January 11, 2006
Spitzer Reveals Unexpected Disks Around Interacting Stars
NOAO -- New Spitzer Space Telescope observations of an unusual class of interacting binary stars detected excess amounts of infrared radiation, suggesting that these odd objects are surrounded by large disks of cool dust.
The results reported today in Washington, DC, at the 207th meeting of the American Astronomical Society (AAS) were produced by one of six teams of professional astronomers and high school teachers participating in a unique program co-sponsored by the Spitzer Science Center and the National Optical Astronomy Observatory (NOAO).
The type of cataclysmic variable system being studied by the team consists of a highly magnetic white dwarf star (a "dead" remnant star formed from the core of a star like our Sun when it exhausts the available fuel to support nuclear fusion) and a very low mass, cool object similar to a brown dwarf.
The two objects orbit so closely"”about the distance from Earth to the Moon -- that they make a complete revolution about each other in only 80-90 minutes. The white dwarf is Earth-sized but weighs about 60 percent of the mass of the Sun, while the companion star is Jupiter-sized but has about 40-50 times the mass of Jupiter.
The high mass of the white dwarf and the closeness of the companion result in mass exchange between the two stars. The gravitational influence of the white dwarf squeezes the companion star into a teardrop shape, and matter squirts from its pointed end toward the white dwarf, like water from the nozzle of a garden hose. This material eventually falls onto the white dwarf, causing tremendous heating of its atmosphere and the emission of a large amount of energy from X-rays to the far infrared.
A team of astronomers and teachers led by Steve B. Howell of NOAO observed four of these types of binaries with NASA's Spitzer Space Telescope in an attempt to study the cool, low-mass object in the pair: EF Eridanus, V347 Pav, GG Leo and RX J0154.
To their surprise, excess infrared emission was discovered around all four. The team's current best model for its origin is a large, cool circumbinary dust disk with a temperature of about 800-1,200 Kelvin (980-1,700 degrees Fahrenheit).
"Our explanation at this point is that the emission originates from a large, relatively cool disk of dust encircling the entire binary system," Howell says. "The discovery of dust disks around these old interacting binaries is very exciting. We have shown our initial results to a variety of specialists, and nobody yet has a better idea of what we are seeing."
Such circumbinary disks have been predicted on theoretical grounds and a few observational studies have attempted to find them, with mixed results. The disks may be the remains of the large "mass-loss" episode that occurred during the formation of the white dwarf. They also could be composed of material spewed from the binary in the form of strong winds (like a very dense version of our Sun's solar wind), or material that was ejected during one or more previous nova explosions. Cyclotron emission due to the large magnetic field of the white dwarfs in these particular binaries cannot be eliminated completely as another potential source of at least part of the infrared emission.
"A number of ideas are on the table, as well the possibility of some still-unknown process," Howell adds. "These objects are ripe for further study."
Only two other white dwarfs (including one newly discovered) are known to be encircled by a dust disk"”stars named G29-38 and GD362. Unlike the cataclysmic variables studied by Howell's team, both of these are single white dwarfs, and the source of their dust disks is not known for certain. Dust disks made up of "left over" material from the star formation process are known to exist around very young stars and have been discovered around Sun-like stars as well. Some of these latter disks are known to harbor planetary-type objects, orbiting in cleared out "rings" within the disk.
"While we have no evidence for planetary objects in our disks, the possibility does exist," Howell adds. "More work must be done to prove the infrared excess is from a disk and, if true, to discover its properties such as density and composition. We also would like to see if these disks exist in every interacting binary of this type or only in some. Their presence would greatly change our concept of the evolution of such systems."
These types of systems are important because they give astronomers insight into the accretion, or "mass transfer," process that also plays a role in the formation of stars and planets, according to team member Donald W. Hoard, an astronomer at the Spitzer Science Center in Pasadena, California.
"Cataclysmic variable accretion is one of the least complicated forms of mass transfer in the Universe," Hoard says. "These systems are great to observe, because unlike accretion during the formation of stars and planets, or around supermassive black holes in far off galaxies, the process in cataclysmic variables happens on relatively short, human timescales."
Other members of the research team reporting in poster 70.17 today at the AAS meeting include Carolyn Brinkworth of the Spitzer Science Center, and physics teachers Howard Chun from Cranston High School in East Cranston, Rhode Island; Beth Thomas of Great Falls Public Schools in Great Falls, Montana; and, Linda Stefaniak of Allentown High School, Allentown, New Jersey.
Chun, Thomas and Stefaniak are graduates of NOAO's Teacher Leaders in Research Based Science Education (TLRBSE), a teacher professional development program funded by the National Science Foundation. Twelve TLRBSE teachers were competitively selected in the fall of 2004 to work in six teams that were awarded three hours of Director's discretionary observing time with Spitzer.
"This opportunity has allowed my students and myself to participate in authentic astronomy research," Thomas says. "It has given me a deeper understanding of the world of infrared and astronomy, while reinforcing how we teach the process of problem solving and how answers are sought in the science community. The experience has been very enlightening, extremely rewarding and genuinely stimulating."
Another six TLRBSE teachers were just selected for a second round of the program, and these teachers met with their astronomer partners during this AAS meeting to begin planning new research. Background information on the Spitzer-TLRBSE program and the experience of the teachers and their students in this group can be found on the Web at the Spitzer Web Site.
The National Optical Astronomy Observatory is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the National Science Foundation.
The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech.