Christine CHEN : NOAO
Spitzer IRS Spectroscopy of IRAS-Discovered Debris
We have obtained Spitzer Space Telescope IRS 5.5 - 35 micron spectra of 59 main sequence stars that possess IRAS 60 micron excess. The spectra of five objects possess spectral features that are well-modeled using micron-sized grains and silicates with crystalline mass fractions 0% - 80%, consistent with T-Tauri and Herbig AeBe stars. Our fits require the presence of a cool black body continuum, Tgr = 80 - 200 K, in addition to hot, amorphous and crystalline silicates, Tgr = 290 - 600 K, suggesting that multiple parent body belts in debris disks, analogous to the asteroid and Kuiper belts in our solar system, may be more common than previously believed. The spectra for the majority of objects are featureless, suggesting that the emitting grains probably have radii a > 10 micron. We have modeled the excess continua using a continuous disk with a uniform surface density distribution, expected if Poynting-Robertson and stellar wind drag are the dominant grain removal processes, and using a single temperature black body, expected if the dust is located in a narrow ring around the star. The IRS spectra of many objects are better modeled with a single temperature black body, suggesting that the disks possess inner holes. The distribution of grain temperatures, based on our black body fits, peaks at Tgr = 110 - 120 K. Since the timescale for ice sublimation of micron-sized with Tgr > 110 K is a fraction of a Myr, the lack of warmer material may be explained if the grains are icy. If all of the debris disks in our sample are dynamically stirred by giant planets, we estimate that the majority of these systems possess parent body masses, MPB < 1 Mearth. The low inferred parent body mass and the ubiquity of inner holes suggest that planet formation may be an efficient process.Last modified: Sun Jul 9 17:33:11 2006