NiCMOS images of the GG Tauri circumbinary disk

We present deep, near-infrared images of the circumbinary disk surrounding the pre-main-sequence binary star GG Tau A, obtained with NICMOS aboard the Hubble Space Telescope. The spatially resolved protoplanetary disk scatters similar to1.5% of the stellar flux, with a near-side to far-side flux ratio of similar to1.4, independent of wavelength, and colors that are comparable to the central source [DeltaM(F110W) - M-F160W) = 0.10 +/- 0.03, Delta(M-F160W-M-F205W) = -0.04 +/- 0.06]; all of these properties are significantly different from the earlier ground-based observations. New Monte Carlo scattering simulations of the disk emphasize that the general properties of the disk, such as disk flux, near-side-to-far-side flux ratio, and integrated colors, can be approximately reproduced using interstellar medium like dust grains, without the presence of either circumstellar disks or large dust grains, as had previously been suggested. A single-parameter phase function is fitted to the observed azimuthal variation in disk flux, providing a lower limit on the median grain size of a > 0.23 mum. Our analysis, in comparison to previous simulations, shows that the major limitation to the study of grain growth in T Tauri disk systems through scattered light lies in the uncertain interstellar medium dust grain properties. Without explicit determination of the scattering properties, it is not possible to differentiate between geometric, scattering, and evolutionary effects. Finally, we use the 9 yr baseline of astrometric measurements of the binary to solve the complete orbit, assuming that the binary is coplanar with the circumbinary ring. We find that the estimated 1sigma range on the disk inner edge to semimajor axis ratio, 3.2 < R-in/a < 6.7, is larger than that estimated by previous smoothed particle hydrodynamic simulations of binary-disk interactions.