Spitzer IRS Spectroscopy of IRAS ‐discovered Debris Disks

We have obtained Spitzer Space Telescope IRS 5.5 - 35 micron spectra of 59main sequence stars that possess IRAS 60 micron excess. The spectra of fiveobjects possess spectral features that are well-modeled using micron-sizedgrains and silicates with crystalline mass fractions 0% - 80%, consistent withT-Tauri and Herbig AeBe stars. With the exception of eta Crv, these objects areyoung with ages <50 Myr. The spectra for the majority of objects arefeatureless, suggesting that the emitting grains probably have radii a > 10micron. We have modeled the excess continua using a continuous disk with auniform surface density distribution, expected if Poynting-Robertson andstellar wind drag are the dominant grain removal processes, and using a singletemperature black body, expected if the dust is located in a narrow ring aroundthe star. The IRS spectra of many objects are better modeled with a singletemperature black body, suggesting that the disks possess inner holes. Thedistribution of grain temperatures, based on our black body fits, peaks at Tgr= 110 - 130 K. Since the timescale for ice sublimation of micron-sized grainswith Tgr > 110 K is a fraction of a Myr, the lack of warmer material may beexplained if the grains are icy. If planets dynamically clear the centralportions of debris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systems possessparent body masses, MPB < 1 Mearth. The low inferred parent body masses suggestthat planet formation is an efficient process. (abridged abstract)Comment: 66 pages, 18 figures (including 12 color figures), ApJS, in pres