Resolving the disk rotation of HD 97048 and HD 100546 in the [OI] 6300Å line:: evidence for a giant planet orbiting HD 100546

Aims. We intend to spatially and spectrally resolve the [O I] emission region in two nearby Herbig stars. Methods. We present high- resolution (lambda/Delta lambda = 80 000) VLT/UVES echelle spectra of the [O I] 6300 angstrom line in the Herbig Ae/Be stars HD 97048 and HD 100546. Apart from the spectral signature, also the spatial extent of the [O I] emission region is investigated. For both stars, we have obtained spectra with the slit positioned at different position angles on the sky. Results. The [O I] emission region of HD 100546 appears to be coinciding with the dust disk, its major axis located at 150 +/- 11 degrees east of north. The SE part of the disk moves towards the observer, while the NW side is redshifted. The [O I] emission region rotates counterclockwise around the central star. For HD 97048, the position angle of the emission region is 160 +/- 19 degrees east of north, which is the first determination of this angle in the literature. The southern parts of the disk are blueshifted, the northern side moves away from us. Our data support the idea that a gap is present at 10 AU in the disk of HD 100546. Such a gap is likely planet-induced. We estimate the mass and orbital radius of this hypothetical companion responsible for this gap to be 20 M-Jupiter and 6.5 AU respectively. Conclusions. Based on temporal changes in the [O I] line profile, we conclude that inhomogeneities are present in the [ O I] emission region of HD 100546. These "clumps" could be in resonance with the suggested companion, orbiting the central star in about 11 yr. If confirmed, these observations could point to the existence of an object straddling the line between giant planet and brown dwarf in a system as young as 10 million years.