Cambridge, MA — Since its discovery in 1998, the “winking star” called KH
15D has baffled astronomers seeking to explain its long-lasting (24-day)
eclipses. Many hypothesized that the eclipses were caused by intervening
blobs of material within a protoplanetary disk surrounding a single, young
Sun-like star.

By examining the past history of these eclipses and how they are changing
with time, astronomer Joshua Winn (Harvard-Smithsonian Center for
Astrophysics) and colleagues have overturned this hypothesis and devised a
new theory that explains nearly everything about the system.

They found that the “winking star” is actually a double star system.
Something in the foreground, possibly a dusty disk of material surrounding
the binary, intermittently blocks the light from one or both stars, as the
stars orbit each other. Eventually, both stars will be completely covered by
the dust curtain, and the “winking star” system will disappear from view.

“These two stars have been playing hide and seek with us. The second star
used to peek out briefly, but now is completely obscured. Soon, it will be
joined by the first star and both will remain hidden for decades,” says
Winn.

** Archives Reveal The Truth **

The vital clues to understanding the “winking star” were found in archival
sky photographs from Harvard College Observatory, in Massachusetts, and
Asiago Observatory, in Italy. Examination of the Harvard photographs showed
that during the first half of the 20th century, there were none of the total
eclipses that are observed today. Asiago photographs taken between 1967 and
1982 held evidence of eclipses, but with a key difference: the system was
brighter than it is today, both during eclipses and outside of eclipses.
This extra light must have come from a second star that was visible in the
1970s, but is completely hidden today.

This insight was the key to unlocking the mystery of KH 15D. Before 1960,
neither star was being eclipsed. Then, a curtain of dust drifted into the
foreground as seen from the Earth, blocking part of the orbit of one of the
stars. Throughout the 1970s, that star underwent eclipses as its orbital
motion carried it behind the curtain. By 1998, the curtain had advanced
enough to completely hide one of the stars-and the other star periodically
drops out of sight as its orbit takes it behind the curtain. By about 2012,
both stars will be completely hidden from view.

Radial velocity measurements currently being made by John Johnson (UC
Berkeley), a co-author of this study, will be able to test whether the
visible star is moving back and forth, tugged by the gravity of a
stellar-mass companion.

“The Asiago plates give very convincing evidence, but the radial velocity
measurements will be the clincher,” says Johnson.

** The New Picture of KH 15D **

Assembling the observations of KH 15D like pieces of a jigsaw puzzle reveals
two stars no older than 10 million years. (Our Sun, by contrast, is 5
billion years old.) They revolve around each other every 48 days in highly
elliptical orbits, which explains the 48-day eclipse period. Their average
distance apart is approximately 0.25 astronomical units (23 million miles),
or two-thirds the distance from Mercury to the Sun. Yet their eccentric
orbits take them as close to each other as only 0.07 AU (6.5 million miles).

“As binaries go, their orbit is not unusual” says co-author Krzysztof Stanek
(CfA).

Winn agrees, adding, “The weird thing about this system is that there’s
something blocking the light from these stars-something opaque, with a sharp
edge.” The identity of this curtain is unknown, but it may be the edge of a
disk of dust that surrounds both of the stars.

“Dust disks have been seen around other binary star systems,” says Matthew
Holman (CfA), a co-author of the study. “We imagine that the disk in this
system is inclined, relative to the plane of the orbit of the two stars.
That would cause the disk to wobble, the way a Frisbee sometimes wobbles in
the air after a bad throw.”

According to Holman’s calculations, the dust may exist in a ring located 2.6
AU (240 million miles) from the stars. The material in the ring itself makes
one complete orbit about every 4 years, but the wobbling (or “precession”)
of the ring has a much longer period of about 1000 years. A similar theory
has been proposed independently by Eugene Chiang and Ruth Murray-Clay of UC
Berkeley.

“Starting around 1960, the edge of this precessing disk happened to start
blocking our view of the stars,” says Holman. “After another decade, the
disk will precess a little further and completely block our view.” Some time
after that, depending on how thick the ring is, the process will reverse
itself as the stars are gradually uncovered, and the eclipses will stop.

Many questions about KH 15D still remain. For example, what is the nature of
the disk? Why is it inclined to the orbital plane of the binaries? Why does
it have such a sharp edge? The winking stars of KH 15D are likely to
confound astronomers with these and other riddles for years to come.

This research will be published in the March 1, 2004 issue of The
Astrophysical Journal Letters. The authors of the study are Joshua Winn
(CfA), Matthew Holman (CfA), John Johnson (UC Berkeley), Krzysztof Stanek
(CfA), and Peter Garnavich (University of Notre Dame).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists, organized
into six research divisions, study the origin, evolution and ultimate fate
of the universe.