January 10, 2008
Disks Around Black Holes and Binary Stars Just Got Bigger
Observations of the interacting binary star using telescopes at Kitt Peak National Observatory and NASA's Spitzer Space Telescope suggest that the disks of hot gas that accumulate around a wide variety of astronomical objects"”from degenerate stars in energetic binary systems to supermassive 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 Sagittae (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 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 Kitt Peak, and the Infrared Array Camera (IRAC) on Spitzer.
"We were very surprised to see the contrasting results obtained with the optical telescopes on the ground and the infrared telescope in space," says Howell, an astronomer at the National Optical Astronomy Observatory (NOAO) and leader of the team who made the discovery being reported today in Austin, TX, at the 211th meeting of the American Astronomical Society (AAS). "The much larger size of the infrared-emitting portion of the accretion disk around WZ Sge was immediately obvious in the data. Our observations strongly imply the presence of dark matter in these structures, which are ubiquitous throughout the Universe."
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, usually a low-mass star or a brown dwarf, has material ripped off its surface by the stronger gravity of the white dwarf. This material flows toward the more massive star and, in the process, forms a disk surrounding the white dwarf, known as an accretion disk.
Stars such as WZ Sge are called cataclysmic variables due to their rapid and often large changes in brightness, all caused by variations in the accretion disk. The two stars in such systems orbit about each other at a similar distance to that between Earth and the Moon, but with tremendous angular momentum that results in orbital periods ranging from a few hours down to as short as tens of minutes (the period of WZ Sge is 81 minutes).
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. Accretion disk models, bolstered by observation, are generally composed of hot gas having a temperature distribution within them, being hottest near the center and falling off in temperature toward the outer edge.
In order to confirm the general accretion disk models and extend them into the mid-infrared portion of the spectrum, Howell's team obtained the first time series observations of an accretion disk system at 4.5 and 8 microns with the Spitzer Space Telescope. At nearly the same time, they obtained optical observations of WZ Sge at Kitt Peak. The optical observations confirmed the standard view of the accretion disk size and temperature, values known for over a decade.
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.
"This discovery suggests that our current model for accretion disks of all kinds is wrong," says team member Donald Hoard of the Spitzer Science Center. "We will need to rethink and recast these models for accretion disks, not only in interacting binary stars but also in distant, highly luminous active galaxies."
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.
In addition, the dust disk (which is thicker than the known gaseous disk) blocks infrared light emitted by the compact central object and the inner hot regions of the gaseous disk. Not knowing that some mid to far infrared light is blocked by the newly discovered outer dust ring can lead observers to significantly underestimate the total luminosity of the central object. "The amount of this underestimation is not yet accurately known from our initial discovery, but may be as large as 50 percent," Howell says.
An artist's concept comparing the previous view and the new view of the accretion disk around WZ Sge is available above.
The observational program making this discovery was a joint effort between research scientists Howell, Hoard, and Carolyn Brinkworth of Spitzer Science Center, and high school teacher Beth Thomas and student Kimmerlee Johnson (Great Falls Public Schools, Great Falls, MT), teacher Jeff Adkins and student John Michael Santiago (Deer Valley High School, Antioch, CA), and teacher Tim Spuck and student Matt Walentosky (Oil City High School, Oil City, PA).
The work was funded by Spitzer Science Center as part of a joint project with NOAO to expand and extend the national observatory's Research Based Science Education (RBSE) teacher professional development program to include observations with the Spitzer Space Telescope. RBSE has been training groups of 20 teachers in the research process (including regular observations at Kitt Peak National Observatory) every year for more than a decade, using funding support from NSF.
Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, based in Tucson, AZ, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF.
NASA's Jet Propulsion Laboratory, Pasadena, CA, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology. Caltech manages JPL for NASA.
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