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Epsilon Aurigae
Mystery and Opportunity
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How Do You Say Epsilon
Aurigae?
Ep’ si lon Au ry’ gee
Where is Epsilon Aurigae?
Courtesy Jerry Lodriguss www.astropix.com
Eclipsing Binaries
Eclipsing binary light curves
• A brightness versus time plot for a variable star is
known as light curve. For close binary systems, time
is usually expressed as phase, for which one unit of
time is the orbital period.
• Measured light curves for periodic variable stars are
usually "folded", which means that successive cycles
are plotted atop one another.
• The shape of the light curve for an eclipsing binary
star system depends mostly on the relative
brightness and size of the two stars as well as their
orbital inclination as seen from Earth.
Eclipsing binary light curves
What can we learn from
eclipsing binaries?
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p = Period
i = Orbital Inclination
M1, M2 = Masses of the Stars
L1, L2 = Luminosities of the Stars
R1, R2 = Radii of the Stars
Epsilon Aurigae
The History of the Mystery
• Johann Fritsch was the first to note the variability of
epsilon Aurigae in early 1821, when the star was
likely in the midst of a deep eclipse.
• The German astronomers Argelander and Heis both
began "regular" observing once every few years
around 1842-1843, and the data from both men
showed that the star became significantly fainter
around 1847.
• Observers later in the 19th Century recorded another
dimming event in 1874-1875, and another in 19011902.
Massive Mystery
• Although they didn't know it at the time, what they had
observed was an extremely long-period eclipsing binary,
and one that was interacting as well.
• In 1928, Harlow Shapley correctly concluded the
secondary was as massive as the primary, an F0
supergiant star. The companion should be as bright as
the primary.
• But the spectrum of the system showed no light from the
companion at all.
• The primary was being eclipsed by a massive, invisible
secondary!
Pieces to the puzzle
• A 1937 paper by three of the greats of
observational astronomy, Gerard Kuiper, Otto
Struve, and Bengt Strömgren, suggested the
system was an eclipsing binary composed of an
F2 star and an extremely cool and tenuous star
that they described as "semitransparent".
• According to this model, the F star was being
eclipsed by this ‘transparent shell star’, and its
light was scattered by the extremely thin
atmosphere of the eclipsing star.
Observation leads to new
ideas
• A 1965 paper by Su-Shu Huang
introduced the suggestion of an edge-on
thick disk as the eclipsing body.
• In 1971, Robert Wilson introduced a tilted,
thin disk with a central opening,
suggesting that this model could most
easily describe all of the observed effects
of the eclipses, particularly the mid-eclipse
re-brightening.
Illustration by Brian Thieme
Moving Target
• There is a slight brightening during mid-eclipse,
suggesting the disk has a hole in it which the F
star shines through.
• The central brightening was stronger in 1954-56
than in earlier eclipses. Was the hole growing?
• The time of minimum light lengthened by about
64 days.
• The overall duration of the eclipse had
decreased by 44 days!
The last eclipse
• During the 1982-84 eclipse the central
brightening was the brightest ever.
• The duration of minimum was the longest,
and the fading and brightening happened
fastest.
• The F star’s companion is changing on
timescales of decades.
• From 1901 to 1983 the time of minimum has
increased from 313 to 445 days.
• The overall eclipse duration has declined
from 727 to 640 days.
Changing eclipse profiles
As if that weren’t enough…
• Precise measurements out of eclipse
revealed a quasi-periodic low amplitude
variation of 96 days from 1984-87.
• During the 2003-2004 observing season
this variation had sped up to 71 days.
• In 2007-2008 the period became 65
days.
The scale of the system is astronomical!
The primary is 300 times the diameter of the Sun.
The secondary orbits at the distance of Uranus from
the Sun. Both components are 14-15 solar masses.
The Center of Controversy
• What is the nature of the object or
objects at the center of the disk?
• It could be two B type stars in a tight
orbit. This would account for the mass.
• A pair of stars would act as a
gravitational eggbeater, keeping the
center clear.
Is there a giant planet
involved?
• One or more proto-hot-Jupiters would
affect the distribution of matter in the
disk.
• A hot Jupiter spiraling inward to meet its
death might account for the low
amplitude variations and their
decreasing periodicity.
IYA 2009
• International Year of
Astronomy
• Celebrating the
400th anniversary of
Galileo turning a
telescope to the
heavens
• AAVSO Citizen
Science ProjectCitizen Sky Project
Citizen Science
What are citizen scientists?
• Volunteers, many of whom have no prior
scientific training, who work with trained
scientific researchers to answer real-world
questions.
• This means YOU!
In August 2009 the next eclipse will
begin!
AAVSO Citizen Sky Project
www.citizensky.org
• We need you to help us collect data so that we
can better understand this star.
• Because the star is very bright, it can be
observed by anyone regardless of background,
training, or equipment.
• With just good eyesight and a finder chart, you
can monitor this eclipse.
We will guide you through the process.
• How to observe the star- (Ten Star Training
program for beginners)
• How to send us your observations of the star
• How to see your results and analyze them
• And even publish them in a scientific journal!
• We hope that this will be the largest citizen
science project in modern history that involves
real, active research!
Workshops
• A 3-day workshop, focused on observing and
education/public outreach, will be held at the
Adler Planetarium in Chicago, August 5-7, 2009.
The workshop will occur just days before the
eclipse of epsilon Aurigae is predicted to begin.
• Video of talks at the workshops will be placed
online and available via DVD upon request.
• It is very likely that we will have travel grants
available for workshop participants. Stay tuned
to www.citizensky.org for workshop/grant
application instructions.
Visual Observers
• At 3rd magnitude, epsilon Aurigae is bright
enough to be seen from most urban areas. This
makes it an ideal target for those interested in
learning how to observe variable stars.
• No equipment is needed.
• By following the Ten Star Tutorial, a new
observer will be trained in the technique needed
to make and report a variable star estimate.
Data Analysis
• All data collected by observers will be available
to the public as it is submitted.
• AAVSO is developing data analysis software.
• This software will come with tutorials to help
train participants in the basics of astronomical
data analysis.
• A special edition of the peer-reviewed Journal of
the AAVSO will be dedicated to papers written
about epsilon Aurigae by amateur astronomers.
Education and Public
Outreach
• The brightness of the star provides a rare opportunity to
engage the general public in citizen science.
• Participants are needed to help write newsletter and
newspaper articles, prepare talks and slide shows,
develop artwork, to give talks and participate in other
forms of community outreach.
• Teams of interested participants with complementary
skill sets are being assembled right now, and will
continue to be assembled throughout the project.
What are the science
questions?
• Is the F star a massive supergiant or an early
post-asymptotic giant branch star?
• What is at the center of the eclipsing disk?
• Is the disk tilted or warped?
• How massive is the disk?
Time Line
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Eclipse begins Aug. 11, 2009
Minimum light begins Dec. 19, 2009
Mid-eclipse Aug. 4, 2010
Minimum light ends Mar.19, 2011
Eclipse ends May 13, 2011
All dates are approximate
You can follow updates on
Twitter
http://twitter.com/epsilon_Aurigae
www.citizensky.org