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the ε Eridani debris disk

the ε Eridani debris disk. Jane Greaves St Andrews, Scotland with Wayne Holland, Mark Wyatt & Bill Dent and cast of thousands. β Pic. Fomalhaut. Vega. discovery of the disk. nearest Solar type star with IRAS excess Aumann et al. 1985

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the ε Eridani debris disk

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  1. the εEridani debris disk Jane Greaves St Andrews, Scotland with Wayne Holland, Mark Wyatt & Bill Dent and cast of thousands...

  2. β Pic Fomalhaut Vega discovery of the disk • nearest Solar type star with IRAS excess • Aumann et al. 1985 • only 7 nearby K-dwarfs even had their photospheres detected by IRAS at 12-60 μm, at this stage

  3. further photometry • far-IR SED extended to 200 μm with ISO • Habing et al. 1999, 2000; Walker & Heinrichsen 2000 • mm photometry: why discrepancies? • looking through a hole in a disk?? • missing ingredient was imaging Chini et al. 1990, 1991 Zuckerman & Becklin 1993 Weintraub & Stern 1994

  4. why worry about it? • imaging the disk could be equivalent to a Solar System ‘time machine’ • ‘late heavy bombardment’ environment of the Earth up to 0.75 Gyr • ... ε Eri is 0.85 Gyr old • di Folco et al. 2004; VLTi stellar radius data image courtesy online Encyclopedia of Astrobiology Astronomy & Spaceflight

  5. SCUBA observations • started Aug 1997 • 850 μm • 450 μm (effectively from 2000) • why SCUBA? • first submillimetre camera • resolution of 8-15” • to ~mJy-rms see Holland et al. 1999

  6. from the start... • by night 1.... • 1 hour frame “another Solar System !? ”

  7. first image • mystery solved! • there really is an inner hole in a disk seen ~face-on • cavity extends to ~Neptune’s orbit • but only half-cleared, so any exo-Earth likely to be massively bombarded Greaves et al. 1998

  8. major outcomes • really a dust disk • size ~ of Solar System • radius of 65 AU • with large cleared cavity • to ~ 30 AU • inclination ~ tilt of stellar pole • i ~ 25° • ‘evolved’ dust, i.e. debris • β ~ 1.0 • LUMPS!

  9. need for more • SCUBA upgrades in late 1999 -> better 450 μm filter • higher resolution view of clumps • better spectral index, temperature and mass Sheret et al. 2004

  10. 1997-2002 images • at 850 μm • clumps confirmed Greaves et al. 2005

  11. first results at 450 • at 450 μm • clumps similar to 850 • independent data, different detectors • ~20 hours integration • clump to west has 350 μm counterpart? (SHARC II)

  12. 3:2 e = 0.3 e = 0.2 e = 0.1 planetary resonances? • dust caught in resonances with a planet forms characteristic patterns - > pinpoint planet location • why it’s hard in practice: • eccentricity changes patterns • multiple resonances overlapping Wyatt 2003; Kuchner & Holman 2003

  13. interpretations Ozernoy et al. 2000: 0.2 MJup at a = 60 AU, e = 0 Quillen & Thorndike 2002: 0.1 MJup at a = 40 AU, e = 0.3

  14. any hard evidence? • the inner planet: • ~2 MJup, a ~ 3.5 AU, e ~ 0.4 • SCUBA: disk centre offset from star • by ~1-2” • evidence of eccentricity forced on planetesimals?

  15. an outer planet? • clearing and clumps seen • all by one outer planet? • rotation expectations: • period ~ 180-570 years! • from just inside cavity, to embedded in dust ring • but in a few years, high S/N clump centre should move by detectable amount (~arcsec)

  16. what’s really disk? • first submm proper motion experiment! • background objects have not moved 4.5” with star 850 μm: colour, 1997/8 data contour, 2000/1/2 data

  17. rotation? • tentative! • but systematic pattern • proper motion plus rotation signature seen • background blends are a problem

  18. simulations • method: • simulate a dust ring with clumps, for 15” beam • add random noise • add realistic background galaxy population • simulate SCUBA chopping on/off field-of-view • produce ‘old’ and ‘new’ images, with known proper motion and chosen rotation • χ2-fitting to recover proper motion & rotation • simplifications: • circular, star-centred, face-on ring; point-like clumps

  19. detecting rotation • scatter found in simulations is 5° (1σ) • when no rotation included • errors may differ with rotating clumps • χ2-fitting gives 11° for real 4.5-yr dataset old/new example: 5° error

  20. implications • if detected: • planet at ~27 AU with ~150-yr period • hence is both clearing and perturbing dust ring • furthest-out exo-planet detection! • still need to model resonances to identify: • planet’s present position, mass, and orbital eccentricity

  21. how to extend the technique • could be a major planet-finding tool, with • higher resolution • e.g. 1-month experiments for ALMA! • more sensitivity • ε Eri is close, and ~median mass of detected debris disks, for Sun-like stars • unique for distant planets?

  22. discovery space • SCUBA-2 Legacy Survey • JCMT, from 2007 • 500 stars, to the 850 μm sub-mJy confusion limit • Large Submm Telescope • plans for ~30-100 m-class • to 200 μm? - > order of mag in resolution

  23. summary • ε Eri is the nearest young-Solar System analogue • history of cometary bombardment • demonstrates detection of outer planet perturbing cometary belt • new robust method of using rotation

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