The AU Mic Debris RingDensity profiles & Dust Dynamics

J.-C. Augereau & H. BeustGrenoble Observatory (LAOG)

Scattered light images

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AU Mic: M0/M1 star ~10pc ~12+8

-4Myr UV & X-ray flares

The AU Mic disk: Seen edge-on Resolved at visible &

near-IR wavelengths

Disk surface density (r) ? Previous attempts to estimate (r) used classical fitting methods:

(r) : radial power law (or a combination of power laws) => synthetic surface brightness profiles

Limitations: Power laws cannot account for the local brightness enhancements Effect of scattering anisotropy ?

Krist et al. 05 (g~0.3)

Direct inversion of brightness profiles

Surface brightness profile Density profile

Basic integral equation

S(y) : observed brightness profile (r,) : surface density f() : scattering phase function

one free parameter : g

Edge-onview

Pole-onview

r

y

y

Surface density of the AU Mic disk

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Isotropicscattering

Anisotropicscattering

g=0.0 g=0.2 g=0.4 g=0.6

H-band image (Liu 2004)

Density profiles

Asymmetric ring-like structure peaked around 35AU

Sub-structures inside 35AU

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Similarity between the Pic and AUMic brightness profiles

Both disks show2 power-law profileswith similar slopes

Coincidental??

Positions of the breaks Pic : ~ 120AU AUMic: ~ 35AU

Liu 2004

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slope ~ -1

Slope~ -4.5…-5

Pictoris brightness profile :controlled by radiation pressure

A scenario for the Pic disk (Augereau et al. 2001):

break around 120AU = outer edge of planetesimal disk r-4.5..-5 law : diffusion of the smallest grains by radiation pressure Predicts more small grains at large distance Explains the butterfly asymmetry

AUMic : Radiation pressure is inefficient (M-type star)=> Scenario proposed for Pic does not readily apply to AUMic

Planetesimals

Dust

AU Mic brightness profile :controlled by wind pressure

Evidences for a stellar wind Young late-type star => coronal solar-like wind expected UV & X-ray excesses & flares Roberge et al. 2005 : short lifetime of the gas and blue shifted CII ( H2 ?)

Wind pressure force dM/dt ~ 5x10-12 Msun/year (Parker 1958 solar-like coronal wind model) Behaves (almost) like a radiation pressure force but is stronger Effect enhanced due to stellar flares Grains smaller than 0.1-1µm are expelled.

Slightly larger grains remain bound but are placed on eccentric orbits

=> explain the r-4.5..-5 brightness profile

Grain size distribution

Implications

Grains observed at r>35-40AU originate from the ring peaked around 35AU and are diffused in the outer disk by wind pressure

Minimum grain size ~ 0.1-1µm=> consistent with the blue color measured by Krist et al. 2005 in the visible (HST/ACS)

Disk color in scattered light should continuously increase with the distance from the star=> observational test to our scenario

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Additional disk properties

Age of the star

Disk surface density& grain size distribution Disk mass ~ 10-4 Mearth mass of Ceres asteroid

Vertical optical thickness < 5x10-3 ; Midplane optical thickness < 0.03

Mean collision time-scale~ a few 104 years at positionof peak density

Conclusion

Direct inversion of brightness profile=> asymmetric dust ring peaked around 35AU

Shape of observed brightness profile can be explained by: a main source of dust located at r~35AU the diffusion of small grains in the outer disk by stellar wind pressure

The wind pressure scenario is in line with the blue color of the disk in scattered light and the disk color depends on r

Collision time-scales are 3 orders of magnitude smaller that stellar age => collisional evolution can happen

Augereau & Beust, submitted to A&A in Sep. 2005