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VStar, Citizen Sky and
the intriguing Epsilon
Aurigae
David Benn
Feb 2010
Outline
• VStar
• multi-platform, open source, easy-touse, variable star visualisation and
analysis tool
• Citizen Sky
• collaboration between amateurs,
professionals, educators to
understand...
• Epsilon Aurigae (more later...)
Citizen Sky
• Under IYA, but continuing beyond
• Focus: current Epsilon Aurigae eclipse
• citizen (US) = volunteer or amateur
• 10-star tutorial, posts, teams, workshops
• http://www.citizensky.org/
Citizen Sky
VStar Team
• AAVSO staff
• myself (lead developer)
• other Citizen Sky participants
• See VStar About Box for credits
• Jan 2010 VStar AAS poster paper
VStar
• Conversation with Arne Henden (AAVSO
Director) at NACAA 2008.
•
Java replacement for Grant Foster’s DOS program.
• Collaboration with AAVSO and Citizen
Sky participants since May 2009.
• Early demo given in Aug 2009 at first
Citizen Sky workshop at Alder
Planetarium, Chicago.
• Frequent releases since Dec 2009;
formal phase 1 release in near future.
Yeah, that’s me hiding
Michael Umbricht
(Brown University, Rhode Island; tester,
domain expert, evangelist)
CSV,
TSV
files
VStar
AAVSO
Internation
al
Database
VSX
(Variable
Star
Index)
VStar
• Demo of
VStar features using these
stars:
• U Scorpii
• Chi Cygni
• W Uma
• Delta Dephei
• (added plot images on subsequent slides
since I gave this talk for people not
present at the ASSA meeting)
Barbara Harris
Not U Sco, but another similar system:
Artist’s rendition of recurrent nova RS Oph
Image credit: David Hardy and PPARC
Shawn Dvorak
https://sites.google.com/site/aavsocvsection/
U Scorpii
Recurrent Nova in outburst
Predicted by Dr Bradley Schaefer
Rapid magnitude rise and fall
1999
2010
1979
1987
All U Sco data annotated with outbursts in
VStar
Jan 28
Feb 2
Jan 27
2010 U Sco outburst
http://en.wikipedia.org/wiki/Chi_Cygni
Mira type pulsating
variable
post-Asymptotic
Giant Branch Star
(late stage red giant)
5th to 13th
magnitude
period: ~407 days
http://www.youtube.com/watch?v=hhMeumI36BM
Chi Cygni
http://www.citizensky.org/forum/w-uma-another-eclipsing-binary-star
W Uma (W Ursae Majoris)
Contact binary star
8 hour period!
VStar’s Future
• Period analysis, time panning (in
database mode), better zoom control,
search, ...
• NACAA 2010 workshop and poster
• Use it! Free Citizen Sky sign up.
• Give feedback.
• Ask for features, fixes!
• It’s open source dude... Develop, test,
document... Volunteers welcome!
Epsilon Aurigae
• Bright (~3rd mag) star in northern
constellation Auriga.
• 11º altitude from SA on Dec 19 2009.
• Variability first noticed by Johann Fritsch,
a German amateur in 1821.
• Distance: 625 parsecs
Epsilon Aurigae (circled) from Adelaide, 20 Dec 2009 at 0030
Capella & The Kids
Epsilon Aurigae
•
Eclipsing binary variable
•
•
•
•
inclination: 89º
orbital separation: 18 to 20 AU
period: 27.1 years
duration: almost 2 years (more on this later)
Current Epsilon Aurigae
eclipse began in August 2009
Minimum: December 2009 (maybe)
Mid-eclipse: August 2010
Minimum light ends: March 2011
Eclipse ends: May 2011
courtesy www.citizensky.org
Epsilon Aurigae
• Harlow Shapley (1928) concluded:
• primary F star being eclipsed by a
companion of similar mass with little
out-of-eclipse spectroscopic signature
• (i.e. an almost unseen companion)
Epsilon Aurigae
• Out-of-eclipse F-star magnitude
variations
• 1984..1987: 96 days
• 2003..2004: sped up to 71 days
• 2007..2008: 65 days
• Out of Eclipse variations make it hard to
determine when totality is reached; not
completely understood.
• “Where’s my totality dude?” (Dr Bob,
Citizen Sky)
Epsilon Aurigae
• Companion changing on timescale of
decades
• 1901..1983
• time of minimum increased from 313 to
445 days
• eclipse duration decreased from 720 to
640 days
Text
Epsilon Aurigae: last
eclipse
Epsilon Aurigae
By Brian Thieme and courtesy www.citizensky.org
By Nico Camargo and courtesy www.citizensky.org
Epsilon Aurigae
• Ludendorff (1912):
• companion: a swarm of meteorites
• Kuiper, Struve, Stromgren (1937):
• Primary F star eclipsed by large semitransparent IR star
• Flat-bottomed light curve caused by
scattering in IR star’s atmosphere.
• But, electron density too low to account
for such scattering.
Epsilon Aurigae
• Schoenberg and Jung (1938)
• Proposed cool companion star.
• Permitted solid particles to cool during
star’s convective process, before
falling, reheating, and breaking apart.
• But, spherical shell of such particles
wouldn’t lead to flattening during
totality.
Epsilon Aurigae
• Kopal (1954)
• First to propose a flat semi-transparent
disk as F-star’s companion:
• radius: ~6 AU
• opacity: 0.8
• water or light hydrocarbons
• Good agreement with observations.
Epsilon Aurigae
• Huang (1965)
• First analytical model supporting disk
as F-star’s companion.
• Gas block-like structure, physically and
optically thick.
• Model agreed with light curve shape.
• Pointed to possible disk asymmetry.
• Subsequent theories extend or slightly
modify Huang’s.
Epsilon Aurigae
• Cameron (1971)
• Tried to explain why disk exists and is
stable by suggesting a black hole in
the system (paper in Nature).
• No evidence from X-ray studies.
Epsilon Aurigae
• Wilson (1971)
• Simulation of light curve from Huang’s
model.
• Noted that a thick disk produces flat
bottomed light curve during eclipse.
• 1955: mid-eclipse brightening.
• Proposed physically thin, optically thin
disk with central opening.
Epsilon Aurigae
• Eggleton & Pringle (1985)
• Proposed binary star system inside
disk to explain long-term disk stability.
• Suggested two mass models:
• High: F-star is super-giant.
• Low: F-star is more like AGB and
disk is overflow remnant.
Epsilon Aurigae
• Schmidtke (1985)
• Considered possibility of gravitational
lense as cause of ~0.2 magnitude mideclipse brightening.
• Found that even in high-mass case
(Eggleton & Pringle) angle observed
from Earth is not sufficient to explain
mid-eclipse brightening.
Epsilon Aurigae
• Kemp & Henson (1983, 1986)
• Out-of-eclipse variation analysis
(Henson)
• evidence of F-star non-radial
pulsations
• In-eclipse variation analysis (Kemp)
• disk tilted with respect to orbit
• orbit crosses F-star just above star’s
middle.
Epsilon Aurigae
• Ferluga (1990)
• Tweaked Huang’s model by proposing
that disk is series of rings with Cassinilike division to explain mid-eclipse
brightening.
• Doesn’t explain older observations.
• Instead, disk may be undergoing rapid
changes. Investigation required.
Epsilon Aurigae
• Hoard et al (2010 AAS)
• Spitzer space telescope observations.
• F-star “only” 2 to 3 solar masses
• still nearly 300 suns across
• so, actually a post-Asymptotic Giant
Branch star
• Companion
• Middle-B dwarf star, 15,000K, 5.9
solar masses.
http://www.nicolascretton.ch/Astronomy/images/HR_post_MS_sun_track.jpg
Epsilon Aurigae
• Hoard et al (2010 AAS)
• Disk:
• < 1 solar mass
• 550K
• 8 AU x 0.5 AU
• Better handle on physical characteristics,
but questions remain.
http://www.skyandtelescope.com/news/80730537.html
In this artist's concept, Epsilon Aurigae (the supergiant
star at right) is starting to be eclipsed by the dust disk
circling a single, much dimmer B star. A new model
explains the decades-old paradoxes of this system by
assuming that its stars are relatively old, not young.
NASA / JPL-Caltech
Epsilon Aurigae
• Some open questions:
• What is the source of the out of
eclipse variations? F-Star? Disk?
• What is the disk made of and exactly
how massive?
• What is the interface between inner
part of disk and the B star.
Epsilon Aurigae:
Summary
• “The history of Epsilon Aurigae is
basically the history of modern
astrophysics” (Brian Kloppenborg).
• Many of the history slides above taken
from Brian’s Citizen Sky disk history
article (see references).
• A rare glimpse into early system
formation.
• Small variations during totality can be
used to map out disk structure.
VStar Links
• http://dbenn.wordpress.com/2009/08/21/
citizensky-workshop-and-vstar-software/
• https://sourceforge.net/projects/vstar/
• http://dbenn.wordpress.com/2009/12/31/
vstar-development-update/
• http://www.citizensky.org/forum/citizensky-or-eps-aur-posters-presentedjanuary-2010-aas-meeting
Other Tools
• http://www.aavso.org/data/software/
• http://www.assa.org.au/sig/variables/soft
ware.asp
• http://www.peranso.com/
Epsilon Aurigae References
• Hopkins & Stencel, 2008, Epsilon
Aurigae
• http://www.aavso.org/vstar/vsots/eps_aur
.shtml
• http://www.citizensky.org/forum/historyand-evolution-disk-theory-epsilonaurigae
• http://www.citizensky.org/forum/unanswe
red-questions-one-step-journeythousand-steps
• http://www.citizensky.org/ (in general)
Epsilon Aurigae References
• See also
http://www.spacedaily.com/reports/Close
_Up_Photos_Of_Dying_Star_Show_Our
_Sun_Fate_999.html
• http://media.skyandtelescope.com/docu
ments/MysteryOfEpsilonAur.pdf
• http://www.skyandtelescope.com/news/8
0730537.html
• http://www.springerlink.com/content/l7vr2
674168p3760/
U Sco References
• http://www.skyandtelescope.com/news/
83025892.html
• http://simostronomy.blogspot.com/
• https://sites.google.com/site/aavsocvse
ction/