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Accretion Disk (Image 2)

An artist's conception of the accretion disk in the binary star system WZ Sagittae (WZ Sge). [A previous version is available Here. This new concept was created using data from Kitt Peak National Observatory (KPNO) and NASA's Spitzer Space Telescope. The picture reveals a larger, thicker disk of cool dusty material surrounding much of the gaseous accretion disk. A small piece of the cool, orange companion star is seen on the left. The hot, white dwarf star is seen in the center of the accretion disk. Material from the cool stars flows into the accretion disk and then onto the white dwarf.

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Observations of an interacting binary star using telescopes at KPNO and NASA's Spitzer Telescope suggest that the disks of hot gas that accumulate around a wide variety of astronomical objects--from white dwarf stars in energetic binary systems to super-massive black holes at the hearts of active galaxies--are likely to be much larger than previously believed.

The target of this specific investigation, named WZ Sge, is an interacting binary star located in the constellation Sagitta, the arrow of the archer Sagittarius. As part of a program, called the Spitzer-NOAO Observing Program for Teachers and Students, Steve B. Howell of the National Optical Astronomy Observatory and a team of astronomers and educators imaged WZ Sge using the National Science Foundation's 2.1-meter telescope and the WIYN 0.9-meter telescope, both located at KPNO; and the infrared array camera (IRAC) on the Spitzer.

Interacting binary stars such as WZ Sge contain a white dwarf star (a compact star about the size of the Earth, but with a mass near that of the sun) and a larger but less massive and much cooler companion star. The companion star has material ripped off its surface by the stronger gravity of the white dwarf. The material flows toward the white dwarf and forms a disk that surrounds it called an accretion disk.

Whether they form in cataclysmic variable systems or they surround the massive black-hole hearts of active galaxies, accretion disks have been well observed and modeled using measurements obtained across much of the electromagnetic spectrum, from X-rays to the near-infrared. The derived picture of the "standard accretion disk" model is a geometrically thin disk of gaseous material surrounding the white dwarf or black hole.

Howell's team obtained the first time-series observations of an accretion disk system at 4.5 and 8 microns with the Spitzer. At nearly the same time, they obtained optical observations of WZ Sge at KPNO. The optical observations confirmed the standard view of the accretion disk size and temperature.

The mid-infrared observations, however, were completely unexpected and revealed that a larger, thicker disk of cool dusty material surrounds much of the gaseous accretion disk. This outer dust disk likely contains as much mass as a medium-sized asteroid. The newly discovered outer disk extends about 20 times the radius of the gaseous disk.

The implications from such a discovery are far reaching, affecting not only the theoretical models (since the formation and evolution of the disks are modeled based on their size, temperature and composition--all quantities that now need to be revised), but also nearly all previous observations of systems containing accretion disks.

Credit: P. Marenfeld/NOAO/AURA/National Science Foundation

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