CfA Press Release
Release No.: 03-19|
For Release: August 7, 2003
Harvard Archives Show "Winking Star" Started Winking Only Recently
Last year, astronomers at Wesleyan University announced that they had
discovered a "winking" star - a star known as KH 15D that undergoes a
regular, long-lasting (~20 day) eclipse every 48 days. They theorized
those eclipses were caused by intervening blobs of material within a
protoplanetary disk surrounding that young star.
Spurred on by those findings, Harvard astronomer Joshua Winn
(Harvard-Smithsonian Center for Astrophysics) and colleagues decided to
examine the past behavior of KH 15D using sky photographs taken during
first half of the 20th century and stored in the Harvard archives. They
found that the winking star used to not wink. The nearly complete
seen today were not happening several decades ago, meaning that the
now seen are a recent phenomenon that began within the past few decades -
remarkably short time by astronomical standards.
"There are very few cases where astronomers can see a significant change
a star over a single human lifetime," said Winn. "And if the eclipses
caused by material in a protoplanetary disk, as suspected, then that
give us the exciting opportunity to study planet formation on
short time scales."
A Valuable Archival Resource
To probe the eclipse history of KH 15D, Winn and colleagues contacted
Harvard Plate Stacks acting curator Alison Doane, who identified more
60 glass photographic plates containing images of the appropriate region
space. All of the plates were taken between 1913 and 1955, a time long
before the advent of today's widely used charge-coupled device (CCD)
and digital storage media. Harvard's Photographic Plate Collection
a half-million plates spanning a century of research from the 1880s to
making it both the largest such archive in the world and an
resource for astronomers seeking to study time-varying celestial
Winn said, "Using the Harvard Plate Stacks is like having a time
After an exciting object like KH 15D is discovered, you can go into the
stacks and observe it as it was nearly 100 years ago."
Winn and colleagues examined the plates identified by Doane to look for
evidence that the brightness of KH 15D changed over time. Specifically,
hunted for plates where stars of similar brightness could be seen but KH
was absent, indicating that the winking star had dimmed due to an
A brighter star close to KH 15D, combined with the intrinsic faintness
the winking star, made the measurements a challenge. However, the
astronomers were able to identify about 40 photographic plates on which
could measure KH 15D with sufficient accuracy to detect a deep eclipse.
If the star's eclipses took place in the past just as they do today,
approximately 16 plates (40 percent of the total) would have shown a
eclipsed star. Instead, the astronomers found that none of the plates
definitively showed an eclipse.
"Statistically, we showed that it's extremely unlikely that the eclipses
were taking place in the early 20th century with anything like their
characteristics. Either they were much shorter in duration, or not nearly
complete, or, they were not happening at all," said Krzysztof Stanek
co-author of the paper announcing their findings.
Co-author Peter Garnavich (University of Notre Dame) added, "Our most
observations show that the length of the eclipse is evolving rapidly. In
few years, this strange star will spend more time faint than bright."
Clues To Planet Formation
Located about 2,400 light-years away in the constellation Monoceros, the star KH 15D is very much like our Sun, except that it is only a few
million years old versus the Sun's age of 4.6 billion years. Star
theories predict that KH 15D may still be surrounded by a disk of dust
gas left over from its birth. That disk, known as a protoplanetary disk,
a possible source of the eclipses.
A nearly 3-week eclipse is difficult to explain by invoking an
planet or companion star due to the length of the eclipse - the star's
is totally hidden from our view almost half the time. The most plausible cause is
wide swath of disk material sliding in front of the star, thereby
most of the star's light.
One possibility is that this swath is actually a "ripple" in the protoplanetary
recently stirred up by the gravitational influence of an embedded
protoplanet. A Jupiter-sized protoplanet orbiting some 0.2 astronomical
units from the star could create such a ripple. (An astronomical unit is
average distance between the Earth and Sun.) Moreover, the ripple would
evolve on a timescale of 10 to 100 years. This makes protoplanet/disk
interactions an appealing explanation for the existence and evolution of
KH 15D eclipses.
"Still, there is no clear theoretical explanation for the eclipses,"
co-author Dimitar Sasselov (CfA). "Radial velocity measurements can rule
the intervening high-mass companion that has been suggested by some
researchers. That will leave us with two possibilities - either the
are caused by a ripple in a protoplanetary disk, or they are caused by
something we haven't even thought of yet!"
Winn and colleagues now plan to collaborate with additional astronomers
investigate other plate archives for data from the second half of the
century. By studying when and how the eclipses began, they hope to
additional clues to their cause.
This research will be published in the August 20, 2003 issue of The
Astrophysical Journal Letters and is available online at http://www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v593n2/17497/brief/17497.abstract.html.
NOTE TO EDITORS: Images associated with this release are available at: http://www.cfa.harvard.edu/news/archive/pr0319image.html
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