2011A&A...533A.135V

The presence of unresolved binaries on sub-arsecond scales could explain the existence of optically thin inner holes or gaps in circumstellar disks, which are commonly referred to as ``transitional'' or ``cold'' disks, and it is the first scenario to check before making any other assumptions. We aim at detecting the presence of companions inside the inner hole/gap region of a sample of five well known transitional disks using spatially-resolved imaging in the near-IR with the VLT/NACO/S13 camera, which probes projected distances from the primary of typically 0.1 to 7-arcsec. The sample includes the stars DoAr 21, HD135344B (SAO206462), HR4796A, T Cha, and TW Hya, spanning ages of less than 1 to 10Myr, spectral types of A0 to K7, and hole/gap outer radii of 4 to 100AU. In order to enhance the contrast and to avoid saturation at the core of the point-spread function (PSF), we use narrow-band filters at 1.75 and 2.12µm. The ``locally optimized combination of images'' (LOCI) algorithm is applied for an optimal speckle noise removal and PSF subtraction, providing an increase of 0.5-1.5mag in contrast over the classic method. With the proviso that we could have missed companions owing to unfavorable projections, the VLT/NACO observations rule out the presence of unresolved companions down to an inner radius of about 0.1" from the primary in all five transitional disks and with a detection limit of 2 to 5mag in contrast. In the disk outer regions the detection limits typically reach 8 to 9mag in contrast and 4.7mag for T Cha. Hence, the NACO images resolve part of the inner hole/gap region of all disks with the exception of TWHya, for which the inner hole is only 4 AU. The 5σ sensitivity profiles, together with a selected evolutionary model, allow to discard stellar companions within the inner hole/gap region of T Cha, and down to the substellar regime for HD135344B and HR4796A. DoAr21 is the only object from the sample of five disks for which the NACO images are sensitive enough for a detection of objects less massive than ∼13M_Jup that is, potential giant planets or low-mass brown dwarfs at radii larger than ∼76AU (0.63"). These new VLT/NACO observations further constrain the origin of the inner opacity cavities to be owing to closer or lower-mass companions or other mechanisms such as giant planet formation, efficient grain growth, and photoevaporation (for DoAr21 and HR4796A).