High Temperature Silica in Protoplanetary Disks


First Author:
WilliamJ. Forrest
Email: forrest AT pas.rochester.edu
Univ. of Rochester
Dept. Physics Astronomy, U. Rochester
Rochester, NY 14637 USA
Coauthors:
Sargent, B.A., Univ. of Rochester
Tayrien, C., Univ. of Rochester
McClure, M.K., Univ. of Rochester
Li, A., Univ. of Missouri, Columbia
Basu, A.R., Univ. of Rochester
Manoj, P., Univ. of Rochester
Watson, D.M., Univ. of Rochester
Bohac, C.J., Univ. of Rochester
Furlan, E., JPL
Kim, K.H., Univ. of Rochester
Green, J.D., Univ. of Rochester
Sloan, G.C., Cornell Univ.

Abstract
Silica is identified in the spectra of protplanetary accretion disks through its prominent emission features at 9.1, 12.6, and 20 microns. Silica is largely absent from the ISM (and spectra of many protoplanetary disks), so it must have been produced locally. Various polymorphs of silica are possible, each has a distinctive infrared spectrum. We fit the spectra of five T Tauri stars from the Taurus, Ophiuchus, and Chamaeleon clouds which show prominent silica emission features in high quality IRS spectra from Spitzer. We developed a simple, 2 temperature, ``reversing layer'' physical model to analyze the emission features arising from the optically thin, superheated upper layers of accretion disks. Our models include emission from small and large amorphous silicate grains, small forsterite and enstatite grains, and small silica grains (alpha and beta quartz, amorphous quartz, and annealed silica). In every case, the best fit was given by the ``annealed silica'' produced by Fabian et al. (2000) by baking silica at 1220 K for 5 hours and found to be mostly Cristobalite with some indications of Tridymite. We conclude that these high temperature forms of silica have been produced in these protoplanetary disks. Perhaps even more significant than the high formation temperature is the implied cooling history of these grains. The high temperature silica polymorph must be cooled rapidly enough to "quench" the metastable structure, otherwise it would revert to the lower temperature alpha and beta quartz polymorphs. This argues for localized, transient processing in regions where the equilibrium, steady state temperatures were well below 845 K. It is interesting that the silica grain "Ada" found in the dust from the comet Wild 2 returned by the Stardust mission was Tridymite (Zolensky et al. 2006), a high temperature polymorph.
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