Spitzer Space Telescope Imaging of Spatially- Resolved Debris Disks. Karl Stapelfeldt Jet Propulsion Laboratory MSC d2p: Mar

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1 Spitzer Space Telescope Imaging of Spatially- Resolved Debris Disks Karl Stapelfeldt Jet Propulsion Laboratory MSC d2p: Mar

2 In collaboration with Jet Propulsion Laboratory: Michael Werner, Chas Beichman, Nick Gautier, Geoff Bryden University of Arizona : George Rieke, Kate Su, John Stansberry, David Trilling NOAO : Christine Chen Harvard/Smithsonian : Tom Megeath, Massimo Marengo Univ. of Rochester : Dan Watson UCLA : Michael Jura Space Science Institute : Dean Hines Ball Aerospace : Jeff van Cleve NASA Ames: Dana Backman 2

3 What are the Fabulous Four? Four nearby main sequence stars with strong far-infrared excess emission Three are A stars: Fomalhaut, Vega, beta Pictoris; all spatially resolved by IRAS One is a K star: epsilon Eridani Dust removal timescale much shorter than stellar ages: replenishment by ongoing parent body collisions & comet passages: Debris disks Disks also spatially resolved by JCMT/SCUBA IRAC MIPS MIPS MIPS 8 μm 24 μm 70 μm 160 μm FWHM ( ) Spitzer diffraction-limited spatial resolution 3

4 Science Goals for Spitzer Observations of the Fab4 1. Resolve disk spatial structures that may indicate planetary perurbations on the disks: central holes, clumps, asymmetries, radial gaps, warps, 2. Study the dust grain composition and search for a gas component using IRS and MIPS SED mode 3. IRAC 4.5 μm search for substellar companions which may be perturbing the disks 4. Provide a proving ground for disk models that will be broadly applied to other Spitzer disk survey datasets (GTO/Legacy/GO) 4

5 Background on Fomalhaut disk 850 μm 450 μm A3 V star, distance= 8 pc Disk resolved with IRAS & KAO (Gillett et al. 1986; Harvey et al. 1996) Submm detection of edge-on ring by Holland et al. 1998, AU ring radius; slightly asymmetric to the SE First debris disk science target for Spitzer, November

6 Fomalhaut MIPS 24 μm (Stapelfeldt et al. 2004) Left: Reference star image Center: Fomalhaut direct image 160 FOV Right: Dust disk revealed by PSF subtraction Kurucz photosphere model fit determines scale factor About 80% of 24 micron excess from unresolved core 6

7 Fomalhaut MIPS 70 μm (Stapelfeldt et al. 2004) 90 = 700 AU Left: 70 micron fine scale image Right: Deconvolution with 20 iterations of the HIRES algorithm Aumann, Fowler & Melnick (1992); implemented at JPL by Velusamy, Backus, and Thompson Asymmetric bar of 70 μm emission overlies the submm ring 7

8 New Fomalhaut MIPS 70 μm results November 2004 dataset: Deeper exposures, better calibration than in Stapelfeldt et al. (2004) Simple ring morphology is now seen clearly after deconvolutions with the HIRES algorithm; material interior to the ring near the SE ansa may not be needed Iteration number CSO map: Marsh et al

9 Fomalhaut Results Summary No obvious spectral features detected grainsizes > 5 microns 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 Spectra show warmer, brighter excess on star than disk ansae 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? 9

10 Eccentric ring model (Wyatt et al. 1999) Outer disk is perturbed by eccentric interior planet Brightness asymmetry induced by warmer dust temperature at periastron Disk e~ 0.07 would account for observed brightness asymmetry, would not be geometrically discernable 10

11 New evidence supports the eccentric ring model for Fomalhaut s asymmetry Marsh et al argue that their CSO Sharc II 350 μm continuum image shows the ring center displaced from the star The new MIPS 70 μm image now resolves the SE ansa into a simple ring with azimuthal brightness variations Kalas et al. (Nature, in press) have detected the ring in scattered light using HST; definite elliptical morphology and 2 offset of star from ring center CLEAREST CASE of a debris disk structure that requires a planetary perturber to maintain it 11

12 Fomalhaut epilogue Additional work needed to tune/explore the secular perturbation models for the eccentric ring Beth Holmes ( ) Link between inner & outer dust clouds should be clarified by spatially resolved spectra (IRS, MIPS SED) Inner dust cloud should be excellent target for Keck nulling interferometry 12

13 Background on Vega disk A0 V star, 50 L, 8 pc distant Prototype main sequence star with IR excess Large particle population, T 80 K, resolved diameter of 160 AU at 60 μm, central hole (IRAS; Aumann et al. 1984) Fractional infrared luminosity is 1.8x10-5, about 200 times that of Sun s zodiacal cloud Disk undetectable in scattered light 1990 s artist rendering of the Vega disk 13

14 Vega dust disk dynamical model: Resonant trapping in a face-on disk Wilner et al. (2002): Large dust grains, 3 M J planet, e=

15 Vega observed with MIPS Spitzer images here are 160 square (Su et al. 2005) Reference star Vega direct image Vega PSF-subtracted 24 μm results: Emission extends to r> 30 Dark hole = saturation artifact 70 μm results: Source has ~25 FWHM Fine scale HIRES deconvolution SCUBA 850 μm map by Holland et al. (1998) 15

16 Vega disk: observed radial profile (Su et al. 2005) 24 μm 70 μm 160 μm Inner hole inferred from radial profiles 16

17 Vega disk color temperature profile: outer disk is too warm to explain with large dust grains (Su et al. 2005) 17

18 Vega radial profile models (Su et al. 2005) = data 24 μm 70 μm 160 μm 850 μm 18

19 Vega Results Summary Population of small (2 μm) grains is required to account for profile & extent of the 24 μm emission. Lifetime < 1000 yrs. Best-fit radial surface density profile is Σ= Σ 0 r -1 consistent with steady-state escape of the grains The inferred mass loss rate is probably unsustainable: (8x10 14 gm/s) x (350 Myrs) = 4.4 Jupiter masses of dust would be lost over the age of the system. Recent breakup of moderate-sized asteroid is good alternative model Distinct population of much larger (200 μm) grains is needed to account for the (sub)millimeter continuum emission Issue of resonant trapping of dust by a planet remains open: small particles dominating the Spitzer images can t become trapped, due to the dominance of radiation pressure (Su et al. 2005) 19

20 ε Eri MIPS 70 μm (Megeath et al. 2005) Left: 70 μm fine scale Right: HIRES deconvolution SCUBA 850 μm (Greaves et al. 1998) 70 μm source has 15 FWHM, and fills the interior of the submillimeter ring No extended 24 μm emission 160 μm data is still pending All images shown at the same linear scale 20

21 ε Eridani Companion Search: IRAC 3.5 μm Roll-subtraction of two epochs (Marengo et al. 2005) Single direct image Red circle diameter = 40 21

22 ε Eridani companion search: Nearby objects to photometer (Marengo et al. 2005) 22

23 AU Mic Debris Disk Liu et al M0 star at 10 pc distance, age 12 Myrs Submillimeter excess led to imaging of the disk in scattered light Kalas 2004 Liu 2004 Krist et al Metchev et al Spitzer does not resolve the disk, but does refine the SED somewhat MIPS 70 μm image; Chen et al

24 β Pictoris MIPS Results MIPS 70 μm default scale, 5 FOV 24 μm direct image 24 μm HIRES deconvolution 850 μm SCUBA image (Holland et al. 1998) All images shown at the same linear scale 24

25 Other nearby IRAS debris disks not spatially resolved in MIPS 70 μm fine scale images 61 Cyg B α CrB β Leo β UMa δ Vel η Tel γ Oph τ Ceti ζ Lep 25

26 MIPS 70 μm coarse scale results: Resolved disk of HD G0 star, d= 17 pc, L d /L *, = 7x x18.7, PA 72 deg Major axis diameter = 240 AU HIRES deconvolution 26

27 Conclusions Major differences are seen between outwardly similar disks: Fomalhaut, Vega. SED can mask this diversity. Emission at different wavelengths (24 μm vs. 70 μm vs. submm) can arise from very different grain populations, at different radial locations in the disk. Beware of debris disk models that postulate a single emission region! To Spitzer, spatially resolved debris disks remain a rarity. Stay tuned for the full story on ε Eri and β Pic later this year. Fomalhaut 24 & 70 μm 27