The fine dust detected by infrared (IR) emission around the nearby β Pic analog star HD172555 is very peculiar. The dust mineralogy is composed primarily of highly refractory, nonequilibrium materials, with approximately three quarters of the Si atoms in silica (SiO 2 ) species. Tektite and obsidian lab thermal emission spectra (nonequilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a −3.95±0.10 . While IR photometry of the system has stayed stable since the 1983 IRAS mission, this steep a size distribution, with abundant micron-sized particles, argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 ± 0.6 AU (equivalent to 1.9 ± 0.2 AU from the Sun), within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 × 10 19 -2 × 10 20 kg, equivalent to a 150-200 km radius asteroid. Significant emission features centered at 4 and 8 μm due to fluorescing SiO gas are also found. Roughly 10 22 kg of SiO gas, formed by vaporizing silicate rock, is also present in the system, and a separate population of very large, cool grains, massing 10 21 -10 22 kg and equivalent to the largest sized asteroid currently found in the solar system's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong stellar X-ray activity, and an extended disk of β meteoroids argues that the source of the observed circumstellar materials is a giant hypervelocity (>10 km s −1 ) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.

We present an analysis of the gas-poor circumstellar material in the HD 113766 binary system (F3/F5, 10 - 16 Myr), recently observed by the Spitzer Space Telescope. For our study we have used the infrared mineralogical model derived from observations of the Deep Impact experiment. We find the dust dominated by warm, fine (~1 um) particles, abundant in Mg-rich olivine, crystalline pyroxenes, amorphous silicates, Fe-rich sulfides, amorphous carbon, and colder water-ice. The warm dust material mix is akin to an inner main belt asteroid of S-type composition. The ~440 K effective temperature of the warm dust implies that the bulk of the observed material is in a narrow belt ~1.8 AU from the 4.4 L_solar central source, in the terrestrial planet-forming region and habitable zone of the system (equivalent to 0.9 AU in the solar system). The icy dust lies in 2 belts, located at 4-9 AU and at 30 - 80 AU. The lower bound of warm dust mass in 0.1 - 20 um, dn/da ~ a^-3.5 particles is very large, at least 3 x 10^20 kg, equivalent to a 320 km radius asteroid of 2.5 g cm^-3 density. Assuming 10m largest particles present, the lower bound of warm dust mass is at least 0.5 M_Mars The dust around HD 113766A originates from catastrophic disruption of terrestrial planet embryo(s) and subsequent grinding of the fragments, or from collisions in a young, extremely dense asteroid belt undergoing aggregation. The persistence of the strong IR excess over the last two decades argues for a mechanism to provide replenishment of the circumstellar material on yearly timescales.Comment: 46 Pages, 7 Figures, 3 Tables; accepted 07 Sept 2007 for publication in Ap

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We have analyzed Spitzer and NASA/IRTF 2-35 μm spectra of the warm, ∼350 K circumstellar dust around the nearby MS star η Corvi (F2V, 1.4 ± 0.3 Gyr). The spectra show clear evidence for warm, water-and carbon-rich dust at ∼3 AU from the central star, in the system's terrestrial habitability zone. Spectral features due to ultraprimitive cometary material were found, in addition to features due to impact produced silica and high-temperature carbonaceous phases. At least 9 × 10 18 kg of 0.1-100 μm warm dust is present in a collisional equilibrium distribution with dn/da ∼ a −3.5 , the equivalent of a 130 km radius Kuiper Belt object (KBO) of 1.0 g cm 3 density and similar to recent estimates of the mass delivered to the Earth at 0.6-0.8 Gyr during the late-heavy bombardment. We conclude that the parent body was a Kuiper Belt body or bodies which captured a large amount of early primitive material in the first megayears of the system's lifetime and preserved it in deep freeze at ∼150 AU. At ∼1.4 Gyr they were prompted by dynamical stirring of their parent Kuiper Belt into spiraling into the inner system, eventually colliding at 5-10 km s −1 with a rocky planetary body of mass M Earth at ∼3 AU, delivering large amounts of water (>0.1% of M Earth's Oceans) and carbon-rich material. The Spitzer spectrum also closely matches spectra reported for the Ureilite meteorites of the Sudan Almahata Sitta fall in 2008, suggesting that one of the Ureilite parent bodies was a KBO.

Spectral modeling of the large infrared excess in the Spitzer IRS spectra of HD 172555 suggests that there is more than 10 19 kg of sub-micron dust in the system. Using physical arguments and constraints from observations, we rule out the possibility of the infrared excess being created by a magma ocean planet or a circumplanetary disk or torus. We show that the infrared excess is consistent with a circumstellar debris disk or torus, located at ~ 6 AU, that was created by a planetary scale hypervelocity impact. We find that radiation pressure should remove submicron dust from the debris disk in less than one year. However, the system's mid-infrared photometric flux, dominated by submicron grains, has been stable within 4% over the last 27 years, from IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of the radiation pressure on fine dust in HD 172555 provide a self-consistent explanation for this apparent contradiction. We also explore the unconfirmed claim that ~10 47 molecules of SiO vapor are needed to explain an emission feature at ~8 m in the Spitzer IRS spectrum of HD 172555. We find that unless there are ~10 48 atoms or 0.05 of atomic Si and O vapor in the system, SiO vapor should be destroyed by photo-dissociation in less than 0.2 years. We argue that a second plausible explanation for the ~8 m feature can be emission from solid SiO, which naturally occurs in submicron silicate "smokes" created by quickly condensing vaporized silicate.