NaCo polarimetric observations of Sz 91 transitional disk: a remarkable case of dust filtering

Abstract

We present polarized light observations of the transitional disk around Sz 91 acquired with VLT/NaCo at $H$ (1.7$\mu$m) and $K_s$ (2.2$\mu$m) bands. We resolve the disk and detect polarized emission up to $\sim$0."5 ($\sim$80 au) along with a central cavity at both bands. We computed a radiative transfer model that accounts for the main characteristics of the polarized observations. We found that the emission is best explained by small, porous grains distributed in a disk with a $\sim$45 au cavity. Previous ALMA observations have revealed a large sub-mm cavity ($\sim$83 au) and extended gas emission from the innermost (<16 au) regions up to almost 400 au from the star. Dynamical clearing by multiple low-mass planets arises as the most probable mechanism for the origin of Sz 91's peculiar structure. Using new $L'$ band ADI observations we can rule out companions more massive than $M_p$ $\geq$ 8 $M_\mathrm{Jup}$ beyond 45 au assuming hot-start models. The disk is clearly asymmetric in polarized light along the minor axis, with the north side brighter than the south side. Differences in position angle between the disk observed at sub-mm wavelengths with ALMA and our NaCo observations were found. This suggests that the disk around Sz 91 could be highly structured. Higher signal-to-noise near-IR and sub-mm observations are needed to confirm the existence of such structures and to improve the current understanding in the origin of transitional disks.