  We present new mass models for the gravitational lens system B1938+666, using
multi-wavelength data acquired from Keck adaptive optics (AO) and Hubble Space
Telescope (HST) observations. These models are the first results from the
Strong-lensing at High Angular Resolution Program (SHARP), a project designed
to study known quadruple-image and Einstein ring lenses using high-resolution
imaging, in order to probe their mass distributions in unprecedented detail.
Here, we specifically highlight differences between AO- and HST-derived lens
models, finding that -- at least when the lens and source galaxies are both
bright and red, and the system has a high degree of circular symmetry --
AO-derived models place significantly tighter constraints on model parameters.
Using this improved precision, we infer important physical properties about the
B1938+666 system, including the mass density slope of the lensing galaxy (gamma
= 2.045), the projected dark matter mass fraction within the Einstein radius
(M_dark/M_lens = 0.55), and the total magnification factor of the source galaxy
(~ 13). Additionally, we measure an upper-limit constraint on luminous
substructure (M_V > 16.2), based on the non-detection of bright satellite
galaxies in all data sets. Finally, we utilize the improved image resolution of
the AO data to reveal the presence of faint arcs outside of the primary
Einstein ring. The positions and orientations of these arcs raise the
intriguing possibility that B1938+666 has a second source galaxy, located at a
more distant redshift. However, future work is needed to verify this
hypothesis.
