Taming the Invisible Monster: The Spectral Energy Distribution of Epsilon Aurigae

1. Taming the Invisible Monster: The Spectral Energy Distribution of Epsilon Aurigae (From the Far-Ultraviolet to the Far-Infrared) Steve B. Howell (NOAO) Don Hoard (Spitzer Science Center Bob Stencel (U. of Denver)

2. 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? Epsilon (ε) Zeta (ζ) Eta (η) Capella Beta (β) Theta (θ) Image credit: Babak Tafreshi (TWAN) [Astronomy Picture of the Day, 5 Dec 2009]

3. 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

4. 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..

5. 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

6. Things are not always as they appear… +  + 

7. SED to the Rescue

8. Optical spectra (1982;1990-92) UBVRI (2008) JHK (1997-2000) Spitzer IRAC (2009) IUE-LWP (1986) Spitzer IRS (2005) IUE-SWP (1985) Spitzer MIPS-24, -70 and MIPS-SED (2005) HST-GHRS (1996) FUSE (2001)

9. Investigate the Dust Disk

10. 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!! IRS MIPS IRAC

11. What an image of a star REALLYlooks like…

12. IRAC Observing Strategy Take advantage of reduced sensitivity at pixel corners. Spread brightest part of remaining stellar image over four pixels.

13. 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

14. 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

15. Spectral Energy Distribution of Epsilon Aurigae

16. Synthesized images from the 2009 observations. B Kloppenborg et al.Nature464, 870-872 (2010) doi:10.1038/nature08968

17. 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)

18. The changing view of the disk pre-eclipse view Lissauer et al. 1996, ApJ, 465, 371

19. 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

20. The changing view of the disk mid-cycle view pre-eclipse view Lissauer et al. 1996, ApJ, 465, 371

21. The changing view of the disk – Anti-Eclipse “Front” Side T = 1100 K disk

22. END