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Paper text:
a small sample of disks , the transition disks , shows a peculiar dip at infrared wavelengths , suggesting a depletion of warm dust around the central star ( ( * ? ? ?
* strom et al . 1989 ) ) .
disk@xmath1companion interaction @xcite , photoevaporation @xcite , and particle growth @xcite are possible clearing processes .
polarimetric differential imaging ( pdi ) is allowing high - resolution imaging of circumstellar disks ( e.g. * ? ? ?
* , * ? ? ?
* ) with unprecedented inner working angle ( @xmath3 ) .
recently , comparisons of pdi images with sub - millimeter images ( e.g. * ? ? ?
* ) have revealed different spatial distribution for small and large dust grains ( see e.g.  * ? ? ?
pdi observations of the herbig ae / be sao 206462 ( hd 135344b ) were obtained with the high - resolution naco ( * ? ? ? * , * ? ? ?
* ) at the very large telescope ( vlt ) , in @xmath4 and @xmath5 band .
the basic principle of pdi is the simultaneous imaging of the linear polarization of the source along two orthogonal directions .
the detailed observation setting and data reduction can be found in * ? ? ? * and avenhaus et al .
( in prep . ) .
the final products are radial stokes @xmath6 parameter images of the source described in garufi et al .
( in prep . ) and shown in fig.[images ]
. the disk is revealed in scattered light in both bands .
the images show three main peculiarities : an inner cavity ( inside @xmath7 ) with light depleted by a factor down a few tenths , a quasi - circular rim surrounding the cavity , and two spiral arms extending from the rim outward
.    apart from the spiral structure ( probably due to a companion orbiting at large scale , ( * ? ? ?
* muto et al . 2012 ) ) , the most tantalizing aspect suggested by these images is the different cavity size of small grains ( 28 au ) with respect to large grains ( 39 au , ( * ? ? ?
* brown et al .
similar discrepancies were recently pointed - out by ( * ? ? ? * dong et al .
( 2012 ) ) .
the cavity for small grains is likely to be due to tidal interaction with a companion .
photoevaporation and dust grain growth are indeed ruled - out : because of high accretion rate ( ( * ? ? ?
* sitko et al . 2012 ) ) and sub - au inner dust belt ( ( * ? ? ?
* fedele et al . 2008 ) ) the former , abrupt radial profile ( this work ) and absence of mm - size grains ( ( * ? ? ?
* brown et al . 2009 ) ) the latter .
the scenario with clearing by a companion orbiting inside the cavity may also explain the observed dissimilarity in the cavity sizes .
in fact , the pile - up of large grains due to a giant planet can occur at up to 10 tidal radii ( ( * ? ? ?
* pinilla et al . 2012 ) ) , whereas the outer edge of the gaseous halo can not exceed 5 tidal radii ( ( * ? ? ?
* dodson - robinson & salyk 2011 ) ) .
we suggest a scenario ( see fig.[sketch ] ) where a giant planet is generating a pressure bump at 39 au , which holds back mm - size grains but allows @xmath0m - size grains to be dragged inward along with the gas as down as 28 au .
non - keplerian flows of gas and small dust grains can still be present in the cavity , thus to sustain the inner dust belt and the high accretion rate of the source .
we analytically find that a 5 to 15 @xmath8 at 17 to 20 au is consistent with the observed cavity sizes .
however , a multiple - planets system is not ruled - out .
dissimilarities in the spatial distribution of small ( @xmath0m@xmath1size ) and large ( mm@xmath1size ) dust grains at the cavity edge of transition disks have been recently pointed out and are now under debate . 
 we obtained vlt / naco near - ir polarimetric observations of sao 206462 ( @xmath2 ) . 
 the disk around the star shows very complex structures , such as dips and spirals . 
 we also find an inner cavity much smaller than what inferred from sub - mm images . 
 the interaction between disk and orbiting companion(s ) may explain this discrepancy .