spitzer space telescope observations of the fomalhaut debris disk michael werner, karl stapelfeldt,...
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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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