The protoplanetary system HD 100546 in H$\alpha$ polarized light from SPHERE/ZIMPOL. A bar-like structure across the disk gap?.

HD 100546 is one of the few known pre-main-sequence stars that may host a planetary system in its disk. We analyze new VLT/SPHERE/ZIMPOL polarimetric images of HD 100546 with filters in H$\alpha$ and the adjacent continuum. We have probed the disk gap and the surface layers of the outer disk, covering a region < 500 mas (< 55 au at 109 pc) from the star, at an angular resolution of ~ 20 mas. Our data show an asymmetry: the SE and NW regions of the outer disk are more polarized than the SW and NE. This can be explained from a preferential scattering angle close to 90$^o$, consistent with previous polarization images. The outer disk extends from 13 $\pm$ 2 to 45 $\pm$ 9 au, with a position angle and inclination of 137 $\pm$ 5$^o$ and 44 $\pm$ 8$^o$. The comparison with previous estimates suggests that the disk inclination could increase with the stellocentric distance, although the different measurements are still consistent within the error bars. In addition, no direct signature of the innermost candidate companion is detected from polarimetry, confirming recent results based on intensity imagery. We set an upper limit to its mass accretion rate < 10$^{-8}$ M$_{\odot}$/yr for a sub-stellar mass of 15M$_{Jup}$. Finally, we report the first detection (> 3$\sigma$) of a ~ 20 au bar-like structure that crosses the gap through the central region of HD 100546. It is tentatively suggested that the bar could be dust dragged by infalling gas that radially flows from the outer disk. This could represent an exceptional case in which a small-scale radial inflow is observed in a single system. If this scenario is confirmed, it could explain the presence of atomic gas in the inner disk that would otherwise accrete on to the central star on a timescale of a few months/years, as previously indicated from spectro-interferometric data, and could be related with additional (undetected) planets.