A YOUNG PROTOPLANET CANDIDATE EMBEDDED IN THE CIRCUMSTELLAR DISK OF HD 100546

We present high-contrast observations of the circumstellar environment of the Herbig Ae/Be star HD 100546. The final 3.8 mu m image reveals an emission source at a projected separation of 0".48 + or - 0".04 (corresponding to ~47 + or - 4 AU) at a position angle of 8[degrees].9 + or - 0[degrees].9. The emission appears slightly extended with a point source component with an apparent magnitude of 13.2 + or - 0.4 mag. The position of the source coincides with a local deficit in polarization fraction in near-infrared polarimetric imaging data, which probes the surface of the well-studied circumstellar disk of HD 100546. This suggests a possible physical link between the emission source and the disk. Assuming a disk inclination of ~47[degrees], the de-projected separation of the object is [degrees]68 AU. Assessing the likelihood of various scenarios, we favor an interpretation of the available high-contrast data with a planet in the process of forming. Follow-up observations in the coming years can easily distinguish between the different possible scenarios empirically. If confirmed, HD 100546 "b" would be a unique laboratory to study the formation process of a new planetary system, with one giant planet currently forming in the disk and a second planet possibly orbiting in the disk gap at smaller separations.