what came first – the chicken or the egg. A circular magnetic field acts to destabilize the disc and the resulting turbulence generates components of vertical magnetic fields. They in turn reproduce the circular magnetic field because of the special form of the disc’s rotational movement.”
Additional experimental follow-up will be required to verify the findings, but this new magnetic field model within the disk appears to close the gap between theory and observation.
Stefani and Kirillov presented the results of their study in a paper titled “Extending the Range of Inductionless Magnetorotational Instability” published in the latest edition of the journal Physical Review Letters.