Harvard Archives Show "Winking Star" Started Winking Only Recently(Forwarded)

Public Affairs Department
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468


Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016


For Release: August 7, 2003

Release No.: 03-19

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 that 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 the 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 eclipses seen today were not
happening several decades ago, meaning that the eclipses now seen are a recent
phenomenon that began within the past few decades -- a remarkably short time by
astronomical standards.

"There are very few cases where astronomers can see a significant change to a
star over a single human lifetime," said Winn. "And if the eclipses are caused
by material in a protoplanetary disk, as suspected, then that would give us the
exciting opportunity to study planet formation on surprisingly 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 than 60 glass
photographic plates containing images of the appropriate region of 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) cameras and digital storage
media. Harvard's Photographic Plate Collection contains a half-million plates
spanning a century of research from the 1880s to 1989, making it both the
largest such archive in the world and an irreplaceable resource for astronomers
seeking to study time-varying celestial phenomena.

Winn said, "Using the Harvard Plate Stacks is like having a time machine. 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, they hunted for
plates where stars of similar brightness could be seen but KH 15D was absent,
indicating that the winking star had dimmed due to an eclipse.

A brighter star close to KH 15D, combined with the intrinsic faintness of the
winking star, made the measurements a challenge. However, the astronomers were
able to identify about 40 photographic plates on which they 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, then
approximately 16 plates (40 percent of the total) would have shown a dim,
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 present
characteristics. Either they were much shorter in duration, or not nearly as
complete, or, they were not happening at all," said Krzysztof Stanek (CfA),
co-author of the paper announcing their findings.

Co-author Peter Garnavich (University of Notre Dame) added, "Our most recent
observations show that the length of the eclipse is evolving rapidly. In a 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 formation theories predict that
KH 15D may still be surrounded by a disk of dust and gas left over from its
birth. That disk, known as a protoplanetary disk, is a possible source of the
eclipses.

A nearly 3-week eclipse is difficult to explain by invoking an intervening
planet or companion star due to the length of the eclipse -- the star's face is
totally hidden from our view almost half the time. The most plausible cause is a
wide swath of disk material sliding in front of the star, thereby blocking most
of the star's light.

One possibility is that this swath is actually a "ripple" in the protoplanetary
disk, 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 the 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 the KH 15D eclipses.

"Still, there is no clear theoretical explanation for the eclipses," said
co-author Dimitar Sasselov (CfA). "Radial velocity measurements can rule out the
intervening high-mass companion that has been suggested by some researchers.
That will leave us with two possibilities -- either the eclipses 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 to
investigate other plate archives for data from the second half of the 20th
century. By studying when and how the eclipses began, they hope to gather
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....abstract.html.

NOTE TO EDITORS: Images associated with this release are available at
http://cfa-www.harvard.edu/press/pr0319image.html

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for
Astrophysics (CfA) 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.

Andrew Yee wrote in message ...
Public Affairs Department
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468


Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016


For Release: August 7, 2003

Release No.: 03-19

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 that 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 the 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 eclipses seen today were not
happening several decades ago, meaning that the eclipses now seen are a recent
phenomenon that began within the past few decades -- a remarkably short time by
astronomical standards.

"There are very few cases where astronomers can see a significant change to a
star over a single human lifetime," said Winn. "And if the eclipses are caused
by material in a protoplanetary disk, as suspected, then that would give us the
exciting opportunity to study planet formation on surprisingly 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 than 60 glass
photographic plates containing images of the appropriate region of 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) cameras and digital storage
media. Harvard's Photographic Plate Collection contains a half-million plates
spanning a century of research from the 1880s to 1989, making it both the
largest such archive in the world and an irreplaceable resource for astronomers
seeking to study time-varying celestial phenomena.

Winn said, "Using the Harvard Plate Stacks is like having a time machine. 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, they hunted for
plates where stars of similar brightness could be seen but KH 15D was absent,
indicating that the winking star had dimmed due to an eclipse.

A brighter star close to KH 15D, combined with the intrinsic faintness of the
winking star, made the measurements a challenge. However, the astronomers were
able to identify about 40 photographic plates on which they 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, then
approximately 16 plates (40 percent of the total) would have shown a dim,
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 present
characteristics. Either they were much shorter in duration, or not nearly as
complete, or, they were not happening at all," said Krzysztof Stanek (CfA),
co-author of the paper announcing their findings.

Co-author Peter Garnavich (University of Notre Dame) added, "Our most recent
observations show that the length of the eclipse is evolving rapidly. In a 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 formation theories predict that
KH 15D may still be surrounded by a disk of dust and gas left over from its
birth. That disk, known as a protoplanetary disk, is a possible source of the
eclipses.

A nearly 3-week eclipse is difficult to explain by invoking an intervening
planet or companion star due to the length of the eclipse -- the star's face is
totally hidden from our view almost half the time. The most plausible cause is a
wide swath of disk material sliding in front of the star, thereby blocking most
of the star's light.

One possibility is that this swath is actually a "ripple" in the protoplanetary
disk, 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 the 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 the KH 15D eclipses.

"Still, there is no clear theoretical explanation for the eclipses," said
co-author Dimitar Sasselov (CfA). "Radial velocity measurements can rule out the
intervening high-mass companion that has been suggested by some researchers.
That will leave us with two possibilities -- either the eclipses 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 to
investigate other plate archives for data from the second half of the 20th
century. By studying when and how the eclipses began, they hope to gather
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....abstract.html.

NOTE TO EDITORS: Images associated with this release are available at
http://cfa-www.harvard.edu/press/pr0319image.html

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for
Astrophysics (CfA) 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.


1) If it is a planet causing the eclipse, isn't it going way too
fast? It would need to be an appreciable ratio of diameter to the star
to block the light, and therefore be in almost a binary situation (is
there an observed wobble?) Must be a dust cloud.

2) Can the protoplanetary disc be rotating on a second axis, causing
the eclipses to be intermittent??
Jim G

Steve Willner wrote:

In article ,
(Jim Greenfield) writes:

1) If it is a planet causing the eclipse, isn't it going way too
fast? It would need to be an appreciable ratio of diameter to the star
to block the light, and therefore be in almost a binary situation (is
there an observed wobble?) Must be a dust cloud.


I think the argument is that since the light curve is near minimum
for nearly half the time, the eclipsing object must occupy that
fraction of its orbit.


2) Can the protoplanetary disc be rotating on a second axis, causing
the eclipses to be intermittent??


You mean precessing? That would require a third body in the system.
I don't know whether there are any limits on whether such a body
could exist. It would be worth looking if nobody has done so yet.


I've wondered why they have not thought in terms of "star spots".
The RS Cvn stars frequently have such large spot groups that nearly
the light curve is "near minimum" for nearly half the time.

John Oliver wrote in message news:v2y_a.10860$qf.6513@lakeread06...
Steve Willner wrote:

In article ,
(Jim Greenfield) writes:

1) If it is a planet causing the eclipse, isn't it going way too
fast? It would need to be an appreciable ratio of diameter to the star
to block the light, and therefore be in almost a binary situation (is
there an observed wobble?) Must be a dust cloud.


I think the argument is that since the light curve is near minimum
for nearly half the time, the eclipsing object must occupy that
fraction of its orbit.


2) Can the protoplanetary disc be rotating on a second axis, causing
the eclipses to be intermittent??


You mean precessing? That would require a third body in the system.
I don't know whether there are any limits on whether such a body
could exist. It would be worth looking if nobody has done so yet.

No. Actually I had not thought of that. What I had in mind was the
simple idea of a disc of material (such as our solar system) also
rotating around a diameter of the plane. This is problematical with
the gyroscopic tendency to resist such rotation (conservation of
angular momentum?)- such a rotation would therefore needs have been
produced by a "hit" from outside the system. Over time, the two spins
would tend back to one (as a gyroscope with an enforced wobble)

Jim G


I've wondered why they have not thought in terms of "star spots".
The RS Cvn stars frequently have such large spot groups that nearly
the light curve is "near minimum" for nearly half the time.

Good thougth. Are star spots so regular in frequency?
Jim G

Jim Greenfield wrote:

John Oliver wrote in message news:v2y_a.10860$qf.6513@lakeread06...

Steve Willner wrote:


In article ,
(Jim Greenfield) writes:


1) If it is a planet causing the eclipse, isn't it going way too
fast? It would need to be an appreciable ratio of diameter to the star
to block the light, and therefore be in almost a binary situation (is
there an observed wobble?) Must be a dust cloud.


I think the argument is that since the light curve is near minimum
for nearly half the time, the eclipsing object must occupy that
fraction of its orbit.



2) Can the protoplanetary disc be rotating on a second axis, causing
the eclipses to be intermittent??


You mean precessing? That would require a third body in the system.
I don't know whether there are any limits on whether such a body
could exist. It would be worth looking if nobody has done so yet.


No. Actually I had not thought of that. What I had in mind was the
simple idea of a disc of material (such as our solar system) also
rotating around a diameter of the plane. This is problematical with
the gyroscopic tendency to resist such rotation (conservation of
angular momentum?)- such a rotation would therefore needs have been
produced by a "hit" from outside the system. Over time, the two spins
would tend back to one (as a gyroscope with an enforced wobble)

Jim G

I've wondered why they have not thought in terms of "star spots".
The RS Cvn stars frequently have such large spot groups that nearly
the light curve is "near minimum" for nearly half the time.


Good thougth. Are star spots so regular in frequency?
Jim G

In the RS Cvn systems a large spot group can persist for years.