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Catastrophic Variables |
Catastrophic variables, also called white dwarf accreters or cataclysmic variables, have had little fanfare. For a period <15 years, there has been little or no literature about the subject, with one exception to this being estimates as to mass, which was used for studies in accretion disk structure and stability. On June 3, 1999 Astronomers released new 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 Cataclysmic Variables.
Cataclysmic Variables typically involve a white dwarf 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 - then the process begins all over again.
Estimates of mass and core composition are usually only discussed in the context of the thermonuclear runaway theory. Space Spectroscopy enabled us to view the exposed white dwarfs in CVs. This has to be done during dwarf nova quiescence or low-brightness states of nova-like variables and CVs - when accretion rates are low. Of the 37 CVs scanned:
If pre-CV dwarf had time to cool
10-3 solar luminosities prior to onset of Roche lobe
overflow, then it would have an average lower lifetime = 2.5 X 108
years, and would be accretion heated to average luminosity log (L
/ L
) = 1.90 ; ~ 11,000 oK since
end of pre-CV evolution.61a
White dwarfs are embers of dying stars shrunken to planetary dimensions - only by accumulation of fresh Hydrogen from molecular cloud passages can an isolated dwarf hope to avoid a typical end. In close binary systems, white dwarfs may hope to escape the typical thermal extinction, and may be reincarnated briefly as a giant star. When a massive star collapses, the outer parts of the star explode at speeds of up to 10,000 kilometers per second and more. A similar amount of energy is released when a white dwarf star undergoes complete disintegration in a thermonuclear detonation.
In close binary systems hydrogen-rich matter arrives from a main-sequence type star filling its Roche lobe. This transfers gas with the proper angular momentum, and forms an accretion disk that enshrouds the dwarf. The object experiences low ongoing accretion rates which are interrupted every few weeks to months by intense accretion which lasts a few days to weeks (dwarf nova accretion event). Accretion heats the white dwarf, adds mass with processed or solar composition chemical abundances, and injects angular momentum into its envelope. Thermonuclear explosions (classical novae) every few thousand years also interrupts low accretion periods.
Far-UV spectroscopic observations on underlying white dwarf accreters (explosive central engine) may hold clues to
insight into
NICMOS has imaged thick, clumpy gas shells - the remnants of Cataclysmic Variable novae - from three systems: QV Vul, and QU Vul (both in the constellation Vulpecula), and V1974 Cyg (in the constellation Cygnus). The images show filaments, blobs, streams, and other structures that can only be seen by the Hubble. These observations imply that a great deal more material is ejected in a nova explosion than was predicted, and there are certain important isotopes that can only be produced by novae. The ejected material of this particular nova is very unlike the gas on the surface of our sun.
It appears the nova gas is enriched in elements such as carbon, nitrogen, oxygen, neon, magnesium, and aluminum. It is possible that some of the aluminum in our own solar system came from nova explosions, as there is evidence that novae eject radioactive aluminum, and that radioactive aluminum once existed in our solar system but has since decayed.
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. Dust grains often condense in the shells ejected by such novae, it is therefore possible that novae are among the stars that produce the solid grains that are the building blocks of planets.
Two of the three novae imaged, QV Vul and QU Vul, formed dust in their gas shells. 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 - similar to material found in our solar systems meteorites, and material that was probably present in our solar system when the planets were forming. 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.
"Be" Type Stars: GM Sgr and Phi Persei
A known variable star apparently is acting up, feeding materials to a massive, compact companion. The star - GM Sgr - is in the constellation Sagittarius, in the direction of the center of our galaxy. GM Sgr is a visible star discovered in 1927. It's actually part of a binary system. Orbiting GM Sgr is a massive, compact body, most likely a neutron star or a black hole, that's causing all the fuss, starting with a modest announcement early this year.
The double-star system Phi Persei.15 has trimmed an aging, once massive star to a lean one-solar mass, and the moderate-sized companion has accreted up to nine-solar masses and is spinning so violently that it's flinging gas from its surface. After consuming most of its hydrogen the aging star swelled up and began jettisoning its mass until only its bare core was left. The companion star cannibalized the discarded material, thereby increasing in size. The roughly 10-million-year-old companion had potentially doubled its lifetime because it has gained a vast amount of hydrogen fuel.
The companion also has changed its identity from a normal, moderately massive star to a "Be" star, a type of hot star with a broad, flattened disk of hydrogen gas swirling around it, much like the rings of Saturn. Based on measurements taken by astronomers at the U.S. Naval Observatory, the disk is eight times wider than the star.
As long as the "Be" star doesn't break apart, it will live for another 10 million years because of the hydrogen fuel it acquired from its companion. Then it will swell during the expansion phase and possibly dump some of its mass back onto the subdwarf, which will have evolved into a white dwarf. At this point it may be possible that the subdwarf then might grow in mass and eventually explode as a supernova, or the companion might swell up so much that it would engulf the white dwarf.
These stripped-down stars are called subdwarfs - a type of aging star that has passed the expansion phase by swelling and puffing away its outer layers, and on its way to becoming a fading white dwarf. The type "Be" companion has the same mass as the Sun, but is nine times hotter than the Sun at 95,000 degrees Fahrenheit. This particular subdwarf would be the brightest object of its class in the sky (a sixth-magnitude star) if it could be seen alone - howener, its companion is ten times brighter in visible light than the subdwarf, and so eluded detection for many years.
What causes the fast spin of "Be" stars ? This orbiting planet also transfers angular momentum to the star, causing it to "spin up" to a much faster rate than it would normally have. Observations of Phi Persei prove that if the gas discarded from a nearby swelling star strikes the companion off-center, it net effect will cause it to spin faster. It begins to rotate so fast (one million mph / 450 kilometers per second at its equator) that the star is distorted into a flattened oblate spheroid.
Scientists can only speculate about their past. It is thought that before the exchange of material, the subdwarf was the more massive of the two (6 Msolar), while its companion was slightly less bulky (5 Msolar). Such massive stars usually end their lives in a supernova event.
However, stars in binary systems live differently. When the once massive subdwarf entered its twilight years about one million years ago, it swelled in size as it began using up its hydrogen fuel. At this point, a single massive star would have reached Chandrasekhars Limmit, and eventually exploded. The presence of the companion prevented the star from suffering such an end - the once-massive star dumped most of its outer layers onto its companion, and now may be heading to a quiet demise.
Images taken with Earth-based telescopes have shown the larger, hourglass-shaped nebula. This picture, taken with NASA's Hubble Space Telescope, reveals a small, bright nebula embedded in the center of the larger one. Astronomers have dubbed the entire nebula the "Southern Crab Nebula" (He2-104). From ground-based telescopes, it looks like the body and legs of a crab.
The nebula, located in the Southern Hemisphere constellation of Centaurus, is a few thousand light-years from Earth and is several light-years long, with a pair of aging stars buried in the glow of the tiny, central nebula. One of them is a red giant, a bloated star that is exhausting its nuclear fuel and is shedding its outer layers in a powerful stellar wind. Its companion is a hot, white dwarf, a stellar zombie of a burned-out star. This odd duo of a red giant and a white dwarf is called a symbiotic system - the red giant is also a Mira Variable, a pulsating red giant, that is far away from its partner. It has been estimated that it could take as much as 100 years for the two to orbit around each other. Astronomers also speculate that the interaction between these two stars may have sparked episodic outbursts of material, creating the gaseous bubbles that form the nebula.
They interact by playing a celestial game of "catch": as the red giant throws off its bulk in a powerful stellar wind, the white dwarf catches some of it. An accretion disk of material forms around the white dwarf and spirals onto its hot surface. Gas continues to build up on the surface until it sparks an eruption, blowing material into space. This explosive event may have happened twice in the "Southern Crab." Astronomers believe that the hourglass-shaped nebulae represent two separate outbursts that occurred several thousand years apart. In the image the jets of material in the lower left and upper right corners may have been accelerated by the white dwarf's accretion disk and probably are part of the older eruption.
The discovery of a protoplanet/protostar, made by Susan Terebey of the Extrasolar Research Corporation in Pasadena, CA, and her team using Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS), offers new insights into the formation of our own Solar System. Located in the sky within a star-forming region in the constellation Taurus, the object, called TMR-1C, appears to lie at the end of a strange filament of light that suggests it has apparently been flung away from the vicinity of a newly forming pair of binary stars. At a distance of 450 light-years, the same distance as the newly formed stars, the candidate protoplanet would be ten thousand times less luminous than the Sun.
If the object is a few hundred thousand years old, the same age as the newly formed star system which appears to have ejected it, then it is estimated to be 2-3 times the mass of Jupiter, the largest gas giant planet in our Solar System.Also possible is that the object is up to ten million years old, the same age as other young stars nearby, in which case it may be a giant protoplanet or a brown dwarf star. The candidate protoplanet is now 130 billion miles from the parent stars and predicted to have a proper motion equivalent to 20,000 miles per hour (10 kilometers/sec), and is destined to forever drift among the Milky Way's starry population.
Hubble researchers estimate the odds at two percent that the object is instead a chance background star.Current models predict that very young giant planets are still warm from gravitational contraction and formation processes. This makes them relatively bright in infrared light compared to old giant planets such as Jupiter. Young planets are difficult to find in new solar systems because the glare of the central star drowns out their feeble glow. These young planets ejected from binary systems would therefore represent a unique opportunity to study extrasolar planets with current astronomical technology. The discovery also challenges conventional theories that predict gas giant planets take millions of years to coagulate from dust in space - it favors more recent ideas that large, low-density planets may condense out of gas very quickly, at the same time their parent star does.
This observation might have been dismissed as a background star if not for the presence of a bizarre 130-billion-mile-long filamentary structure that bridges the space between the binary pair and the candidate protoplanet.It is believed that this could be a tunnel the runaway object burrowed through a dust cloud surrounding the stars. This created a "light tube" which channels light from the stars deep inside their dusty cocoon - like a light beam traveling through a length of fiber optic cable, and highlights the tantalizing possibility that the planet had been flung into deep space by a gravitational "slingshot" effect from its parent stars. This could have happened if the planet's orbit allowed it to rob momentum from the stars and pick up so much speed that it escaped the system, similar to the way spacecraft perform gravitational "slingshot" maneuvers to pick up speed by flying close by a planet.
Cataclysmic Variables & Related Objects
These are systems where two stars are in close orbit around one another and gas is being transferred from one star to the other. This may involve two normal stars (for example in an Algol system) or a normal star and a compact star, such as a white dwarf, neutron star or black hole. Systems involving neutron stars or black holes are studied by the X-ray binaries group.
When the gas is flowing onto a white dwarf, the system is called a cataclysmic variable. The normal star in these systems is usually a small red dwarf, of perhaps half the mass of the Sun. This is distorted into a teardrop shape by the nearby presence of the white dwarf (with a mass similar to the Sun, but about the same diameter as the Earth.) Gas flows out from the pointed end of the red dwarf in a stream towards its companion. This will normally form an accretion disc spiralling down towards the surface, but if white dwarf is magnetic then the stream will follow the magnetic field down onto the poles of the white dwarf. Such strongly magnetic systems are known as Polars or AM Her stars. If the magnetic field is weaker then the accretion disc may reach part way to the surface before being cut off by the magnetic field. These are called Intermediate Polars or DQ Her stars.
Computer Modeling
Models the chemical evolution of the secondary stars of interacting binaries through common envelope and post common envelope phases using rapid ultraviolet spectroscopy with the Hubble Space Telescope have been recently completed in a study of the intermediate polar YY Dra, and scientists are now working on data obtained this year on WZ Sge. Other Targets include the intermediate polar, XY Ari, the dwarf nova HS 1804+53 and the novalike system TT Ari. Scientists have noted similarities between CVs and low-mass X-ray binaries (analogous systems containing neutron star or black hole compact objects) and in particular between dwarf novae and black hole X-ray transients.18 During its last outburst a QPO was observed in optical wavelengths that could come from the inner part of the accretion disc and be explained with a model where the accretion stream overflows the disc.
The least well understood phenomenon in the evolution of interacting binaries is the stage of mass transfer between the components involving mass and angular momentum loss from the system. Algol systems have undergone one such stage of evolution and so may be the key to understanding the processes involved. Obtaining accurate fundamental parameters for Algol systems, including the chemical composition of the fainter star, which is an important constraint on models of the evolution of these systems, is an as-yet unrealized goal of scientists studying these unusual stellar objects.
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