Steve B. Howell (NOAO)Don Hoard (Spitzer Science Center

Bob Stencel (U. of Denver)

Image credit: Babak Tafreshi (TWAN) [Astronomy Picture of the Day, 5 Dec 2009]

What is Epsilon Aurigae?

-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)

-What is the “invisible” object that causes the eclipses?

-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?

Capella Eta (η)Zeta (ζ)

Epsilon (ε)

Beta (β)

Theta (θ)

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 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

What we knew at the start of the current eclipse

+

+

Things are not always as they appear…

SED to the Rescue

IUE-SWP(1985)

FUSE (2001)

UBVRI(2008)

JHK (1997-2000)

Spitzer IRS (2005)

Spitzer MIPS-24, -70and MIPS-SED

(2005)HST-GHRS(1996)

Spitzer IRAC(2009)

IUE-LWP(1986)

Optical spectra(1982; 1990-92)

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!!!

Well, so they thought!!

IRACMIPS

IRS

What an image of a star REALLY looks like…

Take advantage of reduced sensitivity at pixel corners.

Spread brightest part of remaining stellar imageover four pixels.

IRAC Observing Strategy

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 AurigaeUltraviolet Infrared

F0 type post-AGB star

normal B5 type star

Cool dust disk

B Star - Hubble Space Telescope 1 Sept, Dec, & Mar 2011

Spectral Energy Distribution of Epsilon Aurigae

B Kloppenborg et al. Nature 464, 870-872 (2010) doi:10.1038/nature08968

Synthesized images from the 2009 observations.

R = 135 Rsun = 0.63 AU

R = 3.8 AU

h = 0.95 AU

R = 3.9 Rsun

B5 starT = 15,000 KM = 5.9 Msun

F0 post-AGBT = 7750 KM = 1-3 Msun

Dust DiskT = 550 KM ~ Mearth?

1 Astronomical Unit (AU) = Sun-Earth separation

Stellar Separation = 18 AU (~ Sun-Uranus distance)

Lissauer et al. 1996, ApJ, 465, 371

The changing view of the disk

pre-eclipse view

JHKs(2MASS; 1999)

MSX (unfilled diamonds;1996-97)

ground LM (white squares; 1997-2000)

The changing view of the disk - Eclipse

T = 550 K disk

Lissauer et al. 1996, ApJ, 465, 371

The changing view of the disk mid-cycle view

pre-eclipse view

T = 1100 K disk

The changing view of the disk – Anti-Eclipse

“Front” Side

END