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. 1998

– 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 spectrumNew ! IRS SH+LH from improved pipelines

(Stapelfeldt et al. 2006)

photosphere

Fomalhaut thermal continuum emission

JCMT/ SCUBA450 μm(Holland 2003)

MIPS24 μm

(PSF-subtracted)

MIPS 70 μm

MIPS160 μm

JCMT/SCUBA850 μm

(Holland 1998)

CSO / Sharc II350 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. 2005 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 modelKalas et al. ring parameters

MIPS 70 m native

Model convolved with native PSF

Model convolved withHIRES 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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