Dead — but not duds

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Amid the liveliest stars in the cosmos lie stellar corpses. Of these dead stars, the most abundant are white dwarfs — stars that in their prime were similar to the sun. These dense corpses foreshadow what will become of most of the stars in the universe.

Although white dwarfs are dead, they aren’t useless. Postmortem examination shows they have different masses and different chemical makeups. Some are strongly magnetic. Others pulsate. A few even have orbiting planets and debris disks. “Understanding why these cadavers are all different might help us understand the lives of normal stars,” says Patrick Dufour, an astronomer at the University of Arizona in Tucson. “And although we think we know how stars evolve and die, there are still many things we don’t understand in detail.”

That is why Dufour and others perform what he calls a “kind of autopsy” on these stars. But unlike human pathologists who can carry out up-close and personal inspections, white dwarf astronomers are limited to their sense of sight and telescopes to dissect their corpses.

Among the most intriguing of white dwarfs are the ones that pulsate, says Donald Winget, an astrophysicist at the University of Texas at Austin. Lately new telescopes have provided astronomers a better glimpse of these particular corpses, opening a window to unexplored, exotic physics. Studies of pulsating white dwarfs promise insight into the dark matter hiding in the cosmos, provide valuable data on the expansion of the universe and offer clues about whether exoplanets can survive the death of their parent star.

Taking a pulse

So far, astronomers have discovered three families of white dwarfs, which, on average, are about the mass of the sun and fit into a space about the size of Earth. That mass-to-space ratio makes these stars extremely dense, about 200,000 times Earth’s density. What distinguishes one type of white dwarf from another is its veneer. This outer coat may be composed of hydrogen, helium or, as Dufour discovered last year, carbon. Then in May, Winget and his colleagues found that one of these new carbon-veneered white dwarfs pulsates too.

Thinking about a pulsating white dwarf might elicit imagery of a star expanding and contracting. But pulsating white dwarfs do not throb this way. Because of their strong gravity, much of the motion is a “sloshing” of material around the star, not an in-and-out motion, says astrophysicist Agnes Kim of Georgia College & State University in Milledgeville.

When the material “splashes” together on the star’s surface and that region becomes hot, astronomers see the pulsations as a brightening, Kim says. Where the stellar material pulls apart, the temperature drops, and astronomers see the region dim.

Measuring white dwarf pulsations is similar to measuring seismic waves traveling through the Earth. Because parallels exist between seismological vibrations and white dwarf vibrations, astronomers dubbed the technique asteroseismology. Winget thinks of the stars’ vibrations in terms of hearing a bell. “From its ring you can tell the size of the bell and what it is made of,” he says. “How it shakes or vibrates tells about its structure.” Similarly, measuring light vibrations reveals data about a white dwarf’s structure and chemical composition.

“But white dwarfs aren’t doing any nuclear reactions. They have no energy source,” notes Michael Montgomery, an astrophysicist and a colleague of Winget’s at UT Austin. “So, it’s like when you take a hot lump of coal out of the fire; it just sits there and cools off. These stars are like those lone lumps of coal. They are just sitting there cooling off.”

Montgomery explains that of the white dwarfs that pulsate, a subset of them pulsate so regularly that astronomers can use the dead stars as precision timepieces. “They hardly lose any time at all,” he says. In fact, the record for the least time lost by a white dwarf is one second every 10 million years.

“I think of these stars like a heated bell,” Kim says. “As the bell’s metal cools, it will change temperature and density, and therefore the sound, the pitch of it, will change slightly. The same is true for the white dwarf, so the frequency of the light pulses change slowly too.”

Kim’s research uses pulsating dwarfs to look for clues to the nature of the dark matter that lurks unseen in space. Digging through the observational literature, she found 30 years’ worth of data tracking one dwarf’s pulsation and cooling. She observed the star and correlated how fast its pulse periods were changing with how fast it was cooling and determined how much energy the star should be losing.

The measurements allowed room for extra cooling from unseen particles, she says. For her doctoral thesis last year, Kim argued that the star was giving off subatomic particles including neutrinos and the theorized, but undetected, axion. Axions were “invented” to solve a problem in the standard model of particle physics, which describes nature’s forces and the basic building blocks of matter. Axions could also make up the dark matter needed to explain unexpected motions observed in galaxies and galaxy clusters.

Not much is known about the particles, though, since, if they exist, they have so far eluded physicists’ traps. To place a limit on how massive axions could be, Kim began by assuming that they were released as a white dwarf lost heat. Axions cannot contribute too much to cooling. Otherwise, “we would definitely see the pulse periods change faster than we do,” she says. She concluded that the axion mass should be less than 25 milli-electronvolts, or meV. An electron’s mass is around 500 kiloelectronvolts, roughly 20 million times greater. The findings appeared in The Astrophysical Journal in March. (The preprint of the paper is online at arxiv.org/abs/0711.2041.)

More recently, Jordi Isern of the Blanes Advanced Studies Centre’s Institute of Space Sciences in Bellaterra, Spain, and colleagues lowered Kim’s upper limit on the axion mass to 5 meV, using a different method still based on light from white dwarfs. The results appeared in the Aug. 1 Astrophysical Journal Letters.

“The jury is still out on these results, and the Spanish authors admit there are many things they did not take into account,” Montgomery says.

Techniques used to put mass limits on particles are not limited to axions. Astronomers can predict upper mass limits for neutrinos and other proposed dark matter candidates too, Kim says. Finding a limit for the particle candidates’ masses is a first step to figuring out what dark matter is, Winget says.

Ending in style

White dwarfs have already played a role in discovering another of the universe’s dark secrets — dark energy. That’s the term astronomers use to describe a mysterious force causing the universe to expand at an accelerating rate.

Evidence for dark energy came in the 1990s from studies of type 1a supernovas, believed to occur when a white dwarf explodes after stealing matter from a nearby companion. So much mass piles up on the white dwarf that its core can no longer support its mass, leading to nuclear detonation in a flash as bright as a billion suns.

Because white dwarfs should all explode after reaching a similar mass — about 1.4 times the sun’s mass —the explosions should all produce about the same brightness. So astronomers consider white dwarf supernovas “standard candles” for measuring distances to far-off galaxies. Since light from distant galaxies represents conditions in the universe long ago, studying these explosions can reveal details of cosmic history. To astronomers’ surprise, supernova data showed the universe’s expansion was accelerating, not slowing as previously believed.

Further explanations for the cause of the acceleration require more precise data about white dwarf supernovas. It turns out that they do not really all explode with the same brightness, so astronomers need more information on the mass and makeup of white dwarfs, and on how they explode, to make appropriate corrections in the calculations.

“We have come to realize that these supernovae explode by the same mechanism, but there are different things going on. Some explode at a slightly smaller mass, some at slightly heavier mass,” says astrophysicist Sumner Starrfield of ArizonaStateUniversity in Tempe. “And the truth is, we just don’t know what the stars that lead to supernovae are like because the explosions are hard to observe.”

In February, astronomers from Germany and the Netherlands reported in Nature on a supernova in a binary system including a white dwarf, supporting the standard view that type 1a supernovas involve the dead stars. Determining what the parent stars of such supernovas are made of would be possible if astronomers could find a binary system in which one of the partners was a pulsating white dwarf, and could study the system before it exploded. In such cases, the pulsating dwarf would give astronomers a “long-handled spoon for studying these stars’ interiors,” Starrfield says.

Astronomers might be able to use that long-handled spoon not only to get a taste of white dwarf interiors, but also to investigate the possibility of planets or other objects orbiting the star.

Getting around

About three years ago, astrophysicist Eric Becklin, now at NASAAmesResearchCenter in Moffett Field, Calif., and collaborators found dusty debris disks around five white dwarfs. All five dwarfs showed traces of metal nearby. “Solitary white dwarfs don’t have these metals,” Becklin says. “The metals could have never formed in these stars, so basically this tells us that asteroids and maybe even planets are going around these former sunlike stars.”

Further evidence for whether an object orbits a white dwarf could come from studying the timing of the stellar corpse’s pulsations. Because time can be tracked with such precision, astronomers know exactly how many seconds or minutes should elapse between one pulse and the next — unless there is an exoplanet lurking in the shadows.

If a big object is going around the white dwarf, Montgomery says, it pulls the star slightly closer or slightly farther away from Earth depending on where the planet is in its orbit. When this happens, the light from the white dwarf has to travel either slightly less or slightly more distance to reach Earth. “So some of the pulses will come early,” says Fergal Mullally of Princeton University. “And sometimes, pulses will be late.”

Looking for those signature changes in pulsations in 15 white dwarfs, Mullally has so far found only one beginning to show the patterns expected when an exoplanet orbits the star. “The problem with white dwarfs is that we should be finding planets like Jupiter, meaning they have an orbit that takes several years,” he says.

Mullally has been tracking his candidate star for about five years but has only seen half to two-thirds of the signature light pattern a planet’s presence should show. Soon the pulsations should show a delay. If they do, that would complete the planet’s signature pattern, he says.

Further prospects for success in finding planets around white dwarfs comes from Roberto Silvotti of the Osservatorio Astronomico di Capodimonte in Naples, Italy, and colleagues. Last year in Nature, they reported that they found a planet three times the mass of Jupiter orbiting a star on the road to white dwarfhood. During death, the star expanded, almost engulfing its planet with its gases. The star lost all its hydrogen, shrunk to half its size and now fuses helium and carbon. The surviving planet suggests that planets like Earth could endure the violent death of their parent stars. To confirm the idea, astronomers would need to find planets around an actual white dwarf, Winget says.

Another way to seek signs of these planets could emerge from the metallic, dusty debris disks surrounding Becklin’s white dwarfs. Three of those stars pulsate. Astronomers would like to find a signature variation in the light pulses that might characterize the debris disks.

Winget says he and others could then look at the light signatures from the roughly 200 pulsating white dwarfs so far discovered and figure out what fraction of them have orbiting asteroids, debris disks and possibly even planets.

“This is an important area,” Winget says, “because we ultimately learn more about the fate of solar systems.” And maybe, he adds, “we could even better understand what will happen to our own.”