Transcript What is Epsilon Aurigae?

Steve B. Howell (NOAO)
Don Hoard (Spitzer Science Center
Bob Stencel (U. of Denver)
What is Epsilon Aurigae?
Epsilon (ε)
Capella
Eta (η)
Zeta (ζ)
-Bright naked eye object. Presence of eclipses first
reported in 1821.
-Eclipses last almost 2 years, and happen
every 27.1 years (mid-eclipse ~5 Aug 2010)
Beta (β)
-What is the “invisible” object that causes the eclipses?
Theta (θ)
-Why do the eclipses last so long and change over
time?
-What is the nature of the primary (i.e., eclipsed) star,
the secondary star, and the disk?
Image credit: Babak Tafreshi (TWAN) [Astronomy Picture of the Day, 5 Dec 2009]
What we knew at the start of the current eclipse
The Eclipsed Star:
• F spectral type, temperature 7800 K
• Very large (radius ~150 Rsun)
• Is it a massive supergiant (20 Msun)?
 Supernova in the future?
• Or a low mass star (up to a few Msun)?
 post-Asymptotic Giant Branch object?
= Dying star, planetary nebula in the next
few thousand years
What we knew at the start of the current eclipse
The Eclipsing Disk:
• Very large, but how large?
•Radius of ~20 AU if the F star is massive
•Radius of ~5 AU if the F star is low mass
• Very massive?, could it form planets?
• Low mass? , could it form planets?
•Is there a central object(s)?
• Disk Morphology?
 Disk, “donut”, ring, gaps?
 Thick, thin? etc..
What we knew at the start of the current eclipse
The Object at the Center of the Disk:
• Maybe it is Nothing?
Only if the disk is very massive (but then the
disk would be too hot)
•Maybe a Black hole?
Nope (no X-ray emission)
• A massive star? Two somewhat less massive stars?
 Only required if the F star is massive
 Too bright (in the UV)?
• A single, normal, B-type star?
 Only if the disk and F star are not massive
Things are not always as they appear…
+
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SED to the Rescue
Optical spectra
(1982; 1990-92)
UBVRI
(2008)
JHK (1997-2000)
IUE-LWP
(1986)
IUE-SWP
(1985)
HST-GHRS
(1996)
FUSE (2001)
Spitzer IRAC
(2009)
Spitzer IRS (2005)
Spitzer MIPS-24, -70
and MIPS-SED
(2005)
Investigate the Dust Disk
Epsilon Aurigae was estimated to…
…exceed IRAC saturation limit for shortest full array
exposure (2-sec) by factor of ~50
…exceed IRAC saturation limit for shortest sub-array
exposure (0.02-sec) by factor of ~3
IMPOSSIBLE to observe
with IRAC!!!
IRS
MIPS
IRAC
Well, so they thought!!
What an image of a star REALLY looks like…
IRAC Observing Strategy
Take advantage of
reduced sensitivity at pixel
corners.
Spread brightest part of
remaining stellar image
over four pixels.
First Spitzer IRAC Observation of
Epsilon Aurigae
•26 April 2009
•used Channel 1 (3.6 microns)
and Channel 2 (4.5 microns)
•256 x 64 exposures,
each 0.02 seconds long
= 5.1 seconds total per channel
Spectral Energy Distribution of Epsilon Aurigae
Ultraviolet
Infrared
F0 type post-AGB star
Cool dust disk
normal B5 type star
B Star - Hubble Space Telescope 1 Sept, Dec, & Mar 2011
Spectral Energy Distribution of Epsilon Aurigae
Synthesized images from the 2009 observations.
B Kloppenborg et al. Nature 464, 870-872 (2010) doi:10.1038/nature08968
B5 star
T = 15,000 K
M = 5.9 Msun
F0 post-AGB
T = 7750 K
M = 1-3 Msun
R = 135 Rsun
= 0.63 AU
R = 3.9 Rsun
R = 3.8 AU
Dust Disk
T = 550 K
M ~ Mearth?
h = 0.95 AU
1 Astronomical Unit (AU) = Sun-Earth separation
Stellar Separation = 18 AU (~ Sun-Uranus distance)
The changing view of the disk
pre-eclipse view
Lissauer et al. 1996, ApJ, 465, 371
The changing view of the disk - Eclipse
JHKs
(2MASS; 1999)
ground LM (white squares;
1997-2000)
MSX (unfilled diamonds;
1996-97)
T = 550 K disk
The changing view of the disk
mid-cycle view
pre-eclipse view
Lissauer et al. 1996, ApJ, 465, 371
The changing view of the disk – Anti-Eclipse
“Front” Side
T = 1100 K disk
END