Astronomers are announcing today the discovery of
a sun-like star which is eclipsed in a way never before seen – not by
another star, planet or moon, but by dust grains, rocks and maybe
even asteroids orbiting it in a clumpy circumstellar disk.

The international team making the observation was led by William
Herbst and Catrina Hamilton of Wesleyan University in Middletown, CT.
The results are being presented to the Scientific Frontiers on
Research in Exo-Solar Planets meeting sponsored by NASA and the
Carnegie Institution of Washington in Washington, D.C.

This discovery is enabling first time study of the detailed structure
of a disk and to see the evolution of features on time scales of
months and years. It is believed that disks such as this formed Earth
and our solar system. Scientists hope that this discovery will shed
new light on our origins.

The star, named KH 15D, is in the constellation of Monoceros and
located about 2400 light years from the Earth. It is part of a well
known cluster of young stars called NGC 2264 and inhabits a nebulous
region of space close to the famous “Cone Nebula” (recently imaged in
spectacular fashion with the new Advanced Camera System on the Hubble
Space Telescope.) Such regions are known to be the birthplaces of
stars and KH 15D has all the markings of youth. It is estimated to be
about 3 million years old, qualifying it as a cosmic toddler.

Attention was drawn to the star in 1997 by its discoverers, Kristin
Kearns, then a graduate
student at Wesleyan and Herbst. It was star number 15 in an image
which they designated the “D” field, hence the name. “If we knew it
was going to become famous, we would have given it a better name”,
Herbst now laments.

Observations, mostly by undergraduate students, at Wesleyan’s Van
Vleck Observatory during the late 1990’s led Kearns and Herbst to
realize that this was a potentially unique and important object.
“Basically, the star winked at us”, reports Herbst. On most nights it
was at its standard brightness but sometimes it would be nearly gone
– shining by only a tiny fraction of its normal luminosity. After
several years of study, the pair recognized a pattern to the star’s
behavior – it fades out every 48.3 days and stays faint for about 18
days. The strict repetitiveness and other characteristics led to the
realization that something was orbiting the star and blocking its
light on a regular timetable. This is not uncommon in astronomy –
there are many known examples of eclipsing binary stars. What is
uncommon – unique, actually – in the case of KH 15D is the length of
the eclipse as well as its depth. The star was essentially totally
blocked for more than 1/3 of the period of the orbiting matter. No
single object such as a star, planet or moon could do such a thing,
since it would require an object much too large to fit in the space
available. Only a collection of smaller objects – dust grains, rocks
or perhaps asteroids, orbiting together in a strung out, clumpy arc,
could possibly explain such a lengthy eclipse.

To examine this unprecedented phenomenon in greater detail, Herbst
and Wesleyan Physics graduate student Catrina Hamilton, who is also a
senior lecturer at Connecticut College in New London, CT, organized
an international observing campaign during the fall, winter and
spring of 2001/2002. The goal was to keep an eye on this amazing star
for as much of the time as was practical. Astronomers from
Uzbekistan, Germany, Israel, and at several universities in the
United States took part in the observations.

The extensive data set obtained in the past year has confirmed the
basic pattern seen previously and provided tantalizing new facts for
astronomers to study.

Contact Information:

Dr. William Herbst, John Monroe Van Vleck Professor of Astronomy,
Wesleyan University, Middletown, CT 06459 860-685-3672

Prof. Herbst can be reached at the conference in Washington through
the Extrasolar Planets Conference Press Room, telephone numbers 202
939-1123 and 939-1139.

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