Resolving structure of the disc around HD100546 at 7 mm with ATCA

There is much evidence that planet formation is occurring in the disc around the Herbig Be star HD100546. To learn more about the processes occurring in this disc, we conducted high-resolution imaging at 43/45 GHz with the Australia Telescope Compact Array. Multiple array configurations were used, providing a best spatial resolution of similar to 0.15 arcsec, or 15 au at HD100546's distance of similar to 100 pc. Significant structure is revealed, but its precise form is dependent on the u - v plane sampling used for the image reconstruction. At a resolution of <= 30 au, we detected an inner gap in the disc with a radius of similar to 25 au and a position angle approximately along the known disc major axis. With different weighting, and an achieved resolution of similar to 15 au, emission appears at the centre and the disc takes on the shape of an incomplete ring, much like a horseshoe, again with a gap radius of similar to 25 au. The position angle of the disc major axis and its inclination from face-on are determined to be 140 degrees +/- 5 degrees and 40 degrees +/- 5 degrees, respectively. The similar to 25 au gap radius is confirmed by a null in the real part of the binned visibilities at 320 +/- 10 k lambda, whilst the non-axisymmetric nature is also confirmed through significant structure in the imaginary component. The emission mechanism at the central peak is most likely to be free-free emission from a stellar or disc wind. Overall our data support the picture of at least one, but probably several, giant planets orbiting HD100546 within 25 au.