Astronomers today are reporting the first detection of direct radiation from the surface of a white dwarf star in a pre-supernova binary star system using the Hubble Space Telescope. This is a major step forward in identifying the type of star that will become a Type Ia supernova, the type of supernova that is being used to show that the expansion of the universe is accelerating. These binary objects, called AM Cvn stars, have virtually pure helium in their outer layers, and are considered among the strongest progenitor candidates for the occurrence of Type Ia supernovae, the kind of supernova which are used as “standard candles” to measure the size of our Universe, and being used to show that the expansion of the Universe is speeding up instead of slowing down. The Hubble team is led by Dr. Edward Sion of Villanova University and includes Dr. Paula Szkody of the University of Washington, Seattle, Dr.Jan-Erik Solheim of the University of Oslo, Norway, Dr. Boris Gaensicke, the University of Warwick, England and Dr. Steve Howell of the National Optical Astronomical Observatories, Tucson.
The team is presenting the results of their computer modeling of the Hubble data at the 207th meeting of the American Astronomical Society in Washington, DC. Their work has been accepted for publication in an upcoming issue of the Astrophysical Journal Letters.
The extremely dense, planet-sized white dwarf star is normally surrounded by a swirling disk of helium gas from a very close helium-rich lighter weight donor star. The Hubble Team observed the object during a brief time when the helium disk hiding the white dwarf, temporarily goes away.
A mere teaspoonful of the white dwarf’s matter weighs over 100 tons. This heavy weight white dwarf and the lighter weight helium-rich donor star whirl around each other in a stellar dance every 28 minutes. In order for Type Ia supernovae to be used as proper reliable standard candles, we must understand what kind of star exploded. This white dwarf detection helps provide this information.
The Team demonstrates that the pre-supernova object is much cooler and more slowly spinning than predicted by theory. The white dwarf’s surface chemistry is laden with heavy metals. In the pre-supernova binary, helium is being transferred from the lightest, largest star to the heaviest, smallest star as they orbit each other every 28 minutes. In most cases the accretion of mass by the heaviest star proceeds via a nearly pure helium accretion disk, At present, only about 10 such double nearly pure helium white dwarf systems are known. But recent estimates predict enough to account for the observed rate of Type Ia supernovae. This first spectroscopic detection of the white dwarf in a AM CVn system allows us to directly find for the first time the chemical makeup, spin rate, and mass of the white dwarf as well as estimating how fast helium is accumulating onto the primary white dwarf.
These objects can undergo a Type Ia supernova explosions without requiring that the white dwarf star first reach its maximum possible mass, the so-called Chandrasekhar limit. If the incoming helium accumulates slowly enough onto the heavier white dwarf, it will gradually compress or crush the matter below, triggering a helium thermonuclear explosion which will cause the carbon in the core of the white dwarf to detonate 10,000,000 times more violently as a Type Ia supernova. This process called edge-lit detonation (ELD) will occur in an AM CVn system at low enough accretion rates and slow enough rotational velocities.
The AM CVn binary systems are also the only known source of low frequency gravitational waves predicted by Einstein’s theory of general relativity.
When the two compact stars in an AM CV binary system revolve around each other, they lose energy and angular momentum through the emission of gravitational waves, at the expense of their own orbital energy. This causes their orbits to shrink ever further. The orbit shrinkage has been observed in binary radiopulsars, e.g. in the famous Hulse-Taylor pulsar PSR B1913+16 (Nobel Prize in Physics 1993) but there has never yet been a direct detection of gravitational waves. AM CVn systems binary white dwarfs are expected to be the dominant sources of gravitational waves to be detected by LISA, the laser interferometer in space which is due for launch be launch early in the next decade.
