RELEASE: H00-176

HUBBLE SEES LONE NEUTRON STAR STREAKING ACROSS GALAXY

Several hundred million of them may be found in our galaxy, but the  world's most powerful telescope has captured the one thought to be  closest to Earth. NASA's Hubble Space Telescope has caught up with  a runaway neutron star believed to be 200 light years away.

The object known as RX J185635-3754 is expected to swing by the  planet at a safe distance in about 300,000 years. A neutron star is  the remnants left behind after a supernova explosion, as the  material at the core collapses into a dense mass of neutrons. The  star has the mass of the sun packed into an area about 12 miles in  diameter.

Precise observations made with the Hubble telescope confirm the  isolated interstellar traveler is now located in the southern  constellation Corona Australis. Since the object has no companion  star that would affect its appearance, this discovery will allow  future astronomers to more easily confirm stellar theories against  a variety of its physical properties such as size, inherent  brightness and true age.

"Because this is the closest and brightest of the few known  isolated neutron stars, it is the easiest to study and is an  excellent test bed for nuclear astrophysical theories, " said  Frederick M. Walter of the State University of New York (SUNY), in  Stony Brook, NY.

"The scientific importance of this object lies in the fact that the  neutron star is isolated," added Walter. "It appears to be hot, not  because it is accreting hydrogen gas as it moves through space, but  because it is still young and cooling off. Since we know its  approximate age, we can test how fast neutron stars cool off."

The neutron star's wayward trajectory was caught in three Hubble  snapshots taken in 1996 and 1999. The results of the observations  are being presented today by the American Astronomical Society's  High Energy Astrophysics Division (HEAD) in Honolulu, HI.

The images also show the star moves across the sky with an apparent  wobble, caused by a reflection of the Earth's own orbital motion,  called parallax.

In addition, the images reveal that the neutron star is streaking  across the sky from west to east at a rate equal to the diameter of  the Moon every 5,400 years. Although this apparent motion may seem  slow, it is actually one of the fastest-moving stars in the sky.  The apparent motion, combined with the distance, means the  energetic ember is moving at a speed of about 240,000 miles per  hour.

This neutron star may have formed about 1 million years ago when a  massive star in a binary star system exploded as a supernova,  releasing its companion star, an ultra-hot, blue star now known as  Zeta Ophiuchus. Because the neutron star and Zeta Ophiuchus were in  about the same location in space, RX J185635-3754 may be the  remnant of the original binary companion of Zeta Ophiuchus.

The runaway neutron star was first reported in 1992, when  astronomers detected a very bright source of X-ray emission with  the Roentgen Satellite (ROSAT). Because it was not seen in optical  light and appeared to be within 500 light-years of the Earth,  researchers surmised it was likely to be a neutron star.

Four years later, Stony Brook astronomers Walter and L.D. Matthews  reported the optical identification of the star using the Hubble  telescope. The object is very faint (26th magnitude or about 20  billion times fainter than the bright star Vega), and has a blue  color. The blue color indicates that the object is hot, about one  million degrees Fahrenheit, as expected from the bright X-ray  emission. 

In September, images taken with the European Southern Observatory's  Very Large Telescope showed a small, cone-shaped "bowshock" in  front of the neutron star, created as the star plowed through  interstellar space.

The Hubble results have been accepted for publication in the  Astrophysical Journal.

The Space Telescope Science Institute is operated by the  Association of Universities for Research in Astronomy, Inc., for  NASA, under contract with NASA's Goddard Space Flight Center,  Greenbelt, MD. The Hubble Space Telescope is a project of  international cooperation between NASA and the European Space  Agency. 

Image and artwork associated are available at: 
http://oposite.stsci.edu/pubinfo/pr/2000/35