Spitzer Space Telescope Observations of the Fomalhaut Debris Disk Michael Werner, Karl Stapelfeldt, Chas Beichman (JPL); Kate Su, George Rieke, John Stansberry, & James Cadien (Arizona), Dan Watson, K. H. Kim (Univ. of Rochester), Dean Hines (SSI); Michael Jura (UCLA); Massimo Marengo, Tom Megeath (CfA)

Background on Fomalhaut disk A3 V star, distance= 8 pc Disk resolved with IRAS & KAO (Gillett et al. 1986; Harvey et al. 1996) Scattered light undetected until very recently Very nice submm detection of edge-on ring by Holland et al – 110 AU ring radius; slightly asymmetric to the SE First debris disk science target for Spitzer, November 2003

Fomalhaut MIPS 24  m Left: Reference star image Center: Fomalhaut direct image Right: Dust disk revealed by PSF subtraction – Kurucz photosphere model fit determines scale factor – About 80% of 24 micron excess from unresolved core 160” FOV (Stapelfeldt et al. 2004)

Spitzer/IRS High resolution spectrum New ! IRS SH+LH from improved pipelines (Stapelfeldt et al. 2006) photosphere

Fomalhaut thermal continuum emission JCMT/ SCUBA 450 μm (Holland 2003) MIPS 24 μm (PSF-subtracted) MIPS 70 μm MIPS 160 μm JCMT/SCUBA 850 μm (Holland 1998) CSO / Sharc II 350 μm (Marsh 2005)

Spitzer Fomalhaut Results Summary ●No obvious spectral features detected  grainsizes > 5  m ●Disk outer radius (20″= 150 AU) is almost the same in all three MIPS bands, and in the submillimeter (Holland 2003) ●There is a warm disk component inside the submm ring: –Most of 24 μm excess is in compact central core, radius < 20 AU – New IRS results indicate excess at < 15  m (dust in to r~ 4 AU !) – To have gone undetected in the submm, this warm inner dust must have a low optical depth or emissivity (< 10% of the outer dust ring). Tenuous inner dust cloud. ●Asymmetric disk is detected in all three MIPS bands –SE ansa always brighter than NW ansa; difference greater at short wavelengths: 50%, 30%, 10% at 24, 70, and 160 μm respectively –JCMT maps suggested 10% asymmetry at 450 microns –What is the origin of this feature ?

Explaining Fomalhaut’s Disk Asymmetry ●Recent parent body collision creating localized dust cloud ? –Pro: We know these collisions must be happening –Con: Particles should spread fairly rapidly; cloud not long visible ●Dust particles trapped in mean-motion resonance with planet? –Pro: Could produce long-lived asymmetry –Con: May be hard to account for asymmetry variation with wavelength; trapped dust population won’t have big radial extent ●Secular perturbations from planet in eccentric orbit on a continuous disk? –Disk particles will be forced onto eccentric orbits, tend toward apsidal alignment with planetary perturber. Eccentric disk. –Pro: Produces long-lived asymmetry, can account for its variation with wavelength

Eccentric ring model (Wyatt et al. 1999) ●Outer disk is perturbed by eccentric interior planet ●Brightness asymmetry induced by warmer dust temperature at periastron ●Stapelfeldt et al. 2004: Ring e~ 0.07 would account for the observed brightness asymmetry, and not be geometrically discernible to Spitzer  Marsh et al. 2005: suggest ring e~ 0.06 from submm maps Kalas et al find e~0.11 from HST/ACS images This model seems to work well

HIRES Deconvolution at 70  m Native image 70 iterations 10 iterations 100 iterations 40 iterations 130 iterations New ! 5x deeper images (Stapelfeldt et al. 2006)

70  m data/model comparison HST/ACS (Kalas et al.) 70  m emission model Kalas et al. ring parameters MIPS 70  m native Model convolved with native PSF Model convolved with HIRES PSF MIPS 70  m deconvolved (Stapelfeldt et al. 2006) Some 70  m emission from ring interior appears to be needed Next step is model fitting to all Spitzer images & spectra

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