2010 Annual Research Report

   Weidenschilling has investigated a number of topics related to the formation of the solar system and extrasolar planetary systems. One  project has used the PSI multi-zone accretion code to model growth of large bodies from planetesimals. The goal is to determine initial conditions that can produce the observed excess of ~ 100 km bodies in the asteroid belt. Accretion simulations show that this size distribution would not result from the canonical starting condition of km-sized planetesimals. This has led to suggestions by other researchers that asteroids did not accrete in stages, but formed at    their current sizes directly from mm- to m-sized particles concentrated by turbulence in the solar nebula. However, Weidenschilling found that starting with smaller planetesimals of ~ 0.1 km yields good agreement with observations. Deviations of the size distribution from a simple power law are produced by transition of growth regimes from dominance by random velocity to Keplerian shear at different sizes. This result supports the formation of planetesimals by collisional coagulation.

   During 2010 Weidenschilling completed a study of the evolution of small dust grains in circumstellar debris disks, such as those observed by the Spitzer and Hubble space telescopes. Grains are produced by collisional destruction of larger parent bodies, and removed by mutual collisions, radiation pressure, and Poynting-Robertson drag due to light from the central star. A variant of the accretion code was used to model the collisional evolution of the size distribution of particles in a debris disk. The evolution of bodies of meter size and larger is computed explicitly, and the size distribution is extrapolated analytically down to the sizes of the smallest grains. The infrared luminosity of the disk due to absorbed starlight is computed. The evolution of the disk can be computed for model times up to Gyrs. Weidenschilling derived a new constraint on the mass of a debris disk and the sizes of the unseen source bodies that produce the visible dust, based on the observable infrared luminosity. He applied the numerical model to constrain the proprties of a specific debris disk around the star HD 12039 that has been well observed by the Spitzer Space Telescope. It was found that a debris disk reaches an initial peak luminosity, followed by a gradual decay. A low-mass disk evolves more slowly than one with higher initial mass, and may have a higher luminosity at a later time. This effect may explain why observations find no correlation between the brightness of a debris disk and its star's content of heavy elements.

   In addition, Weidenschilling has collaborated with other researchers at the University of Arizona and the NASA Ames Research Center on projects to    investigate formation of chondrules and accretion of the cores of the giant planets.

Papers: 

   Particles in the nebular midplane: Collective effects and relative velocities. Meteoritics & Planetary Science 45, 276-288.

   Collisional and luminosity evolution of a debris disk: The case of HD 12039. Astrophysical Journal 722, 1716-1726.