Spitzer IRS Spectroscopy of IRAS-Discovered Debris Disks Christine H. Chen (NOAO) IRS Disks Team astro-ph/

Mid- to Far-Infrared Spectra of Dust Debris Around Main Sequence Stars IRAS observations discovered more than 100 main sequence stars with unresolved excess and grain temperatures (T gr = 50 – 125 K), similar to the Kuiper Belt in our Solar System, and fractional infrared luminosities (L IR /L * = – ) Dust grain lifetimes are shorter than the ages of the systems suggesting that the material is replenished from a reservoir such as collisions between parent bodies or sublimation of comets. These objects typically have F (10 m) < 1 Jy, making the majority of systems too faint to be studied spectroscopically from the ground. IRS 5.5 – 35 m spectroscopy of 59 main sequence stars with IRAS 60 m excess. (Observations of the first 19 objects observed are published in Jura et al. 2004)

Single Temperature Black Body Fits SED modeling suggests that the dust is located in a thin ring which can be modeled assuming a single temperature distribution

Are Circumstellar Dust Grains Icy? Sublimation temperature of water in a vacuum is T sub = 150 K. The grain temperatures inferred from black body fits to the infrared excess peak at 110 – 130 K Sublimation lifetimes are a sensitive function of grain temperature. For example, dust grains with a = 3.5 micron and T gr = 70 K, have a lifetime of Gyr (!) while grains with a = 16 m and T gr = 160 K have a lifetime of 7.4 minutes.

Observed Decay of Fractional Infrared Luminosity in Debris Disks Sample The upper envelope of the relation between fractional infrared luminosity and age can be fit with a 1/t power law. The 1/t 2 power law does not produce a bad fit (only η Crv is inconsistent). The scaling factor for our fit constrains (L IR /L * ) o t o =0.4 Myr or (L IR /L * ) o t o 2 =60 Myr 2

Collisional Cascades in Planetesimal Disks In a minimum mass solar nebula, 1000 – 3000 km-sized bodies are expected to grow on timescales, t P = 15 – 20 Myr (D/30 AU) 3 (Kenyon & Bromley 2004) If debris disks are self-stirred by forming planets and if dust is generated in collisional cascades, then an outward propagating ring of dust emission should be observed.

Silicate Emission Features Predominantly associated with intermediate-age disks with ages <50 Myr 80% of the systems observed may possess crystalline silicates Warm Dust Component (T gr = 290 K – 600 K): silicate emission features that are well-fit using large grains (radii above the blow- out limit) Cool Dust Component T gr = 80 K –200 K): single temperature black bodies (required to fit the remaining continuum Multiple parent body belts may exist around these objects

HR 7012, η Tel, HD , and η Crv

Conclusions 1.The majority of observed debris disks do not possess spectral features, suggesting that their grains are too cool and/or too large (a > 10 μm) to produce spectral features. Detailed modeling of objects with spectral features requires the presence of large, warm, amorphous silicates with T gr = 290 – 600 K, in addition to cool black bodies with T gr = 80 – 200 K, and the presence of crystalline silicate mass ratios 0-76%. 2.The IRS spectra of debris disks (without spectral features) are generally better fit using a single temperature black body than with a uniform disk. Stellar radiation pressure (in collisionally dominated systems), sublimation if the grains are icy, gas drag, and/or the presence of a perturbing body may contribute to the presence of inner holes in these disks. 3.The peak in the estimated black body grain temperatures T gr = 110 – 120K suggests that sublimation of icy grains may produce the central clearings observed. 4.Since parent body masses typically are less than the mass of the Earth, it appears that planet formation efficiently consumes most of the mass of the primordial disk.