MODEL SPECTRAL ENERGY DISTRIBUTIONS OF CIRCUMSTELLAR DEBRIS DISKS. Ⅰ. ANALYTIC DISK DENSITY DISTRIBUTIONS

摘要

We present results of a study aimed at deriving fundamental properties of circumstellar debris disks from observed infrared-to-submillimeter spectral energy distributions (SEDs). This investigation is motivated by increasing telescope/detector sensitivity, in particular the expected availability of the Space Infrared Telescope Facility (SIRTF) followed by the Stratospheric Observatory for Infrared Astronomy (SOFIA), which will enable detailed studies with large source samples of late-stage circumstellar disk and planetary system evolution. We base our study on an analytic model of the disk density distribution and geometry, taking into account existing constraints from observations and results of theoretical investigations of debris disks. We also outline the effects of the most profound characteristics of circumstellar dust, including the grain size distribution and dust chemical composition. In particular, we find that an increasing iron content in silicates mainly causes an increase of the dust absorption efficiency and thus increases the dust reemission continuum. Furthermore, the influence of the sp~2/sp~3 hybridization ratio in carbon grains on the SED is examined. We investigate the influence of various parameters on the resulting dust scattering and absorplion/reemission SED and discuss the possibility for distinguishing between different disks from their infrared to submillimeter spectra. The strength and shape of amorphous silicate may be particularly diagnostic of debris disk evolutionary stages. Since the appearance of these features at 10 and 20 μm depends on the relative abundance of small grains and therefore the minimum grain size and slope of the grain size distribution, they can be used to trace recent collisional processes in debris disks. Thus, debris disk surveys containing statistically large numbers of objects should reveal the likelihood of collisions and therefore the evolution of dust/ planetesimals in debris disks. The results of our study underline the importance of knowledge of the stellar photospheric flux, especially in the near- to mid-infrared wavelength range, for a proper analysis of debris disk SEDs: while the quality of subtraction of the direct stellar light at far-infrared wavelengths determines the accuracy of the mass estimate in the disk, our simulations show that the remaining stellar contribution due to scattering at near- to mid-infrared wavelengths constrains the dust grain size and chemical composition, for example, the iron abundance in silicate grains.