Discovery of an azimuthal density gradient in a gas-rich debris disk possibly related to a massive collision

The gas-rich debris disk of HD 141569, first discovered in 1999 with the HST in near-IR and later in visible revealed structures such as multiple rings and outer spirals extended as far out as 410 AU. More recently, the direct imaging SPHERE instrument has resolved several non-uniform concentric rings inside the inner cavity (<100 AU) in near-IR. A North-South asymmetry was discovered in the brightest and innermost ringlets at 40 AU, which is aligned with the disk projected major axis. This observed asymmetry cannot be explained by the light scattering properties of dust. We proposed instead an azimuthal variation of the dust density. However, this interpretation is complicated by the post-processing techniques commonly used for scattered light disk observations in total intensity, which particularly impact the shape and local photometry of extended objects. In 2017, we have acquired polarimetric data using the dual polarimetric imaging mode of SPHERE/IRDIS. We discovered that the Lorentzian azimuthal distribution of the intensity reported in the innermost ring in Perrot et al. (2016) is significantly different in total intensity and polarized intensity. Assuming a model based on the massive collisions between planet embryos, we found that both images can be described as a combination of a phase function and an azimuthal density variation which takes a Lorentzian profile peaking to the south-west of the ring. The complementarity of polarimetric and total intensity images has allowed us to constrain the actual dust density distribution and to relate this morphology to a potential massive collision. In this talk, I will introduce HD141569 followed by the imaging modes of SPHERE and the disk structure observed within 100 AU both in total and polarimetric intensity. I will then present the hypothesis that observed asymmetries are produced by an azimuthal variation of the dust density, possibly connected to massive collisions.