  We obtained high angular resolution (~2") images of the 13CO(J=1-0) line and
2.7 millimeter continuum emission, and slightly lower resolution images of
12CO(J=1-0) and C18O(J=1-0) line emission toward the Herbig Ae star AB Aurigae.
We resolve a circumstellar disk of diameter 780 AU (FWHM) with a velocity
pattern consistent with a purely rotational disk at inclination 21.5 degrees
and position angle 58.6 degrees. Using Keplerian disk models, we find a central
source dynamical mass of 2.8+-0.1 Msun and a cutoff radius of 615 AU for the
13CO emission. Inclination, mass, and radius determined from 12CO and C18O
observations agree with those values, given optical depth and abundance
effects. As a result of the high angular resolution of our observations, we
confirm the existence of spiral structure suggested by near-IR scattered light
images and show that the spiral arms represent density contrasts in the disk.
