Scientist At American Museum Of Natural History Helps Prove Theory About Evolution Of dwarf Novae

Enhanced GALEX image showing Z Cam (center) and the wispy remnants of a classical nova explosion (below, right and left of Z Cam) that scientists believe happened thousands of years ago.

NASA/JPL-Caltech/M. Shara (AMNH)

A team of astrophysicists led by Mike Shara at the American Museum of Natural History recently proved a long-standing theory about the behavior of a class of objects known as "dwarf novae." Scientists believe that the group of mildly variable binary stars must also exhibit "classical nova" behavior and explode in spectacular fashion every ten thousand years or so. After decades of observation, however, no one has ever found conclusive evidence that they do so. A new study appearing in the March 8 issue of the journal Nature confirms the theory and reveals the most extensive remnant shell of gas left from a nova explosion.

"The new images are the strongest evidence yet in favor of the cyclic evolution of these binary stars," said Shara, a curator in the Division of Astrophysics. "It's gratifying to see such strong evidence for this theory to finally emerge after all this time." 

Observations of classical novae, composed of a collapsed, dense white dwarf star and an ordinary, sun-like star, have shown that the white dwarf periodically steals enough hydrogen-rich material from its companion to produce a gigantic thermonuclear explosion on its surface. For a few days or weeks classical novae shine so brightly that they have been mentioned in historical literature. 

Though not as bright as classical novae, Z Camelopardalis (Z Cam) is one of the brightest dwarf novae in the sky and, at a little over 530 light years, is one of the closest of the millions of such objects in the Milky Way. Every 20 days or so Z Cam "hiccups," brightening by as much as 40 times for a period of several days. In many ways it's a prototypical dwarf nova, but until now no one has ever detected the remnants of the much more massive classical nova explosions that theory predicts should periodically occur in such objects.

View an animation that shows an artist's concept of Z Camelopardalis (Z Cam), a stellar system featuring a collapsed, dead star, or white dwarf, and a companion star.

Image credit: NASA/JPL-Caltech

In 2003, Shara's colleague Mark Seibert at the Carnegie Institution of Washington was analyzing images from the NASA's orbiting ultraviolet telescope, Galaxy Evolution Explorer (GALEX). His scientific focus was on galaxies, but in the course of his work he noticed an unusual arc of celestial material 2 light years from Z Cam centered on that binary star. Previous observations had failed to reveal the massive shell, the largest known to exist, because it is very faint at visible wavelengths. It is, however, easily detected at the UV wavelengths detected by GALEX. Shara's team obtained additional images from Kitt Peak National Observatory, Palomar Observatory, and the Wise Observatory along with measurements made at the Lick Observatory and confirmed that the structures detected in the GALEX images were indeed a massive shell of stellar gas. These findings support the conclusion that the arc is composed of material ejected from the dwarf nova during a classical nova eruption centuries ago. 

Depending on the velocity of the arc, the team puts the date of the eruption anywhere between 2,400 and 240 years ago, with the older age more likely. The authors also conclude that Z Cam's classical nova explosion must have been quite spectacular. "During that eruption," they write, "it must have become, for a few days or weeks, one of the brightest stars in the sky." 

The study was supported by NASA, Kitt Peak National Observatory, Lick Observatory, the Palomar Observatory, and the Wise Observatory. Caltech leads the GALEX mission and is responsible for science operations and data analysis; NASA's Jet Propulsion Laboratory manages the mission and built the science instrument. 

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