ABUNDANT CIRCUMSTELLAR SILICA DUST AND SiO GAS CREATED BY A GIANT HYPERVELOCITY COLLISION IN THE ∼12 MYR HD172555 SYSTEM

Abstract

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.

CONSTRAINTS ON THE PRESENCE OF SiO GAS IN THE DEBRIS DISK OF HD 172555

“…While IR photometry of the system has remained stable since the 1983 IRAS mission, this steep size distribution, with abundant micron-sized particles, argues for a fresh source of material within the last 0.1 Myr (Lisse et al 2009;Johnson et al 2012). …”

“…Unlike most young debris disks, and debris disks in general, the dust surrounding the primary star of HD 172555 is peculiar. Lisse et al (2009) took Spitzer/IRS spectra with λ/Δλ=90, covering wavelengths λ=5-35 μm and combined these results with additional IR photometry to determine the spectral energy distribution from 5 to 100 μm. They showed that a large amount of fine dust with a temperature of 245 K was at 5.8 ± 0.6 au from the primary (Lisse et al 2007), consistent with the later measurements of Pantin &di Folco (2011) using "Lucky Imaging," andSmith et al (2012) using Very Large Telescope (VLT)/MIDI interferometry that showed that the circumstellar dust emission arises from an axisymmetric torus of radius 1-8 au (Smith et al 2012) (0 2 at 29.3 pc).…”

“…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 laboratory thermal emission spectra (nonequilibrium glassy silicas found in impact and magmatic systems) are required to fit the data (Lisse et al 2009). The best-fit model size distribution for the observed fine dust is a number-size distribution of = - dn da a .…”

“…In a re-analysis of the HD 172555 data, Johnson et al (2012) argued that the 5.8 au torus containing the ∼10 47 molecules of gas phase SiO reported by Lisse et al (2009) would have to be optically thick in the UV to provide self-shielding stability over decades for any circumstellar SiO gas, while being optically thin in the IRto allow detection by Spitzer but reducing any radiation pressure forces on the dust. In addition, from blended 8 μm features, Lisse et al (2009) reported that 10 47 SiO molecules were present in the gas phase and proposed that the most likely source of the fine dust and gas phase SiO was a massive hypervelocity impact between two rocky planetoids which liberated huge amounts of melted and reformed ejecta.…”

“…In addition, from blended 8 μm features, Lisse et al (2009) reported that 10 47 SiO molecules were present in the gas phase and proposed that the most likely source of the fine dust and gas phase SiO was a massive hypervelocity impact between two rocky planetoids which liberated huge amounts of melted and reformed ejecta. Such a torus should also contain an abundance of silicon and oxygen atoms.…”