Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2S045

Porous dust grains in circumstellar disks

Kirchschlager, Florian (University of Kiel, Germany)
Wolf, Sebastian (University of Kiel, Germany)

We investigate the impact of porous dust grains on the structure and observable appearance of circumstellar disks (Kirchschlager & Wolf 2013). Our study is motivated by observations and laboratory studies which indicate that dust grains in various astrophysical environments are porous. In addition, the modeling of the spatial structure and grain size distribution of debris disks reveals that under the assumption of spherical compact grains the resulting minimum grain size is often significantly larger than the blowout size, which might be a hint for porosity. Using the discrete dipole approximation, we compute the optical properties of spherical, porous grains (Draine & Flatau 1994, 2010). Subsequently, we calculate the blowout sizes for various debris disk systems and grain porosities. We find that the blowout size increases with particle porosity and stellar temperature. In addition, the lower dust equilibrium temperature of porous particles results in a shift of the maximum of the thermal reemission of debris disks towards longer wavelengths. For our studies of the impact of dust grain porosity in protoplanetary disks we use the radiative transfer software MC3D, which is based on the Monte-Carlo method and solves the radiative transfer problem self-consistently (Wolf et al. 1999, Wolf 2003). We find that the spectral energy distribution of protoplanetary disks shows significant differences between the cases of porous and compact grains. In particular, the flux in the optical wavelength range is increased for porous grains. Furthermore, the silicate peak at ~9.8 microns exhibits a strong dependence on the degree of grain porosity. We also investigate the temperature distribution in the disk. In the midplane no influence of porosity is detectable, but in the vertical direction minor changes of a few Kelvin are found. To complete our study we outline the differences between the two grain types in maps of the linear polarization. We detect a polarization reversal in selected disk regions, depending on the grain porosity and disk inclination, which might set an observational test for porous grains in circumstellar disks.

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