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Steve Roy
Marshall Space Flight Center, Huntsville, AL
Phone: 256-544-6535

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, MA
Phone: 617-496-7998

Science Contacts:
Sangwook Park, park{at}astro.psu.edu, 814-863-7111

May 26, 2003

CXC Press Release: 05-01

Chandra Adds to Story of the Way We Were

Data from NASA's Chandra X-ray Observatory have enabled astronomers
to use a new way to determine if a young star is surrounded by a
planet-forming disk like our early Sun. These results suggest that
disks around young stars can evolve rapidly to form planets, or they
can be disrupted by close encounters with other stars.

Chandra observed two young star systems, TW Hydrae and HD 98800, both
of which are in the TW Hydrae Association, a loose cluster of 10
million-year-old stars. Observations at infrared and other
wavelengths have shown that several stars in the TW Hydrae
Association are surrounded by disks of dust and gas. At a distance of
about 180 light years from Earth, these systems are among the nearest
analogs to the early solar nebula from which Earth formed.

"X-rays give us an excellent new way to probe the disks around
stars," said Joel Kastner of the Rochester Institute of Technology in
Rochester, NY during a press conference today in Nashville, Tenn. at
a meeting of the American Astronomical Society. "They can tell us
whether a disk is very near to its parent star and dumping matter
onto it, or whether such activity has ceased to be important. In the
latter case, presumably the disk has been assimilated into larger
bodies -- perhaps planets -- or disrupted."

Kastner and his colleagues found examples of each type of behavior in 
their study. One star, TW Hydrae, namesake of the TW Hydrae
Association, exhibited features in its X-ray spectrum that provide
strong, new evidence that matter is accreting onto the star from a
circumstellar disk. They concluded that matter is guided by the
star's magnetic field onto one or more hot spots on the surface of
the star. 

In contrast, Chandra observations of the young multiple star system
HD 98800 revealed that its brightest star, HD 98800A, is producing
X-rays much as the Sun does, from a hot upper atmosphere or corona.
HD 98800 is a complex multiple-star system consisting of two pairs of
stars, called HD 98800A and HD 98800B. These pairs, each of which is
about an Earth-Sun distance apart, orbit each other at about the same
distance as Pluto orbits the Sun.

"Our X-ray results are fully consistent with other observations that 
show that accretion of matter from a disk in HD 98800A has dropped to
a low level," said Kastner. "So Chandra has thrown new weight behind
the evidence that any disk in this system has been greatly diminished
or destroyed in ten million years, perhaps by the ongoing formation
of planets or by the companion stars."

The new X-ray technique for studying disks around stars relies on the 
ability of Chandra's spectrometers to measure the energies of
individual X-rays very precisely. By comparing the number of X-rays
emitted by hot gas at specific energies from ions such as oxygen and
neon, the temperature and density of particles can be determined.
This new technique will help astronomers to distinguish between an
accretion disk and a stellar corona as the origin of intense X-ray
emission from a young star.

Other members of the research team are David Huenemoerder, Norbert 
Schulz, and Claude Canizares from the Massachusetts Institute of 
Technology, and David Weintraub from Vanderbilt University. NASA's 
Marshall Space Flight Center in Huntsville, Ala., manages the Chandra 
program, and TRW, Inc., Redondo Beach, Calif., is the prime
contractor for the spacecraft. The Smithsonian's Chandra X-ray Center
controls science and flight operations from Cambridge, Mass., for the
Office of Space Science at NASA Headquarters, Washington.

The image and additional information are available at:

     http://chandra.harvard.edu/photo/2003/twhy/
and
     http://chandra.nasa.gov

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