"Wonderful" Star Reveals its Hot Nature (Forwarded)

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

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

Science Contacts:
Margarita Karovska, 617-495-7347

April 28, 2005

CXC RELEASE 05-05

"Wonderful" Star Reveals its Hot Nature

For the first time an X-ray image of a pair of interacting stars has
been made by NASA's Chandra X-ray Observatory. The ability to
distinguish between the interacting stars -- one a highly evolved giant
star and the other likely a white dwarf -- allowed a team of scientists
to observe an X-ray outburst from the giant star and find evidence that
a bridge of hot matter is streaming between the two stars.

"Before this observation it was assumed that all the X-rays came from a
hot disk surrounding a white dwarf, so the detection of an X-ray
outburst from the giant star came as a surprise," said Margarita
Karovska of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Mass., and lead author article in the latest Astrophysical
Journal Letters describing this work. An ultraviolet image made by the
Hubble Space Telescope was a key to identifying the location of the
X-ray outburst with the giant star.

X-ray studies of this system, called Mira AB, may also provide better
understanding of interactions between other binary systems consisting of
a "normal" star and a collapsed star such as a white dwarf, black hole
or a neutron star, where the stellar objects and gas flow cannot be
distinguished in an image.

The separation of the X-rays from the giant star and the white dwarf was
made possible by the superb angular resolution of Chandra, and the
relative proximity of the star system at about 420 light years from
Earth. The stars in Mira AB are about 6.5 billion miles apart, or almost
twice the distance of Pluto from the Sun.

Mira A (Mira) was named "The Wonderful" star in the 17th century because
its brightness was observed to wax and wane over a period of about 330
days. Because it is in the advanced, red giant phase of a star's life,
it has swollen to about 600 times that of the Sun and it is pulsating.
Mira A is now approaching the stage where its nuclear fuel supply will
be exhausted, and it will collapse to become a white dwarf.

The internal turmoil in Mira A could create magnetic disturbances in the
upper atmosphere of the star and lead to the observed X-ray outbursts,
as well as the rapid loss of material from the star in a blustery,
strong, stellar wind. Some of the gas and dust escaping from Mira A is
captured by its companion Mira B.

In stark contrast to Mira A, Mira B is thought to be a white dwarf star
about the size of the Earth. Some of the material in the wind from Mira
A is captured in an accretion disk around Mira B, where collisions
between rapidly moving particles produce X-rays.

One of the more intriguing aspects of the observations of Mira AB at
both X-ray and ultraviolet wavelengths is the evidence for a faint
bridge of material joining the two stars. The existence of a bridge
would indicate that, in addition to capturing material from the stellar
wind, Mira B is also pulling material directly off Mira A into the
accretion disk.

Chandra observed Mira with its Advanced CCD Imaging Spectrometer on
December 6, 2003 for about 19 hours. NASA's Marshall Space Flight
Center, Huntsville, Ala., manages the Chandra program for NASA's Science
Mission Directorate, Washington. Northrop Grumman of Redondo Beach,
Calif., was the prime development contractor for the observatory. The
Smithsonian Astrophysical Observatory controls science and flight
operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/photo/2005/mira/
and
http://chandra.harvard.edu/photo/2005/mira/