‘Failed’ Star Rapidly Orbits ‘Dead’ Star in Weird Stellar Pairing

white-dwarf
ESO

The galaxy may be filled with weird stellar wonders, but you’d be hard-pressed to find a binary system stranger than WD1202-024.

First thought to be an isolated white dwarf star approximately 40% the mass of our sun, astronomers studying observational data from NASA’s Kepler space telescope realized the stellar husk has company. In an extremely fast 71-minute orbit, the star has a brown dwarf, 67 times the mass of Jupiter, in tow — an unprecedented find.

White dwarfs are formed after sun-like stars run out of fuel and die. This will be the fate of our sun in about five billion years time, after it becomes depleted of hydrogen in its core and puffs-up into a red giant. Shedding its outer layers after a period of violent stellar turmoil, a planetary nebula will form with a tiny mass of degenerate matter — a white dwarf — in its center. Earth would be toast long before the sun turns into a red giant, however.

But in the case of WD1202-024, it seems that when it was a young star (before it passed through its final red giant phase), it had a brown dwarf in orbit.

Commonly known as “failed stars,” brown dwarfs are not massive enough to sustain sufficient fusion in their cores to spark the formation of a star. But they’re too massive to be called planets as they possess the internal circulation of material that is more familiar to stars (so with that in mind, I like to refer to brown dwarfs as “overachieving planets”). They are the bridge between stars and planets and fascinating objects in their own right.

But the brown dwarf in the WD1202 binary couldn’t have formed with only a 71-minute orbit around the white dwarf; it would have evolved further away. So what happened? After carrying out computer simulations of the system, the international team of researchers found a possible answer.

“It is similar to an egg-beater effect,” said astronomer Lorne Nelson, of Bishop’s University, Canada, during the American Astronomical Society meeting in Austin, Texas on June 6th. “The brown dwarf spirals in towards the center of the red giant and causes most of the mass of the red giant to be lifted off of the core and to be expelled. The result is a brown dwarf in an extraordinarily tight, short-period orbit with the hot helium core of the giant. That core then cools and becomes the white dwarf that we observe today.”

In the future, the researchers hypothesize, the brown dwarf will continue to orbit the white dwarf until energy is depleted from the system via gravitational waves. In less than 250 million years, the orbital distance will be so small that the extreme tidal forces exerted by the white dwarf will start to drag brown dwarf material into the star, cannibalizing it.

This will turn WD1202 into a cataclysmic variable (CV), causing its brightness to flicker as the brown dwarf material is extruded into an accretion disk orbiting the white dwarf.

What a way to go.

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This Black Hole Keeps Its Own White Dwarf ‘Pet’

The most compact star-black hole binary has been discovered, but the star seems to be perfectly happy whirling around the massive singularity twice an hour.

Credits: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss

A star in the globular cluster of 47 Tucanae is living on the edge of oblivion.

Located near a stellar-mass black hole at only 2.5 times the Earth-moon distance, the white dwarf appears to be in a stable orbit, but it’s still paying the price for being so intimate with its gravitational master. As observed by NASA’s Chandra X-ray Observatory and NuSTAR space telescope, plus the Australia Telescope Compact Array, gas is being pulled from the white dwarf, which then spirals into the black hole’s super-heated accretion disk.

47 Tucanae is located in our galaxy, around 14,800 light-years from Earth.

Eventually, the white dwarf will become so depleted of plasma that it will turn into some kind of exotic planetary-mass body or it will simply evaporate away. But one thing does appear certain, the white dwarf will remain in orbit and isn’t likely to get swallowed by the black hole whole any time soon.

“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” said Arash Bahramian, of the University of Alberta (Canada) and Michigan State University. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.” Bahramian is the lead author of the study to be published in the journal Monthly Notices of the Royal Astronomical Society.

It was long thought that globular clusters were bad locations to find black holes, but the 2015 discovery of the binary system — called “X9” — generating quantities of radio waves inside 47 Tucanae piqued astronomers’ interest. Follow-up studies revealed fluctuating X-ray emissions with a period of around 28 minutes — the approximate orbital period of the white dwarf around the black hole.

So, how did the white dwarf become the pet of this black hole?

The leading theory is that the black hole collided with an old red giant star. In this scenario, the black hole would have quickly ripped away the bloated star’s outer layers, leaving a tiny stellar remnant — a white dwarf — in its wake. The white dwarf then became the black hole’s gravitational captive, forever trapped in its gravitational grasp. Its orbit would have become more and more compact as the system generated gravitational waves (i.e. ripples in space-time), radiating orbital energy away, shrinking its orbital distance to the configuration that it is in today.

It is now hoped that more binary systems of this kind will be found, perhaps revealing that globular clusters are in fact very good places to find black holes enslaving other stars.

Probing Variable Black Holes

Artist impression of a black hole feeding off its companion star... and a rogue Higgs particle (ESO/L. Calçada)
Artist impression of a black hole feeding off its companion star... and a rogue Higgs particle (ESO/L. Calçada/Particle Zoo)

Black holes are voracious eaters. They devour pretty much anything that strays too close. They’re not fussy; dust, gas, plasma, Higgs bosons, planets, stars, even photons are on the menu. However, for astronomers, interesting things can be observed if a star starts to be cannibalized by a neighbouring black hole. Should a star be unlucky enough to have a black hole as its binary partner, the black hole will begin to strip the stars upper layers, slowly consuming it on each agonizing orbit. Much like water spiralling down a plug hole, the tortured plasma from the star is gravitationally dragged on a spiral path toward the black hole’s event horizon. As stellar matter falls down the event horizon plug hole, it reaches relativistic velocities, blasting a huge amount of radiation into space. And now, astronomers have taken different observations from two observatories to see how the visible emissions correlate with the X-ray emissions from two known black hole sources. What they discovered came as a surprise
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