Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998exdu.work..286n&link_type=abstract
Exozodiacal Dust Workshop, p. 286
Astronomy and Astrophysics
Astronomy
Optical Properties, Stellar Envelopes, Size Distribution, Infrared Astronomy, Cosmic Dust, Interplanetary Dust, Thermal Emission, Scattering Cross Sections, Absorption Cross Sections, Albedo
Scientific paper
Beta Pictoris is the best studied Vega-type star with the optically resolved circumstellar dust disk. However, the typical size of dust particles in the disk is still an open question. Several optical observations have indicated that the color of the disk is same as the beta Pic itself within the observational uncertainties. This result has been interpreted that dust size, responsible for the scattered light, is larger than 1 micron. On the other hand, infrared observations suggest smaller typical size less than 1 micron. Telesco et al. (1988) observed thermal emission from the disk at 10 and 20 microns. As the deduced color temperature is much higher than blackbody, they concluded that the dust particle, which emit most of thermal infrared radiation, must be smaller than 1 micron. Mid infrared spectroscopy also infers that submicron-sized particles dominate thermal radiation from the disk because the observed silicate feature would diminish if the typical dust size exceeds several microns. Based on the similarity to the Interplanetary Dust Particles (IDPs), we present a new model of the dust particles around Beta Pictoris and calculate the optical properties by Discrete Dipole Approximation with a1-term method. The scattering cross sections in optical region becomes neutral when the mass-equivalent sphere exceeds 0.6 micron independent of the monomer size. The infrared silicate feature is present even for large aggregates whose mass-equivalent radius is 6 micron. These results indicate that our model can explain the coexistence of the neutral scattering and silicate feature with no tuning of the size distribution or compositions of dust particles. The visible absorption cross sections are also higher compared with the mass-equivalent sphere due to the enhancement of geometrical cross section, while the absorption cross sections in the mid infrared are comparable. These properties are also consistent with the observed superheat and high albedo of the dust particles in the beta Pic disk.
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