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Current efforts for modeling exozodiacal disks

Current efforts for modeling exozodiacal disks. Jean-Charles Augereau & Olivier Absil LAOG, Grenoble, France & U. Liège , Belgium Barcelona, September 2009. Various kind of models. Radiative transfer

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Current efforts for modeling exozodiacal disks

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  1. Current efforts for modeling exozodiacal disks Jean-Charles Augereau & Olivier AbsilLAOG, Grenoble, France &U. Liège, BelgiumBarcelona, September 2009

  2. Various kind of models • Radiative transfer • To reproduce the scarce exozodi measurements, in particular IR interferometric data (CHARA/FLUOR, KIN) • To reproduce extreme Spitzer spectra showing unusually large amount of warm dust and clear spectral features (e.g. Lisse et al. 2009) • Dynamical modeling • Sculpting of an asteroid/dust belt by a planet: capture in mean motion resonances (e.g. Stark et al. 2008) • Larger scale models, with outward planet migration toward a Kuiperbelt • Sudden event, e.g. Nice LHB model (Booth et al. 2009) • More progressive (Vandeportalet al., in prep.) • Collisional models to evaluate lifetime of asteroid belts due to collisions (works by Krivov, Löhne, Thébault)

  3. ISSI working group on exozodis • ISSI international team assembled in order to • Interpret the pioneering observations of exozodis with (near-)IR interferometers • Identify the dominant source of dust (dust transport, collisions, comet evaporation) • Predict the structure and amount of dust that can be expected around nearby stars • Help to prepare future missions aiming at finding planets in the habitable zone • Two one-week meetingswere held (August 2007,April 2009) • Next and last meetingin 2010Anyone interestedto attend should contactJean-Charles Augereauaugereau@obs.ujf-grenoble.fr ISSI team: Exozodiacal dustdiks and Darwin

  4. Exozodis: are they zodi analogs? • Fit of the 2.2μm excess with the Solar system zodiacal model : • Vega : 3000 zodisto reproduce the CHARA K-band excess • Fomalhaut : 5000 zodis to reproduce the VLTI/VINCI K-band excess • Problem: it predicts too much flux in the mid-IR (by ~ an order of magnitude) • Our zodi is not representative : detected exozodis have dust much closer to the star ZODIPIC package [Kuchner] Flux 2.2 μm 10 μm Exozodiemissionspectrum Wavelength

  5. Exozodis : radiative transfer • Development of an optically thin exozodiacaldisk model, based on debris disk model by Augereau et al. (1999) • Scattered light and thermal emission • Parametric surface density profiles and size distribution • Variable dust composition (silicates, carbonaceous grains, porosity, …) • Specific treatment of the exozodidisk inner edge to account for thesize-dependent sublimation radius • Outcome of the model: • Peak position of surface density • Grain properties (size, composition) • Mass, optical thickness • Typical dynamical time-scales (vs collisions, radiation pressure) - Solid line : 50% silicates + 50% carbons- Dashed line: 100% carbons

  6. Exozodis : radiative transfer • Result for Vega(Absil et al. 2006) • All parameters go in the same direction: shifting the exozodi spectrum to shorter wavelengths compared to our zodi • Smallgrains (mostly < 1 µm) at distances ~ 0.2 – 0.5 AU • Highly refractive grains, no silicate feature  carbons > 50% • Steepdensity profile: S(r) ~ r-4 (or steeper) Total Thermal emission only MMT/BLINCupperlimit

  7. Exozodis : radiative transfer • Tau Ceti (Di Folco et al. 2007)Solar-type star (G8V at 3.65pc) Spitzerupperlimits

  8. Exozodis : radiative transfer • Zeta Aql(Absil et al. 2008): • More and more similar cases foundwith CHARA/FLUOR • ~20% of stars? • Depends on spectral type 0.6-0.65 Msun 5.5-8AU

  9. Other types of detections • Spitzer/IRS (see Chas’ talk) • Mid-IR imaging on 10-m telescopes • VISIR, T-ReCS, Michelle, etc • Only in favourable cases (~5 so far)due to limitedresolution/sensitivity • JWST/MIRI will do muchbetter • Mid-IR interferometry (see Chas’ talk) • BLINC, KIN, (MIDI) • All detections are showing large amounts of warm/hot dust HD 32297 (Moerchen et al. 2007)

  10. Vega’s exozodi : origin of the dust? • Exozodiacal dust grains have very short lifetimehigh dust production rate,~10-8Mearth/yr for Vega • Equivalent to 1 medium-sizedasteroid every year • Equivalent to a dozen ofHale-Bopp-like cometspassing every day(400 tons of dust per second) • Equivalent to 10-3 the mass ofthe Vega Kuiper Belt per Myr • If we exclude the case ofa dramatic event, the questionis: where does all thisdust come from?

  11. Vega’s exozodi : origin of the dust? • Inward migration of grains due to Poynting-Robertson drag is excluded : too slow compared to other dynamical timescales (collisions, radiation pressure) • Steady-state evolution of an asteroid belt excluded: km-sized bodies in a 10-3MEarth belt at around 0.2-0.5 AU do survive ~105years (Löhne& Krivov, priv. communication – see also Wyatt et al. 2007) • Reservoir of mass inVega’s Kuiper Belt(~85AU, ~ 10MEarth). • How to extract 10-3 of this mass every Myr and transportit well inside 1AU? Planets can help… Schematic representation of the Vega system

  12. Vega: planet migration • Structures in Vega’s Kuiper belt : migrating planet trapping the parent bodies of the dust grains in MMR • Wyatt (2003): Neptune-mass planet,migrating from 40 AU to 65 AU,at a rate of 0.5 AU/Myr. Circular orbits. • Reche et al. (2008): Saturn mass planet, with e<0.05,and relatively cold disk (<e> < 0.1) 2:1 MMR 3:2 MMR

  13. Vega: the comet factory • A two planet system … • Migrating Saturn mass planet [Wyatt 2003, Reche et al. (2008)] • 0.5 – 2 Jupiter mass planet inside • … and a Kuiper Belt … • … to produce star-grazing comets.Numerical simulations with SWIFT (symplectic integrator).Counting of the test particles entering the 1AU zone

  14. Vega: the comet factory • A two planet system: • Migrating Saturn mass planet [Wyatt 2003, Reche et al. (2008)] • A 0.5 Jupiter mass planet at ~20 – 25 AU does the job well: Sufficient rate of cometsin the 1AU zone for 40Myr Resonant structuresat 85AU preserved

  15. More extrasolar LHBs? • See M. Booth’s poster • Constrain: MIPS statistics • ~12% of debrisdisksystemsaffected by LHB event? • Mid-IR excessshort-lived • Expect (very) few detections

  16. How does all this help? ~100 zodi • Asymmetries • Most problematicexozodifeature (but alsointeresting on theirown) • Couldtheybepredicted? • Dust sources need to beknown • Planetary system architecture matters • Extrapolate the Bryden exo-Kuiper belthistogram? • Alsovalid for exozodis? • How deepshouldwesurvey to make sure that XX % of exozodis are < 10 zodi? • Can werely on statisticsonly? • Do weneed to observe the exozodis of actual candidate targets? ( wait for SIM’s input)

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