Jan. 12, 2001

Scientists zero in on rapid stars



Astronomers have found 154 rapidly moving stars toward the center of our galaxy and our brightest neighboring galaxy. The findings were presented Monday by Lab researcher Andrew J. Drake for the Massive Compact Halo Objects (MACHO) collaboration, during the annual meeting of the American Astronomical Society in San Diego.

The results are of special interest because this is the first time scientists have been able to discover such objects in front of the millions of stars seen at the galactic center and our brightest neighbor galaxy, the Large Magellanic Cloud (LMC).

To date, among the thousands of known High Proper Motion (HPM) stars, few have been discovered in the most densely packed regions of the sky, where stars appear to merge together in images because of their extreme density.

"Until now astronomers have been unable to detect HPM stars in the most dense locations because of the extreme density of stars toward the galactic center," said Drake, who works at the Institute for Geophysics and Planetary Physics. "Toward the galactic center, the billions of stars within our galaxy form the bright band in the sky known as the Milky Way."

Another region where the density of stars makes discovery of the moving ones difficult is toward the LMC. To the naked eye, this galaxy appears as a faint nebulous patch in the southern sky. Through a small telescope, the presence of millions of individual stars become recognizable.

Our solar system resides 26,000 light years from the center of the galaxy and rotates once every 240 million years. The great distance to the galactic center means that the slow rotation of the sun has little effect on stars there. However, much closer stars (less than 500 light years) appear to move relative to these distant stars. In order to find HPM stars, Drake looked at images of stars in the galactic center and the LMC taken over seven years.

Using 50,000 astronomical images of 55 million stars, Drake identified the stars that appear to move and measured their motions. From these measurements, he discovered 154 new HPM stars. The yearly motions of these objects are estimated to be accurate to 6 milli-arcseconds, which is equivalent to the width of a human hair seen from a distance of one mile.

These images came from a recent galactic dark matter experiment using the 50-inch Great Melbourne Telescope in Canberra, Australia. During the 1990s, scientists also used the Great Melbourne Telescope to detect MACHOs (Massive Compact Halo Objects) through the gravitational microlensing of stars. Microlensing is a physical phenomenon that causes stars to appear to shift or brighten when two or more of them lie on the same line of sight.

Over the years, techniques such as astrometry have allowed astronomers to produce a picture of the motions of stars within our galaxy. Astrometry is the branch of astronomy that deals with the measurement of positions and movements. Applying this picture to the motions of the HPM stars discovered, Drake was able to predict that most of these objects likely are located at distances between 100 and 1,000 light years.

However, at present, the motions of these newly discovered HPM stars have been based on the motion measured between just two images. More detailed studies of these stars are necessary to determine how the parallax effect, due to the Earth’s motion around the sun, would change the true direction of each HPM star’s motion from that observed.

Although many microlensing events have been discovered, astronomers continue to search for them because they can point out properties of the lensing objects, such as planets, that populate our galaxy. Within the next 10 years, NASA’s Space Interferometry Mission (SIM) telescope will be launched into orbit. One of the goals of this mission is to use astrometry to determine the masses and distances of the stars causing microlensing events. By finding the HPM stars in the foreground of these dense areas of the sky and predicting their paths over future years, astronomers will be able determine when these stars will pass in front of a distant star to cause microlensing.

The MACHO collaboration consists of scientists from the Lab (Kem Cook, Andrew Drake, Stuart Marshall, Cailin Nelson and Piotr Popowski); the University of Pennsylvania; the Australian National Supercomputing Facility; STScI; the Mount Stromlo Observatory; Bell Labs; University of Notre Dame; UC Santa Cruz; UC San Diego; Universidad Catolica; University of Washington; the European Southern Observatory; the University of Oxford; and McMaster University.