+

Current efforts for modeling exozodiacal disks

Jean-Charles Augereau & Olivier AbsilLAOG, Grenoble, France & U. Liège, Belgium

Barcelona, September 2009

+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 Kuiper belt Sudden event, e.g. Nice LHB model (Booth et al.

2009) More progressive (Vandeportal et al., in prep.)

Collisional models to evaluate lifetime of asteroid belts due to collisions (works by Krivov, Löhne, Thébault)

+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 dust diks and Darwin

+

Exozodi emission spectrum

Exozodis: are they zodi analogs? Fit of the 2.2μm excess with the Solar

system zodiacal model : Vega : 3000 zodis to 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

Flux

Wavelength

2.2 μm 10 μm

ZODIPIC package [Kuchner]

+

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 exozodi

disk 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)

Exozodis : radiative transfer

- Solid line : 50% silicates + 50% carbons- Dashed line: 100% carbons

+

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 Small grains (mostly < 1 µm) at distances ~ 0.2 – 0.5 AU Highly refractive grains, no silicate feature carbons > 50% Steep density profile: S(r) ~ r-4 (or steeper)

Thermal emission only

Total

Exozodis : radiative transfer

MMT/BLINCupper limit

+Exozodis : radiative transfer

Tau Ceti (Di Folco et al. 2007)Solar-type star (G8V at 3.65pc)

Spitzer upper limits

+Exozodis : radiative transfer

Zeta Aql(Absil et al. 2008):

More and more similar cases found with CHARA/FLUOR ~20% of

stars? Depends on

spectral type

0.6-0.65 Msun

5.5-8AU

+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 limited resolution/sensitivity JWST/MIRI will do much better

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)

+Vega’s exozodi : origin of the dust? Exozodiacal dust grains have very short lifetime

high dust production rate, ~10-8 Mearth/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?

+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 10-3 MEarth belt at around 0.2-0.5 AU do survive ~105 years (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 transport it well inside 1AU?

Planets can help…

Schematic representation of the Vega system

+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

+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

+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

+More extrasolar LHBs? See M. Booth’s poster

Constrain: MIPS statistics ~12% of debris disk

systems affected by LHB event?

Mid-IR excess short-lived Expect (very) few

detections

+How does all this help?

Asymmetries Most problematic exozodi feature (but

also interesting on their own) Could they be predicted?

Dust sources need to be known Planetary system architecture matters

Extrapolate the Bryden exo-Kuiper belt histogram? Also valid for exozodis? How deep should we survey to make

sure that XX % of exozodis are < 10 zodi?

Can we rely on statistics only? Do we need to observe the exozodis of

actual candidate targets? ( wait for SIM’s input)

~100 zodi