The AU Mic Debris Ring Density profiles & Dust Dynamics J.-C. Augereau & H. Beust Grenoble Observatory (LAOG)

Scattered light images AU Mic: M0/M1 star ~10pc ~ Myr 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-on view Pole-on view r y y

Surface density of the AU Mic disk Isotropic scattering Anisotropic scattering g=0.0 g=0.2g=0.4g=0.6 H-band image (Liu 2004)

Density profiles Asymmetric ring-like structure peaked around 35AU Sub-structures inside 35AU

Similarity between the  Pic and AUMic brightness profiles Both disks show 2 power-law profiles with similar slopes Coincidental?? Positions of the breaks  Pic : ~ 120AU AUMic: ~ 35AU Liu 2004 slope ~ -1 Slope ~ -4.5…-5

 Pictoris brightness profile : controlled by radiation pressure A scenario for the  Pic disk ( Augereau et al ) : break around 120AU = outer edge of planetesimal disk r 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 : short lifetime of the gas and blue shifted CII ( H 2 ?) Wind pressure force dM/dt ~ 5x M sun /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 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 in the visible (HST/ACS) Disk color in scattered light should continuously increase with the distance from the star => observational test to our scenario

Additional disk properties Age of the star Disk surface density & grain size distribution  Disk mass ~ M earth  mass of Ceres asteroid Vertical optical thickness < 5x10 -3 ; Midplane optical thickness < 0.03 Mean collision time-scale ~ a few 10 4 years at position of 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