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We investigate the physical structure of the gas component of the disk aroundthe pre-main-sequence star HD169142. The 13CO and C18O J=2-1 line emission isobserved from the disk with 1.4'' resolution using the Submillimeter Array. Weadopt the disk physical structure derived from a model which fits the spectralenergy distribution of HD169142. We obtain the full three-dimensionalinformation on the CO emission with the aid of a molecular excitation andradiative transfer code. This information is used for the analysis of ourobservations and previous 12CO J=2-1 and 1.3 mm continuum data. The disk is inKeplerian rotation and seen at an inclination close to 13 deg from face-on. Weconclude that the regions traced by different CO isotopologues are distinct interms of their vertical location within the disk, their temperature and theircolumn densities. With the given disk structure, we find that freeze-out is notefficient enough to remove a significant amount of CO from gas phase. Bothobserved lines match the model prediction both in flux and in the spatialstructure of the emission. Therefore we use our data to derive the 13CO andC18O mass and consequently the 12CO mass using standard isotopic ratios. Weconstrain the total disk gas mass to (0.6-3.0)x10(-2) Msun. Adopting a maximumdust opacity of 2 cm2 per gram of dust we derive a minimum dust mass of2.16x10(-4) Msun from the fit to the 1.3 mm data. Comparison of the derived gasand dust mass shows that the gas to dust mass ratio of 100 is only possibleunder the assumption of a dust opacity of 2 cm2/g and 12CO abundance of 10(-4)with respect to H2. However, our data are also compatible with a gas to dustratio of 25, with a dust opacity of 1 cm2/g and 12CO abundance of 2x10(-4).

Gas and dust mass in the disk around the Herbig Ae star HD169142