Silica in Protoplanetary Disks

Abstract

Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope show prominent narrow emission features indicating silica (crystalline silicon dioxide). Silica is not a major constituent of the interstellar medium; therefore, any silica present in the circumstellar protoplanetary disks of TTS must be largely the result of processing of primitive dust material in the disks surrouding these stars. We model the silica emission features in our spectra using the opacities of various polymorphs of silica and their amorphous versions computed from earth-based laboratory measurements. This modeling indicates that the two polymorphs of silica, tridymite and cristobalite, which form at successively higher temperatures and low pressures, are the dominant forms of silica in the TTS of our sample. These high temperature, low pressure polymorphs of silica present in protoplanetary disks are consistent with a grain composed mostly of tridymite named Ada found in the cometary dust samples collected from the STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks may arise from incongruent melting of enstatite or from incongruent melting of amorphous pyroxene, the latter being analogous to the former. The high temperatures of ∼ 1200K-1300 K and rapid cooling required to crystallize tridymite or cristobalite set constraints on the mechanisms that could have formed the silica in these protoplanetary disks, suggestive of processing of these grains during the transient heating events hypothesized to create chondrules.

Transient dust in warm debris disks

“…Additionally, it is possible that other polymorphs (e.g., non-crystalline silica) are present in the debris disk around HD 15407 A, but inferring their presence is a degenerate problem given the strong similarities between the emissivity profiles, especially for the strongest emission feature, at about 9 μm. The β-cristobalite form of silica we used is close to the annealed silica studied by Fabian et al (2000), which is the result of an annealing experiment, for 5 h at a temperature of 1220 K. This indicates that the grains have experienced quite high temperatures, but as discussed in Sargent et al (2009a), once formed, cristobalite grains must be cooled quickly enough to retain their crystalline structure. If not, the grains are expected to revert to the lower-temperature polymorphs of silica (β-and α-quartz).…”

“…6 (as well as Fig. 2 to 4 of Sargent et al 2009a), β-cristobalite (annealed silica) is one of the few polymorphs of SiO 2 that display an emission feature at 16 μm (coesite also displays a similar emission feature at these wavelengths). Therefore, it is difficult to firmly conclude on the exact nature of the SiO 2 polymorph that we detected in the disk around HD 15407 A, but we can narrow down possible polymorphs either to β-cristobalite or coesite.…”

“…Several polymorphs of silica have been studied in detail by Sargent et al (2009a). According to these authors, spectroscopic observations of protoplanetary disks are best reproduced by annealed silica, which contains β-cristobalite.…”

“…Sargent and collaborators used the KBr pellet transmission measurements from Fabian et al (2000), in which β-cristobalite was formed by thermal annealing of SiO 2 during 5 h at a temperature of 1220 K. This motivated our choice for using this particular polymorph to represent silica grains. However, even if Sargent et al (2009a) discussed the possible influence of the KBr medium before concluding it should not have altered the band positions too much, we prefer to use aerosol measurements from Tamanai (2010) to avoid any possible contamination by the pellet medium. The maximum grain size inside the aerosol chamber is again 1 μm.…”

A mid-IR study of the circumstellar environment of Herbig Be stars

“…Rietmeijer et al 1986;Fabian et al 2000). In this scenario, enstatite (MgSiO 3 ), silica (SiO 2 ), and forsterite (Mg 2 SiO 4 ) are connected by the chemical equilibrium reaction Sargent et al (2009) suggested that in protoplanetary disks this reaction takes place via incongruent melting. In this context it is very interesting that we do not find such a correlation between silicate and forsterite for the HBe stars (see Fig.…”