Abstract
HR4796 is a young, early A-type star harbouring a well structured debris
disk, shaped as a ring with sharp inner edges. It forms with the M-type star
HR4796B a binary system, with a proj. sep. ~560 AU. Our aim is to explore the
surroundings of HR4796A and B, both in terms of extended or point-like
structures. Adaptive optics images at L'-band were obtained with NaCo in
Angular Differential Mode and with Sparse Aperture Masking (SAM). We analyse
the data as well as the artefacts that can be produced by ADI reduction on an
extended structure with a shape similar to that of HR4796A dust ring. We
determine constraints on the presence of companions using SAM and ADI on
HR4796A, and ADI on HR4796B. We also performed dynamical simulations of a disk
of planetesimals and dust produced by collisions, perturbed by a planet located
close to the disk outer edge. The disk ring around HR4796A is well resolved. We
highlight the potential effects of ADI reduction of the observed disk shape and
surface brightness distribution, and side-to-side asymmetries. No planet is
detected around the star, with masses as low as 3.5 M_Jup at 0.5" (58 AU) and
less than 3 M_Jup in the 0.8-1" range along the semi-major axis. We exclude
massive brown dwarfs at separations as close as 60 mas (4.5 AU) from the star
thanks to SAM data. The detection limits obtained allow us to exclude a
possible close companion to HR4796A as the origin of the offset of the ring
center with respect to the star; they also allow to put interesting constraints
on the (mass, separation) of any planet possibly responsible for the inner disk
steep edge. Using detailed dynamical simulations, we show that a giant planet
orbiting outside the ring could sharpen the disk outer edge and reproduce the
STIS images published by Schneider et al. (2009).
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