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D. H. Wooden, K. R. Bell, D. E. Harker (NASA Ames), C. E. Woodward (U. Minnesota)
Our HIFOGS (HI-efficiency Faint Object Grating Spectrometer, Witteborn et al.~1991) 10~\micron \ multi-epoch temporal studies have discovered variable dust features in the highly active T Tauri star DG Tauri. The observed dust variability occurs on time scales of months to years while the underlying dust continuum remains relatively constant. Prior to 1996, DG Tau had a featureless 10~\micron \ spectrum. In 1996, a silicate emission feature appeared. A year later, the silicate feature was still present but the shape of the feature had changed. The mineralogy deduced from the silicate emission features indicate that in 1996 amorphous and crystalline forms of olivine and pyroxene were present. By September 1997, the mineralogy was dominated by Mg-rich crystalline olivine. The HIFOGS multi-epoch spectra of DG Tau may be the first observational evidence for in situ crystallization of amorphous silicates in a protoplanetary disk system. Crystallization processes (Hallenbeck et al.~1998) have been suggested as a source of cometary crystals (Irvine et al.~1998) and as the cause of variation in the degree of crystallinity detected amongst Herbig Ae/Be stars by ISO SWS (Nuth 1999; Grady et al.~1999).
DG Tau's silicate feature continued to evolve, diminishing in contrast to the continuum in November 1997, and going into absorption in September 1998. DG Tau's silicate feature continues to be in absorption in April 2000, although its shape has evolved. Photometric monitoring of the 1~--~23~\micron \ spectral energy distribution (SED) suggests changes in disk structure or opacity accompany the changes in silicate mineralogy. The activity in the silicate emission feature preceded the drop in visual magnitude that was concurrent with the silicate feature going into absorption. Our HIFOGS observations motivate a scenario where changing silicate features are attributed to dust in disk atmospheres and continua are attributed to the underlying disk photospheres.
We acknowledge support from NASA, NRC, and NSF.
The author(s) of this abstract have provided an email address for comments about the abstract: wooden@delphinus.arc.nasa.gov