  We present observations of four T Tauri stars using long baseline infrared
interferometry from the Palomar Testbed Interferometer. The target sources, T
Tau N, SU Aur, RY Tau and DR Tau, are all known to be surrounded by dusty
circumstellar disks. The observations directly trace the inner regions (< 1 AU)
of the disk and can be used to constrain the physical properties of this
material. For three of the sources observed, the infrared emission is clearly
resolved. We first use geometric models to characterize the emission region
size, which ranges from 0.04 to 0.3 AU in radius. We then use Monte Carlo
radiation transfer models of accretion disks to jointly model the spectral
energy distribution and the interferometric observations with disk models
including accretion and scattering. With these models, we are able to reproduce
the data set with extended emission arising from structures larger than 10
milliarcseconds contributing less than 6% of the K band emission, consistent
with there being little or no envelope remaining for these Class II sources (d
log(lambda*F_lambda)/d log(lambda) ~ -2 to 0 in the infrared). The radiation
transfer models have inner radii for the dust similar to the geometric models;
however, for RY Tau emission from gas within the inner dust radius contributes
significantly to the model flux and visibility at infrared wavelengths. The
main conclusion of our modeling is that emission from inner gas disks (between
the magnetic truncation radius and the dust destruction radius) can be a
significant component in the inner disk flux for sources with large inner dust
radii.
