Ab Aurigae resolved: Evidence for spiral structure

We have obtained high angular resolution (similar to 2 '') images of the (CO)-C-13 (J = 1 -> 0) line and 2.7 mm continuum emission and slightly lower resolution images of (CO)-C-12 (J = 1 -> 0) and (CO)-O-18 (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 degrees.5 and position angle 58 degrees.6. Using Keplerian disk models, we find a central-source dynamical mass of 2.8 +/- 0.2 M-circle dot and a cutoff radius of 615 AU for the (CO)-C-13 emission. The inclination, mass, and radius determined from (CO)-C-12 and (CO)-O-18 observations agree with these 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-infrared scattered-light images and show that the spiral arms represent density contrasts in the disk.