The architecture of planetary systems revealed by debris disk imaging

Transcription

1 The architecture of planetary systems revealed by debris disk imaging Paul Kalas University of California at Berkeley

2 Collaborators: James Graham, Mark Clampin, Brenda Matthews, Mike Fitzgerald, Geoff Bower, Eugene Chiang, et al. Outline 1. Show the rapid progress of debris disk imaging. 2. Present new HST polarization results for AU Mic. 3. Discuss Fomalhaut s Belt and evidence for a planetary system. 4. Review more disks and suggest some ideas regarding the origin of their architecture.

3 The Coronagraph: Imaging follow-up to IR excess stars Kalas et al. 2004

4 First detection of the solar corona without a lunar eclipse (1932) Voyager 2 reaches Saturn (1981) Bernard Lyot ( ) Brad Smith (head of Voyager imaging team)

5 Introduction: Vega Phenomenon Direct Image of the β Pic Dust Disk as early as 1983 Smith & Terrile 1984 Beta Pic was the Rosetta Stone Debris Disk for 15 years >300 refereed papers

6 Introduction: Vega Phenomenon 0.5 µm 2.2 µm µm 850 µm β Pic Vega Fomalhaut ε Eri 1998 see HR 4796A HD Resolved images of dust structure linked to unseen planets

7 2006

8 2006

9 AU Mic - Past AU Mic (GJ803): Early evidence for circumstellar dust: Tsikoudi 1988, "Flare stars detected by the Infrared Astronomical Satellite" Mathioudakis & Doyle 1991, "Active M-type stars from the ultraviolet to the infrared" One of the closest flare stars: Distance = 9.9 pc SpT = M1Ve Mass = 0.5 M sun Radius = 0.56 R sun T eff = 3500 K Luminosity = 0.1 L sun M v = 8.8 mag Period = d Avg. Mag. Field: B = 4000 G Ha Equivalent Width = 8.70 Quiescient X-ray flux: log 10 (L x ) = 29.8 erg/s Age: Young

10 AU Mic: Stellar Properties beta Pic Moving Group Kalas & Deltorn (1999, unpublished) Barrado y Nav ascues 1999 Zuckerman, Song, et al. 2001

11 AU Mic - Present AU Mic Discovery Image: Kalas, Liu, & Matthews 2004 R-band, UH 2.2 m telescope, 0.4"/pix, 900 s, seeing FWHM = 1.1"

12 Follow-up high resolution imaging Metchev et al (Keck NIR), Krist et al (HST, visible), Liu 2004 (Keck, NIR) Radius: AU Width: AU within 50 AU Dust depletion beyond the ice sublimation boundary Blue scattering throughout the disk Krist et al. 2005

13 AU Mic Origin of the Disk: Grain lifetimes as a function of radius from Backman & Paresce 1993: Poynting-Robertson Drag Timescale Kalas et al Collision Timescale Sublimation Timescale Most of the grains seen in the discovery image are fragments of larger objects, very little mass has been removed from the system. <67 AU: 0.1 mm grains have spiraled into the star in 8 Myr <20 AU: 1.0 mm grains have spiraled into the star in 8 Myr 100 AU: Collision timescale is 1.8 Myr >200 AU:Collisionally unevolved disk, pristine material

14 AU Mic Origin of the Disk? Plavchan, Jura, & Lipscy 2005 Augereau et al (in press) radiation pressure / gravity stellar wind / gravity

15 AU Mic s Blue Color: Birth Ring Theory Strubbe & Chiang (2006) 100 x solar mass loss rate projected radius

16 AU Mic - Present How do disks evolve differently around an A star and an M star? radiation pressure blowout stellar wind blowout Kalas et al. 2004

17 HST ACS: HRC Polarization J. R. Graham, P. Kalas, & Matthews 2006, submitted to The Astrophysical Journal F606W ACS/HRC

18 HST ACS: HRC Polarization Simultaneous fit to optical SB profile and polarization Need high p, and large g. Small grains give high p, but scatter too isotropically. If astronomical silicates, P = 94 ± 6% inner radius ~40 AU, outer radius ~200 AU. If ice, P = 91 ± 9%

19 Is this what we see around AU Mic? No, comet grains have porosity ~70% after sublimation of volatiles. Moreover, AU Mic grains originate far beyond the ice sublimation radius. More likely, particle coagulation via ballistic clustercluster aggregation. To avoid restructuring and compactification, the upper size limit of the parent bodies is ~10 cm. Wurm & Blum 1998

20 Beta Pic Polarization How does AU Mic compare to Beta Pic? Artymowicz 1997

21 Keck Observatory with Adaptive Optics (Fitzgerald et al. 2006) J, H, K

22 AU Mic: Fitting the SED & Color simultaneously Two component model disk (Mie scattering, Monte Carlo radiative transfer code; Duchene et al). Polarization Compact silicates (Drain & Li 2001) do not work. Mathis & Whiffen (1989) model works well. Highly porous aggregates of silicates, carbonaceous and icy elements. H-band (Keck) V-band (HST) Polarization too high in this simple model. Nonspherical grains? Minimum grain size varies with radius? Fitzgerald et al. 2006

23 Fomalhaut Stapelfeldt et al ring eccentricity in model = 0.07 planet orbit: a = 40 AU, e = 0.15 Marsh et al Model fit using Spitzer (24, 70, 160 µm) & 350 µm image suggests 8 AU center of symmetry offset. Planet a = 86 AU, e = 0.07, M > 1 Earth if the inner ring boundary is the location of a 2:3 MMR (Neptune :CKB)

24 HST ACS planet search Fomalhaut Kalas, Graham & Clampin 2005, Nature, Vol. 435, pp Semi-major axis: a =140.7± 1.8 AU Semi-minor axis: b = 57.5 ± 0.7 AU PA major axis: ±0.3 Inclination: i = 65.9 ± 0.4 Projected Offset: 13.4 ± 1 AU F814W: 80 min., 17 May, 02 Aug, 27 Oct, 2004 PA of offset: ± 0.3 F606W: 45 min., 27 Oct Deprojected Offset f = 15.3 AU 25 mas / pix, FWHM = 60 mas = 0.5 AU Eccentricity: e = f / a = 0.11 orbital period at 140 AU = 1200 yr

25 HST ACS planet search Asymmetric Scattering Phase Function Kalas, Graham & Clampin 2006 g = 0.2 Zodiacal Light = +0.2; Forward Scattering Median size ~30 microns (blowout size for Fomalhaut is 7 microns). Integrated light from model gives a total grain scattering cross section of 8.7 x cm 2. Assume 30 mm sized particles and density 2.5 g cm -2, albedo = 0.1, then belt mass is 0.09 Lunar mass --> 17 times smaller than inferred from submm data. Albedo may be much lower --> a dark belt similar to the rings of Uranus. Model subtraction emphasizes inner dust component, 20% of the peak flux in the main belt.

26 HST ACS planet search Wyatt et al Evidence for a planetary system: Center of symmetry offset Kalas, Graham & Clampin 2006 How Observ ations of circumstellar disk asymmetries can rev eal hidden planets:pericenter glow and its application to the HR 4796A disk Wyatt, M.C. et al. 1999, ApJ, 527, 918 G. Schneider, STIS S = stellar position D = center of particle orbit C = center of precession circle P = pericenter of a particle orbit DP = a, semi-major axis of a particle orbit w f = direction of forced pericenter SD = a e SC = a e forced CD = a e proper Torus inner radius = a (1 - e proper ) = 133 AU Torus outer radius = a (1+ e proper ) Particle eccentricity composed of a p rop er (or free) eccentricity, inherent to the particle, and a forced eccentricity due to a perturber. The pericenter also has a free and a forced component. The orbital distribution of particles with common forced elements will be a torus with center, C, offset from the stellar position, S. The forcing is due to an eccentric companion that could be either inside or outside the belt. Infer offset 2 AU for HR 4796A Similarly offset = 0.01 AU for Zodiacal dust disk (e.g. Kelsall et al. 1998). External eccentric perturber can produce the same center of symmetry offset, but not the sharp inner disk boundary.

27 Kalas, Graham & Clampin 2006 Radial cut along 10 segement Q2 (apastron), in the illumination corrected image; cut traces the material surface density of the structure rather than its brightness. Model has a hard edge inner edge, but the integration in the line of sight and the 7 AU vertical scale height means that the edge will not appear sharp in the sky projection. Quillen (2006) argues that the steepness of the edge is consistent with a Neptune to Saturn-mass object at 119 AU semi-major axis.

28 HST ACS planet search Evidence for planets: sharp inner edge Kuiper Belt dust models by Moro-Martin & Malhotra 2002 radial cuts planets 1) Dust produced by KBOs a=35-50 AU, i = ) 1-40 mm, r = 2.7 g cm -3 & mm, r =1 g cm -3 3) 7 planet, or no planets 4) Solar gravity, RP, P-R drag, solar wind drag. 5) b = RP / gravity L * / r s no planets

29 DISK ARCHITECTURE Still more debris disks discovered with ACS HRC Coronagraphy HD SpT=F5V d=17.5 pc age = 300 Myr AU Kalas et al HD SpT=G2V d=28.5 pc age = 100 Myr AU Ardila et al HD SpT=K1V d=18.4 pc age = 1.0 Gyr >110 AU Kalas et al HD SpT=K1V d = 22 pc age = 100 Myr >146 AU Clampin et al. 2006

30 Kalas et al, 2006

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34 Future HST/ACS Observations: Multi-color imaging of the entire belt (July - August 2006) Search for azimuthal asymmetries; e.g. Trojans Measure ring width as a function of azimuth Search for color gradients azimuthally and radially Characterize properties of Zodiacal dust analog; dust interior to the belt. Understand grain properties, source regions More Future Work: Are there planets? Detect the planet(s) directly. Keck II AO run in July, October. Are there external perturbers confining the outer belt boundary? Wide field multiepoch search. What is the origin of the belt? Planet formation theory; migration; resonance vs. ejection. What are the orbital elements of a planet? Is Fomalhaut's belt a mirror of our young Kuiper Belt? What accounts for the factor of three difference in semi-major axis scale?