How cosmic turbulences lead to formation of stars and black holes
Washington: A new study has shown how magnetic fields can also cause turbulences within "dead zones," thus making an important contribution to our current understanding of just how compact objects form in the cosmos.
When Johannes Kepler first proposed his laws of planetary motion in the early days of the 17th century, he could not have foreseen the central role cosmic magnetic fields would play in planetary system formation.
Today, we know that in the absence of magnetic fields, mass would not be able to concentrate in compact bodies like stars and black holes.
One prominent example is our solar system, which formed 4.6 billion years ago through the collapse of a gigantic cloud of gas, whose gravitational pull concentrated particles in its center, culminating in the formation of a large disc.
"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," Helmholtz-Zentrum Dresden-Rossendorf physicist Dr. Frank Stefani, said.
"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," he said.
As early as 1959, Evgenij Velikhov conjectured that magnetic fields are capable of prompting turbulences within stable rotating flows.
Although it wasn`t until 1991 that astrophysicists Steven Balbus and John Hawley fully grasped the fundamental significance of this magneto rotational instability (MRI) in cosmic structure formation.
However, in order to ensure the MRI actually works, the discs have to exhibit a minimum degree of electrical conductivity.
In areas of low conductivity like the "dead zones" of protoplanetary discs or the far-off regions of accretion discs that surround supermassive black holes , the MRI`s effect is numerically difficult to comprehend and is thus a matter of dispute.
The calculations by Dr. Oleg Kirillov and Dr. Frank Stefani have shown that the helical MRI very much applies to the Keplerian rotation profile if only the circular magnetic field is produced not entirely from the outside but at least partly from within the accretion disc.
Regardless of whether with or without a vertical magnetic field, current calculations show that the MRI is possible even in areas of low conductivity like the " dead zones" -- something astrophysicists had not previously thought possible.
The research is published in the scientific journal Physical Review Letters.
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