June 3, 1999 -- Astronomers today released new Hubble Space Telescope images that show gas shells ejected into space at regular intervals by an unusual type of white dwarf star. The images may reveal important information about the role novae play in the evolution of our galaxy, including the distribution of heavier elements and the development of planetary systems.
The images, produced by the Hubble Space Telescope's Near Infrared Camera Multi-Object Spectrograph (NICMOS), are the first images of gas shells produced by novae that are members of a class of double star systems called Cataclysmic Variables.
Cataclysmic Variables typically involve a white dwarf (a sun-mass star that has burnt down its internal fuel and has collapsed to a dense, earth-size sphere) in very close proximity to a larger and cooler star. The strong gravity tides from the white dwarf drag hydrogen gas off the larger star. This gas spirals down onto the surface of the white dwarf and accumulates as a deeper and deeper hydrogen shell around the white dwarf core, which is primarily carbon, oxygen and other heavier elements.
When enough hydrogen has accumulated on the white dwarf's surface (after about 10,000 years), thermonuclear fusion reactions begin and gradually intensify until finally (much later) temperatures rise to the point where they cause a nova explosion in the hydrogen shell, blowing it and part of the white dwarf core into space -- and then the process begins all over again.
NICMOS has imaged thick, clumpy gas shells -- the remnants of Cataclysmic Variable novae -- from three systems, QV Vul, QU Vul (both in the constellation Vulpecula), and V1974 Cyg (in the constellation Cygnus). "Our pictures show filaments, blobs, streams, and other structures that can only be seen by the Hubble," said team member Chick Woodward from the University of Wyoming. "We can analyze these pictures and determine how much gas is blown into space by the various systems that we are studying. Before, we could only guess how the gas was distributed in space and now we can see that our previous guesses were not very good."
In fact, the images provide important data which can be used to test the accuracy of some key theories concerning novae and galactic evolution. In their analysis, the team plans to compare the ejected shells in the images to computer simulations of nova explosions. "Studies of these novae will be critical in improving our calculations and will help us better understand the cause and evolution of these explosions," said team member Sumner Starrfield from Arizona State University.
"For example, these observations imply that a great deal more material is ejected in a nova explosion than was predicted by our calculations, and that could make nova far more important in the evolution of the chemical elements in our galaxy than was previously believed," said Starrfield.
Novae have long been understood as part of the processes which contribute to element production in our galaxy, Astronomers generally believe that only hydrogen and helium were formed in the Big Bang event which formed the universe; all other chemical elements are formed by processes in stars. Thus the proportion of elements such as carbon, oxygen and iron has slowly grown over the lifetime of the galaxy.
According to team member James Truran from the University of Chicago, much of the gaseous material in the galaxy comes from supernova explosions involving stars like the sun or more massive stars, "and there are certain important isotopes that can only be produced by novae."
"We know from other studies of the novae we're looking at that the ejected material is very unlike the gas on the surface of our sun," said Starrfield. "The nova gas is enriched in elements such as carbon, nitrogen, oxygen, neon, magnesium, and aluminum." But, Starrfield argues, that does not mean that material from novae were not involved in the evolution of our solar system.
"It is possible that some of the aluminum in our own solar system came from nova explosions. There is evidence that novae eject radioactive aluminum and there is also evidence that radioactive aluminum once existed in our solar system but has since decayed," said Starfield.
"Furthermore, dust grains often condense in the shells ejected by such novae," noted team member Robert Gehrz from the University of Minnesota. "Their infrared spectral signature shows that dust they make is similar in size to the small dust grains released from comets in our solar system.
"It is therefore possible that novae are among the stars that produce the solid grains that are the building blocks of planets. Like the gas produced by novae, these grains end up in the gas and dust clouds that produce new stellar and planetary systems," Gehrz said.
Two of the three novae imaged, QV Vul and QU Vul, formed dust in their gas shells. "We hope that the structures revealed by the NICMOS novae images will help us to understand where the dust forms in nova shells and why some novae produce grains while others do not," Gehrz added. "During the 1980's, QV Vul was found to produce four types of astrophysical grains at various times during a two-year period following its eruption." The dust that formed contained carbon, silicates, silicon carbide, and hydrocarbons -- "stardust" similar to material found in our solar systems meteorites, and material that was probably present in our solar system when the planets were forming.
"Our earlier models of QV Vul suggested that the carbon dust components formed in fast-moving polar flumes, and that the silicates formed in a slow-moving equatorial ring," Gehrz said. "We hope the NICMOS images will enable us to evaluate the correctness of this model."
The Hubble nova program is also laying the groundwork for greatly expanded further study of novae in the universe. "Most of what astronomers know about how novae effect the chemical evolution of galaxies is limited to studies of novae in the Milky Way," noted team member Matthew Greenhouse from the NASA Goddard Space Flight Center. "The heavy elements ejected by these novae explosions produced bright infrared emission lines -- an infrared signature that will allow the Infrared Spectrograph instrument aboard the upcoming NASA Space Infrared Telescope Facility (scheduled for launch in 2001) to discover and study novae in a wide range of galaxies for the first time."
The international research team which planned and analyzed these observations included Chick Woodward and Kunegunda Belle of the University of Wyoming, Nye Evans and Stuart Eyres of the University of Kaele in England, Robert Gehrz and Michael Schuster of the University of Minnesota, Matthew Greenhouse of NASA Goddard Space Flight Center, Joachim Krautter of the State Observatory and University of Heidelberg in Germany, Sumner Starrfield of Arizona State University, and James Truran of the University of Chicago.
The work was supported by NASA and the Space Telescope Science Institute.
EDITORS: False color gif images can be obtained over the internet at http://wapiti.uwyo.edu/hst-nova on June 3, 1999 at 9:20 a.m. CDT.
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