Resolving the inner dust disks surrounding LkCa 15 and MWC 480 at mm wavelengths

Aims. We constrain the dust distribution and its properties (temperature, emissivity) in inner proto-planetary disks Methods. We performed sub-arcsecond high-sensitivity interferometric observations of the thermal dust emission at 1.4mm and 2.8mm in the disks surrounding LkCa15 and MWC480, with the new 750 m baselines of the IRAM PdBI array. This provides a linear resolution of similar to 60 AU at the distance of Taurus. Results. We report the existence of a cavity of similar to 50 AU radius in the inner disk of LkCa15. Whereas LkCa15 emission is optically thin, the optically thick core of MWC480 is resolved at 1.4 mm with a radius of similar to 35 AU, constraining the dust temperature. In MWC480, the dust emission is coming from a colder layer than the CO emission, most likely the disk mid-plane. Conclusions. These observations provide direct evidence of an inner cavity around LkCa15. Such a cavity most probably results from the tidal disturbance created by a low-mass companion or large planet at similar to 30 AU from the star. These results suggest that planetary system formation is already at work in LkCa15. They also indicate that the classical steady-state viscous disk model is too simplistic a description of the inner 50 AU of proto-planetary disks and that the disk evolution is coupled to the planet formation process. The MWC480 results indicate that a proper estimate of the dust temperature and size of the optically thick core are essential for determining the dust emissivity index beta.