Tag: white dwarfs

Two Stellar Zombie Spinners Are Ripping Up Spacetime

The pair of white dwarf stars are orbiting one another every seven minutes—and future gravitational wave observatories will be able to detect them whirl.

White dwarf binaries are among some of the most fascinating star systems known, and a newly discovered compact binary, located some 8,000 light-years away in the constellation Boötes, has taken the exotic nature of these systems to new spacetime-warping extremes.

The extremely compact eclipsing binary, called ZTF J1530+5027, is one of the most extreme white dwarf systems known to exist [Caltech/IPAC]

Astronomers using Caltech’s Zwicky Transient Facility (ZTF), a precision sky survey at Palomar Observatory near San Diego in Southern California, made the discovery of ZTF J1530+5027 by detecting the dimming effect caused by one of the stars passing in front of the other. Known as an “eclipsing binary,” the cooler (and therefor dimmer) white dwarf blocks the starlight of the hotter (and brighter) star, causing the ZTF to register a periodic dimming event. This dimming occurs once every seven minutes, meaning they are zipping around each other at speeds of hundreds of miles per second! It is the second fastest white dwarf binary known and the most rapid eclipsing binary discovered in our galaxy. The fortunate alignment allows astronomers to not only precisely measure their orbital speed, they can also gauge the stars’ sizes and masses.

White dwarfs are the stellar corpses of sun-like stars that ran out of fuel long ago. Our sun will become a white dwarf in around five billion years, after it has exhausted its hydrogen fuel that maintains fusion in its core. A short period after, it will swell into a red giant (possibly expanding out as far as Earth, incinerating it) and then lose its plasma to space via powerful solar winds. All that will be left of our once glorious star will be a planetary nebula and a tiny and dense white dwarf, approximately the size of our planet, spinning in the middle. The two white dwarfs in ZTF J1530+5027 likely passed through their red giant phase at different times, but now they’re stuck, in a perpetual death spiral that spells doom for one of the objects.

To fully realize just how crazy-extreme this white dwarf binary is, they are only separated by one-fifth of the distance that the moon orbits Earth, meaning both stars would fully fit inside Saturn. They have a combined mass of our entire sun. As they orbit so snugly, it’s likely that the more massive star will start to tidally drag material from the other, cannibalizing it.

“Matter is getting ready to spill off of the bigger and lighter white dwarf onto the smaller and heavier one, which will eventually completely subsume its lighter companion,” said Kevin Burdge, Caltech graduate student and lead author of a study published in the journal Nature. “We’ve seen many examples of a type of system where one white dwarf has been mostly cannibalized by its companion, but we rarely catch these systems as they are still merging like this one.”

While impressive, the real fireworks are invisible—the stars are ripping up spacetime, generating gravitational waves that are sapping energy from the system, hastening the binary’s ultimate demise. What’s more, astronomers are anticipating that the future Laser Interferometer Space Antenna (LISA), which is scheduled for launch by the European Space Agency in 2034, will be able to detect its gravitational pulse.

“These two white dwarfs are merging because they are emitting gravitational waves,” added collaborator Tom Prince, a senior research scientist at Caltech and NASA’s Jet Propulsion Laboratory (JPL, in Pasadena, Calif. “Within a week of LISA turning on, it should pick up the gravitational waves from this system. LISA will find tens of thousands of binary systems in our galaxy like this one, but so far we only know of a few. And this binary-star system is one of the best characterized yet due to its eclipsing nature.”

This system is expected to keep blinking from our perspective for another hundred thousand years, but how will the system ultimately go kaput? Well, the researchers aren’t entirely certain. On the one hand, the more massive white dwarf may suck the other dry like a vampiric parasite, consuming all of its matter until only one, well-fed star remains. Alternatively, the act of cannibalization may cause the reverse; as mass is transferred to one star, the other may be flung outward to a wider orbit, increasing their orbital period.

“Sometimes these binary white dwarfs merge into one star, and other times the orbit widens as the lighter white dwarf is gradually shredded by the heavier one,” said co-author Jim Fuller, an assistant professor at Caltech. “We’re not sure what will happen in this case, but finding more such systems will tell us how often these stars survive their close encounters.”

One early mystery about this extreme binary is the question of X-rays, or lack thereof. The more massive star is really hot, with a temperature nine times that of the sun (50,000 Kelvin). The researchers believe that this is because it has already begun pulling material from its partner, an act that accelerates and heats the plasma that is being stolen, starting to create an accretion disk. But the accreting gas should be so hot that the system would be humming in X-rays, but it isn’t. “It’s strange that we aren’t seeing X-rays in this system. One possibility is that the accretion spots on the white dwarf—the areas the material is falling on—are bigger than what is typical, and this could result in the emission of ultraviolet light and optical light instead of X-rays,” added Burdge.

[Caltech/IPAC]

It’s exciting to think what the next generation of gravitational wave observatories (particularly LISA that will be sensitive to extremely weak spacetime ripples from systems such as these) combined with traditional (re: electromagnetic) observatories will herald for the future of astronomy. Like the emerging “multi-messenger” era for astronomy that combines observations of the electromagnetic spectrum and gravitational wave signals to confirm short gamma-ray bursts are triggered by neutron star collisions, it’s going to blow our minds when we can access more subtle gravitational wave sources such as these and directly see the gravitational energy leaking from compact binaries.

Exoplanets Are Sacrificing Moons to Their White Dwarf Overlords

An artist’s impression of a planet, comet and debris field surrounding a white dwarf star (NASA/ESA)

As if paying tribute, exoplanets orbiting white dwarfs appear to be throwing their exomoons into hot atmospheres of these stellar husks.

This fascinating conclusion comes from a recent study into white dwarf stars that appear to have atmospheres that are “polluted” with rocky debris.

A white dwarf forms after a sun-like star runs out of hydrogen fuel and starts to burn heavier and heavier elements in its core. When this happens, the star bloats into a red giant, beginning the end of its main sequence life. After the red giant phase, and the star’s outer layers have been violently ripped away by powerful stellar winds, a small bright mass of degenerate matter (the white dwarf) and a wispy planetary nebula are left behind.

But what of the planetary system that used to orbit the star? Well, assuming they weren’t so close to the dying star that they were completely incinerated, any exoplanets remaining in orbit around a white dwarf have an uncertain future. Models predict that dynamical chaos will ensue and gravitational instabilities will be the norm. Exoplanets will shift in their orbits, some might even be flung clear of the star system all together. One thing is for sure, however, the tidal shear created by the compact white dwarf will be extreme, and should anything stray too close, it will be ripped to shreds. Asteroids will be pulverized, comets will fall and even planets will crumble.

Stray too close to a white dwarf and tidal shear will rip you to shreds (NASA/JPL-Caltech)

Now, in a science update based on research published late last year in the journal Monthly Notices of the Royal Astronomical Society, astronomers of the Harvard-Smithsonian Center for Astrophysics (CfA) have completed a series of simulations of white dwarf systems in an attempt to better understand where the “pollution” in these tiny stars’ atmospheres comes from.

To explain the quantities observed, the researchers think that not only is it debris from asteroids and comets, but the gravitational instabilities that throw the system into chaos are booting any moons — so-called exomoons — out of their orbits around exoplanets, causing them to careen into the white dwarfs.

The simulations also suggest that as the moons meander around the inner star system and fall toward the star, their gravities scramble to orbits of more asteroids and comets, boosting the around of material falling into the star’s atmosphere.

So there you have it, planets, should your star turn into a white dwarf (as our sun will in a few billion years), keep your moons close — your new stellar overlord will be asking for a sacrifice in no time.

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.