Caer McCabe

Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars

We present a high spatial resolution, 10-20 μm survey of 65 T Tauri binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks around the individual stellar components in these binary systems, this study increases the number of binaries with spatially resolved measurements at 10 μm by a factor of ~5. Combined with resolved near-infrared photometry and spectroscopic accretion diagnostics, we find that ~10% of stars with a mid-infrared excess do not appear to be accreting. In contrast to an actively accreting disk system, these passive disks have significantly lower near-infrared colors that are, in most cases, consistent with photospheric emission, suggesting the presence of an inner disk hole. In addition, there appears to be a spectral type/mass dependence associated with the presence of a passive disk, with all passive disks occurring around M-type stars. The presence of a passive disk does not appear to be related to the fact that these objects are in visual binary systems; the passive disk systems span the entire range of binary separations present in the sample, and a similar fraction of passive disks is observed in a sample of single stars. The possibility that the passive disks are caused by the presence of an as yet undetected companion at a small separation (0.3-3 AU) is possible for any individual system; however, it cannot account for the spectral type dependence of the passive disk sample as a whole. We propose that these passive disks represent a subset of T Tauri stars that are undergoing significant disk evolution. The fraction of observed passive disks and the observed spectral type dependence can both be explained by models of disk evolution that include disk photoevaporation from the central star.

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 ~1.5% of the stellar flux, with a near-side-to-far-side flux ratio of ~1.4, independent of wavelength, and colors that are comparable to the central source [Δ = 0.10 ± 0.03, Δ = -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 μm. 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 1 σ range on the disk inner edge-to-semimajor axis ratio, 3.2 < Rin/a < 6.7, is larger than that estimated by previous smoothed particle hydrodynamic simulations of binary-disk interactions.

The Circumstellar Environment of T Tauri S at High Spatial and Spectral Resolution

We have obtained the first high spatial (005) and spectral (R ~ 35,000) resolution 2 ?m spectrum of the T Tau S tight binary system using adaptive optics on the Keck II telescope. We have also obtained the first 3.8 and 4.7 ?m images that resolve the three components of the T Tau multiple system, as well as new 1.6 and 2.2 ?m images. Together with its very red near-infrared colors, the spectrum of T Tau Sb shows that this T Tauri star is extincted by a roughly constant extinction of AV ~ 15 mag, which is probably the 07 ? 05 circumbinary structure recently observed in absorption in the ultraviolet. T Tau Sa, which is also observed through this screen and is actively accreting, further possesses a small edge-on disk that is evidenced by warm (390 K), narrow overtone CO rovibrational absorption features in our spectrum. We find that T Tau Sa is most likely an intermediate-mass star surrounded by a semitransparent 2-3 AU radius disk whose asymmetries and short Keplerian rotation explain the large photometric variability of the source on relatively short timescales. We also show that molecular hydrogen emission exclusively arises from the gas that surrounds T Tau S and that its spatial and kinematic structure, while providing suggestive evidence of a jetlike structure, is highly complex.