Context : Proto-planetary disks are thought to provide the initial environment for planetary system formation .
The dust and gas distribution and its evolution with time is one of the key elements in the process .

Aims : We attempt to characterize the radial distribution of dust in disks around a sample of young stars from an observational point of view , and , when possible , in a model-independent way , by using parametric laws .

Methods : We used the IRAM PdBI interferometer to provide very high angular resolution ( down to 0.4 ^ { \prime \prime } in some sources ) observations of the continuum at 1.3 mm and 3 mm around a sample of T Tauri stars in the Taurus-Auriga region .
The sample includes single and multiple systems , with a total of 23 individual disks .
We used track-sharing observing mode to minimize the biases .
We fitted these data with two kinds of models : a ” truncated power law ” model and a model presenting an exponential decay at the disk edge ( ” viscous ” model ) .

Results : Direct evidence for tidal truncation is found in the multiple systems .
The temperature of the mm-emitting dust is constrained in a few systems .
Unambiguous evidence for large grains is obtained by resolving out disks with very low values of the dust emissivity index \beta .
In most disks that are sufficiently resolved at two different wavelengths , we find a radial dependence of \beta , which appears to increase from low values ( as low as 0 ) at the center to about 1.7 – 2 at the disk edge .
The same behavior could apply to all studied disks .
It introduces further ambiguities in interpreting the brightness profile , because the regions with apparent \beta \approx 0 can also be interpreted as being optically thick when their brightness temperature is high enough .
Despite the added uncertainty on the dust absorption coefficient , the characteristic size of the disk appears to increase with a higher estimated star age .

Conclusions : These results provide the first direct evidence of the radial dependence of the grain size in proto-planetary disks .
Constraints of the surface density distributions and their evolution remain ambiguous because of a degeneracy with the \beta ( r ) law .