Illustration courtesy Ben Bromley, University of Utah
Published April 4, 2012
A galactic black hole has the same mass as millions to billions of suns. It's long been thought that such black holes grow by consuming matter, including stars. But models show that it's less likely for a lone star to fall into a black hole.
"If you want to bring a star into a black hole, you can try throwing a single star at it, but like a comet that flies past the sun, nothing of interest will happen unless you get a very nearly direct hit," explained study co-author Ben Bromley, an astrophysicist at the University of Utah.
"On the other hand, if you throw a binary at the black hole, the binary is easily [separated] and, while you don't get a direct hit immediately, you at least capture one of the stars onto a bound orbit."
That's because, gravitationally speaking, a stellar binary acts like a single, much more massive object, so the star pairs can be ensnared by the black hole from farther away.
Black Holes' Binary Binge
According to the new simulations, when a binary pair strays too close to a galactic black hole, the two stars get peeled apart. One member of the duo is captured while the other is chucked away at high speed in a gravitational slingshot.
The captured star gets incorporated into a rotating disc of stars and can circle close to the black hole for millions of years. Eventually, though, gravitational jostling brings the star close enough to fall in and get torn apart, or disrupted.
"When the star gets close to the black hole and becomes disrupted, it gets stretched out, and part of the star falls into the black hole," said study team member Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics (CfA) in Massachusetts.
"The falling gas emits energy, and you would see a sort of flash" that can last for up to a year, which is known to astronomers as a tidal disruption event.
Meanwhile, the discarded star gets hurled away at speeds of up to 1.8 million miles (2.9 million kilometers) an hour—fast enough to fly out of its parent galaxy altogether.
The model backs up observations that show about a dozen of these so-called hypervelocity stars speeding out of our Milky Way galaxy. (Related: "Hyperspeed Planets Are Hurtling Out of the Milky Way?")
Our Black Hole a Sluggish Eater
Astronomers think supermassive black holes could also grow via collisions and mergers or by snacking on gas clouds.
But these mechanisms have their own problems—for instance, some still-growing galactic black holes don't appear to have any gas clouds around them. So the team thinks other processes may play lesser roles in black hole growth.
"I believe [binary-star disruption] has got to be the dominant method for growing supermassive black holes," the University of Utah's Bromley said.
Applying their theory to the Milky Way, the team estimates that our galaxy's supermassive black hole consumes a star about every thousand years.
That's enough added material for our black hole to have doubled to quadrupled in mass over the past five to ten billion years.
"It only has to eat a typical star every thousand years or so," the CfA's Kenyon said. "It's not fasting, but it's not exactly eating at a high rate."
Lone Stars Still Key Players?
But astrophysicist David Merritt, of the Rochester Institute of Technology in New York, said he's not yet convinced that binary stars are the primary fuel for galactic black hole growth.
Merritt agrees that binary stars are probably easier to capture. But afterward, he thinks, captured stars don't fall into the black hole readily enough to sustain growth.
Even though throwing single stars at a black hole is inefficient, he thinks it still beats waiting for a separated binary star to fall in.
"If binaries are contributing in an important way, then single stars"—which are thought to outnumber binary stars in galaxies—"are contributing in an even more important way," said Merritt, who was not involved in the study.
"That point was made already ... in the 1970s using similar assumptions that they're using here.
"What we really need is more observations of tidal disruption events," Merritt said. "There's only been a handful observed, and that's not enough to derive statistics from."
Study co-author Kenyon said that more powerful space telescopes are expected to launch in the coming years, and they should be able to provide more precise figures for the rates used in the team's calculations.
The black hole-growth study was detailed in the April 2 issue of Astrophysical Journal Letters.
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