Double-star systems cycle between big and small blasts
Wed Mar 7, 2007 at 20:46 UTC
Certain double, or binary, star systems erupt in full-blown explosions and then flare up with smaller bursts, according to new information gathered by NASA's Galaxy Evolution Explorer (GALEX) and analyzed by a team of astronomers, including postdoctoral researcher Mark Seibert of the Carnegie Observatories.
The data bolster a 20-year-old theory suggesting that double star systems experience both explosion types, rather than just one or the other. It also implies that the systems cycle between blast types, hiccupping every few weeks with small surges and experiencing giant outbursts every 10,000 years or so.
The discovery, appearing in the March 8 issue of the journal Nature, centers around a binary system called Z Camelopardalis (Z Cam). Astronomers have long known Z Cam to be a cataclysmic binary-a system that features a collapsed, dead star, or white dwarf, which sucks hydrogen-rich matter from its companion like a stellar vampire. The stolen material forms an orbiting disk of gas and dust around the white dwarf.
Astronomers divide cataclysmic binaries into two classes-dwarf novae, which erupt in smaller, "hiccup-like" blasts, and classical novae, which undergo huge explosions. Classical novae explosions are 10,000 to one million times brighter than those of dwarf novae, and they leave behind large shells of shocked gas.
In 2003, Seibert examined ultraviolet images collected by GALEX during its Survey of Nearby Galaxies. He noticed a never-before-seen arc and linear features surrounding Z Cam that indicated the presence of a massive shell -- evidence that the dwarf nova had in fact undergone a classical nova explosion a few thousand years ago. The features had remained invisible up to this point because they cannot be easily detected at optical wavelengths. However, they are easily seen at the ultraviolet wavelengths detected by GALEX.
"You could actually see it immediately, but we had to convince ourselves that we were really seeing a nova remnant," Seibert said. "If true, it would represent the largest nova remnant yet known. But it was especially shocking to find it associated with such a diminutive dwarf nova system. Everyone was skeptical and it took a considerable amount of time and effort to be certain."
Image Credit: NASA/JPL-Caltech
High resolution image
This enhanced image from the far-ultraviolet detector on NASA's Galaxy Evolution Explorer shows a ghostly shell of ionized gas around Z Camelopardalis, a binary, or double-star system featuring a collapsed, dead star known as a white dwarf, and a companion star.
The image was processed to enhance the diffuse emissions from the shell. Z Cam is the bright object near the center of the image. Parts of the shell are seen as a lobe-like, light-blue feature below and to the right of Z Cam, and as two large, light blue, perpendicular lines on the left.
The massive shell around Z Cam provides evidence of material ejected during and swept up by a powerful nova eruption, called a classical nova, which likely occurred a few thousand years ago.
In exploding binary systems, one of the two stars steals material from the other until it builds up to a certain level; at that point, the system erupts in a giant inferno. In the case of Z Cam, the white dwarf is pilfering material from its sedate companion.
There are two classes of exploding binary star systems, or cataclysmic variables: recurrent dwarf novae, which erupt in small, "hiccup-like" blasts episodically, and classical novae, which undergo huge explosions thousands of times more powerful than dwarf novae.
Z Cam was the one of the first known recurrent dwarf novae. Yet the shell of ionized gas around Z Cam detected by the Galaxy Evolution Explorer can only be explained as the remnant of a full-blown classical nova explosion. The discovery of the shell provides the first evidence that some binary systems undergo both types of explosions. Previously, a link between the two types of novae had been predicted, but there was no evidence to support the theory.
The Galaxy Evolution Explorer first began imaging Z Cam in 2003; this image was taken on Jan. 25, 2004. The type of emission found around Z Cam is most easily visible at far-ultraviolet wavelengths.
Most of the background galaxies and stars have been eliminated by the image processing, although a few linger as white spots near the top. The light-blue streaky clump in the bottom right corner is created by ultraviolet light reflected by dust. It is uncertain if Z Cam is the source of the dust-scattered light.
Image Credit: NASA/JPL-Caltech
High resolution image
This composite image shows Z Camelopardalis, or Z Cam, a double-star system featuring a collapsed, dead star, called a white dwarf, and a companion star, as well as a ghostly shell around the system. The massive shell provides evidence of lingering material ejected during and swept up by a powerful classical nova explosion that occurred probably a few thousand years ago.
The image combines data gathered from the far-ultraviolet and near-ultraviolet detectors on NASA's Galaxy Evolution Explorer on Jan. 25, 2004. The orbiting observatory first began imaging Z Cam in 2003.
Z Cam is the largest white object in the image, located near the center. Parts of the shell are seen as a lobe-like, wispy, yellowish feature below and to the right of Z Cam, and as two large, whitish, perpendicular lines on the left.
Z Cam was one of the first known recurrent dwarf nova, meaning it erupts in a series of small, "hiccup-like" blasts, unlike classical novae, which undergo a massive explosion. That's why the huge shell around Z Cam caught the eye of astronomer Dr. Mark Seibert of Carnegie Institution of Washington in Pasadena, Calif. - it could only be explained as the remnant of a full-blown classical nova explosion. This finding provides the first evidence that some binary systems undergo both types of explosions. Previously, a link between the two types of novae had been predicted, but there was no evidence to support the theory.
The faint bluish streak in the bottom right corner of the image is ultraviolet light reflected by dust that may or may not be related to Z Cam. Numerous foreground and background stars and galaxies are visible as yellow and white spots. The yellow objects are strong near-ultraviolet emitters; blue features have strong far-ultraviolet emission; and white objects have nearly equal amounts of near-ultraviolet and far-ultraviolet emission.
About 530 light years from Earth, Z Cam was one of the first dwarf novae ever detected. For decades, observers have watched the system hiccup with regular outbursts. It brightens about 40-fold every 3 weeks or so, when an instability causes some of the material drawn by the white dwarf to crash onto its surface. Theory holds that Z Cam and other recurring dwarf novae should eventually accumulate enough matter and pressure from their swirling disks of hydrogen to trigger gigantic classical novae explosions. But no one had found definitive evidence for this until Seibert's discovery in 2003.
Other team members confirmed that the structures detected by GALEX were indeed parts of a massive shell of gas surrounding Z Cam. Narrowband images from Kitt Peak National Observatory near Tucson, Ariz., Palomar Observatory near San Diego, Calif., and the Wise Observatory near Mizpe Ramon, Israel, along with optical spectroscopic measurements made at the Lick Observatory near San Jose, Calif., contributed to this verification.
"The new images are the strongest evidence yet in favor of the cyclic evolution scenario of these binary stars," said lead author Mike Shara of the American Museum of Natural History in New York. "It's gratifying to see such strong evidence for this theory finally emerge after all this time."
Carnegie Institution News Release
