2017A&A...608A.104M

Context. HD 100546 is one of the few known pre-main-sequence stars that may host a planetary system in its disk.
Aims. This work aims to contribute to our understanding of HD 100546 by analyzing new polarimetric images with high spatial resolution.
Methods. Using VLT/SPHERE/ZIMPOL with two filters in Hα and the adjacent continuum, we have probed the disk gap and the surface layers of the outer disk, covering a region <500mas (<55au at 109pc) from the central star, at an angular resolution of ∼20mas.
Results. Our data show an asymmetry: the SE and NW regions of the outer disk are more polarized than the SW and NE regions. This asymmetry can be explained from a preferential scattering angle close to 90° and is consistent with previous polarization images. The outer disk in our observations extends from 13±2 to 45±9au, with a position angle and inclination of 137±5° and 44±8°, respectively. 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 the polarimetric data, confirming recent results that were based on intensity imagery. We set an upper limit to its mass accretion rate <10^–8M_☉/yr for a substellar mass of 15M_Jup. Finally, we report the first detection (>3σ) of a ∼20au bar-like structure that crosses the gap through the central region of HD 100546.
Conclusions. In the absence of additional data, it is tentatively suggested that the bar could be dust dragged by infalling gas that radially flows from the outer disk to the inner region. 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.