Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths:: a non Keplerian disk

We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. These observations allow us to trace the structure of the circumstellar material of AB Aur in regions where optical and IR imaging is impossible because of the emission from the star. These images reveal that the environment of AB Aur is widely different from the proto-planetary disks that surround T Tauri stars like DM Tau and LkCa15 or HAeBe stars like MWC 480 in several aspects. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral- like) feature at about 140 AU from the central star. Little emission is associated with the star itself. The molecular emission shows that AB Aur is surrounded by a very extended flattened structure (disk), which is rotating around the star. Bright molecular emission is also found towards the continuum spiral. The large-scale molecular structure suggests the AB Aur disk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. Analysis of the emission in terms of a Keplerian disk provides a reasonable fit to the data, but fails to give a consistent picture because the inclinations determined from (CO)-C-12 J = 2 -> 1, (CO)-C-13 J = 2 -> 1, (CO)-C-13 J = 1 -> 0 and (CO)-O-18 J = 1 -> 0 do not agree. The mass predicted for the central star in such Keplerian models is in the range 0.9 to 1.2 M-circle dot, much smaller than the expected 2.2 M-circle dot from the spectral type of AB Aur. Better and more consistent fits to the (CO)-C-13 J = 2 -> 1, (CO)-C-13 J = 1 -> 0 data are obtained by relaxing the Keplerian hypothesis. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial motion. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest that the dust is less evolved than in typical T Tauri disks. Both the spiral- like feature and the departure from purely Keplerian motion indicates the AB Aur disk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.