2001A&A...365...78A

The close environment of four old Pre-Main Sequence stars has been observed thanks to the coronagraphic mode of the HST/NICMOS2 camera at λ=1.6µm. In the course of this program, a circumstellar annulus around HD 141569 was detected and has already been presented in Augereau et al. (1999A&A...350L..51A). In this paper, we report the detection of an elliptical structure around the Herbig Be star HD 100546 extending from the very close edge of the co mask (∼50AU) to 350-380AU (3.5-3.8") from the star. The axis ratio gives a disk inclination of 51°±3° to the line-of-sight and a position angle of 161°±5°, measured east of north. At 50AU, the disk has a surface brightness between 10.5 and 11mag/arcsec², then follows a -2.92±0.04 radial power law up to 250-270AU and finally falls as r^–5.5±0.2. The inferred optical thickness suggests that the disk is at least marginally optically thick inside 80AU and optically thin further out. Combined with anisotropic scattering properties, this could explain the shape of a brightness asymmetry observed along the minor axis of the disk. This asymmetry needs to be confirmed. The circumstellar disks around SAO 206462 and MWC 480 are not resolved, leading to constraints on the dust distribution. A tight binary system separated by only 0.32"±0.04" is nevertheless detected in the close vicinity of SAO 206462.