A Self-Consistent Model of the Circumstellar Debris Created by a Giant Hypervelocity Impact in the Hd 172555 System

Abstract

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.

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

“…These were used to provide limits for the abundance of gas phase SiOfor a range of temperatures. The upper limit from our APEX detection, assuming an 8000 K primary star photospheric excitation, falls more than an order of magnitude below the self-shielding stability threshold derived by Johnson et al (2012). Our results thus favor a solid-state origin for the 8.3 μm feature seen in the Spitzer IRS spectrum of the circumstellar excess emissionand the production of circumstellar O I and Si I by SiO UV photolysis.…”

“…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). …”

“…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.…”

“…Such a torus should also contain an abundance of silicon and oxygen atoms. Johnson et al (2012) found that about 10 48 atoms of Si and O vapor would be needed to prevent the photodissociation of SiO. HD 172555 has been re-classified as an A6V-A7V star, which would lower the expected UV flux, and consequently lower the amount needed for stability by about a factor of two.…”

The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars

“…It also has a wide K5 companion, CD-64 1208, with a separation of 71.3" (2040 AU, Feigelson et al 2006). The disk of HD 172555 contains both dust and gas (Riviere-Marichalar et al 2012), possibly resulting from a recent impact event during planet formation (Johnson et al 2012). Our age for HD 172555 is 276 Myr, with a 68% confidence interval between 83 and 462 Myr.…”