Dark Matter Discovered in Accretion Disks (Forwarded)

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EMBARGOED FOR RELEASE: 10:30 a.m. CST, Wednesday, January 9, 2008

RELEASE NO: NOAO 08-02

Dark Matter Discovered in Accretion Disks -- Suggests Major Revisions to
Concepts of Disk Structure and Luminosity

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 at
http://www.noao.edu/outreach/press/pr08/pr0802.html

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.