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Astronomy Now To bring a dead star back to life, first get a black hole

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Astronomy Now


Computer simulations indicate close encounters between intermediate-mass black holes and white dwarf stars could generate heavy elements as the white dwarf is briefly re-ignited. Image: Fragile et al.

Imagine a white dwarf, the dead core of a star like our sun, caught in the gravitational grip of a black hole. If it passes close enough, inside the so-called tidal radius, the black hole’s gravity will rip the dead star apart in a cataclysmic tidal disruption event, or TDE, as it is simultaneously stretched and compressed in opposite directions.

New research shows the compression may be powerful enough to briefly re-ignite nuclear fusion in the white dwarf, bringing the star back to life, in effect, for a few seconds before it is torn apart.

While some of the debris will merge with the black hole, computer simulations show a “significant fraction” will be ejected back into space, providing raw material, including newly formed heavy elements, for future stellar generations.

But not just any black hole will do.

It must be an intermediate=mass black hole, one with 1,000 to 10,000 times the mass of the Sun. The size of a black hole’s tidal radius is directly related to its mass and if the mass is too low, the tidal radius would be smaller than the size of the white dwarf. Conversely, if the mass is too high, the white dwarf would be pulled inside before the tidal forces could compress it enough to trigger fusion and the creation of new elements.

Supermassive black holes are commonplace, lurking in the hearts of countless galaxies including Earth’s Milky Way. Stellar-mass black holes also are known, but its not known how common intermediate-mass holes might be.

“It is important to know how many intermediate mass black holes exist, as this will help answer the question of where supermassive black holes come from,” said Fragile. “Finding intermediate mass black holes through tidal disruption events would be a tremendous advancement.”

The computer simulations indicate the elements created in a white dwarf’s brief re-ignition depend on how close the doomed star comes to the black hole. Close approaches can produce iron while more distant encounters favour calcium. The encounters also can produce gravitational waves that may one day be detectable by instruments more sensitive than those now available.

The study was carried out by Chris Fragile at the University of Charleston in Charleston, South Carolina, Peter Anninos and Robert Hoffman of Lawrence Livermore National Laboratory, Samuel Olivier of the University of California, Berkeley, Bhupendra Mishra of the University of Colorado and Karen Camarda of Washburn University.

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