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HR4796A is surrounded by a debris disc, observed in scattered light as aninclined ring. Past observations raised several questions. First, a strongbrightness asymmetry detected in polarized reflected light recently challengedour understanding of scattering by the dust particles in this system. Secondly,the morphology of the ring strongly suggests the presence of planets, althoughno planets have been detected to date. We obtained high-angular resolutioncoronagraphic images of the circumstellar environment around HR4796A withVLT/SPHERE during the commissioning of the instrument in May 2014 and duringguaranteed-time observations in February 2015. The observations reveal for thefirst time the entire ring of dust, including the semi-minor axis that waspreviously hidden either behind the coronagraphic spot or in the speckle noise.We determine empirically the scattering phase function of the dust in the Hband from 13.6deg to 166.6deg. It shows a prominent peak of forward scattering,never detected before, for scattering angles below 30deg. We analyse thereflectance spectra of the disc from the 0.95 to 1.6 microns, confirming thered colour of the dust, and derive detection limits on the presence ofplanetary mass objects. We confirm which side of the disc is inclined towardsthe Earth. The analysis of the phase function suggests that the dust populationis dominated by particles much larger than the observation wavelength, of about20 microns. Compact Mie grains of this size are incompatible with the spectralenergy distribution of the disc, however the observed rise in scatteringefficiency beyond 50deg points towards aggregates which could reconcile bothobservables. We do not detect companions orbiting the star but ourhigh-contrast observations provide the most stringent constraints yet on thepresence of planets responsible for the morphology of the dust.

Near-infrared scattered light properties of the HR4796A dust ring: a measured scattering phase function from 13.6deg to 166.6deg