Context : Giant planets form in protoplanetary disks while these disks are still gas-rich , and can reveal their presence through the annular gaps they carve out .
HD 100546 is a gas-rich disk with a wide gap between between a radius of \sim 1 and 13 AU , possibly cleared out by a planetary companion or planetary system .

Aims : We want to identify the nature of the unseen companion near the far end of the disk gap .

Methods : We use mid-infrared interferometry at multiple baselines to constrain the curvature of the disk wall at the far end of the gap .
We use 2D hydrodynamical simulations of embedded planets and brown dwarfs to estimate viscosity of the disk and the mass of a companion close to the disk wall .

Results : We find that the disk wall at the far end of the gap is not vertical , but rounded-off by a gradient in the surface density .
Such a gradient can be reproduced in hydrodynamical simulations with a single , heavy companion ( \gtrsim 30…80 M _ { Jup } ) while the disk has viscosity of at least \alpha \gtrsim 5 \cdot 10 ^ { -3 } .
Taking into account the changes in the temperature structure after gap opening reduces the lower limit on the planet mass and disk viscosity to 20 M _ { Jup } and \alpha = 2 \cdot 10 ^ { -3 } ) .

Conclusions : The object in the disk gap of HD 100546 that shapes the disk wall is most likely a 60 ^ { +20 } _ { -40 } M _ { Jup } brown dwarf , while the disk viscosity is estimated to be at least \alpha = 2 \cdot 10 ^ { -3 } .
The disk viscosity is an important factor in estimating planetary masses from disk morphologies : more viscous disks need heavier planets to open an equally deep gap .