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[30.09] Dust and planetesimals in the inner Beta-Pictoris disk. A numerical model

P. Thebault (Observatoire de Meudon), J-C Augereau (CEA-Saclay)

Near-Infrared observations tend to indicate the presence of a significant lack of dust in the inner Beta-Pictoris disk. The total mass of dust in the r<10 AU region is believed to be comprised between 2.10²¹ (Pantin & Lagage, 1997) and 2.10²² g (Li & Greenberg, 1998). Supposing that this dust is produced through a collisional cascade following a classical r^-3.5 differential law leads to less than 2.10^-2 M_earth of parent planetesimals in the 10 to 50 km range. However, analysis of the so called Falling Evaporating Bodies (FEB) process provide a very different estimate, at least 20.M_earth of large planetesimals in the same region (Thebault & Beust, 2001). Are these two estimates incompatible? Could there be a lack of dust despite the presence of numerous kilometre-sized objects? We address this problem by running a Particle-in-a-box statistical code exploring the evolution of the size distribution in the inner Beta-Pic system, taking into account fragmentation, craterization and accretion processes and covering all particle sizes from 100 km down to the minimum cut-off, around a few microns, due to the star's pressure radiation. The specific behaviour of the smallest micron-sized grains, i.e. high a and high e orbits due to pressure radiation and the local depletion in the inner disk that follows, are also taken into account. Several disk densities (comprised between the ``low" dust-observations estimate and the ``high" FEB deduced one) and initial excitations are explored. Preliminary results indicate that size distributions always significantly depart from collisional equilibrium power-law, mainly because of the small-size cutoff and the peculiar dynamics of the smaller grains. An important lack of bodies in the 0.01 to 1 cm range is in particular always observed. But this seems insufficiant to conciliate the observed dust mass and the FEB-deduced planetesimal density. Furthermore, densities as high as those required for the FEB scenario lead to too rapid mass loss in the inner disk, making the FEB mass estimate appear significantly overestimated.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.