HST/NICMOS2 coronagraphic observations of the circumstellar environment of three old PMS stars: HD 100546, SAO 206462 and MWC 480

The close environment of four old Pre-Main Sequence stars has been observed thanks to the coronagraphic mode of the HST/NICMOS2 camera at lambda = 1.6 mum. In the course of this program, a circumstellar annulus around HD 141569 was detected and has already been presented in Augereau et al. (1999b). In this paper, we report the detection of art elliptical structure around the Herbig Be star HD 100546 extending from the very close edge of the coronagraphic mask (similar to 50 AU) to 350-380 AU (3.5-3.8 ) from the star. The axis ratio gives a disk inclination of 51 degrees +/- 3 degrees to the line-of-sight and a position angle of 161 degrees +/- 5 degrees, measured east of north. At 50 AU, the disk has a surface brightness between 10.5 and 11 mag/arcsec(2), then follows a -2.92 +/- 0.04 radial power law up to 250-270 AU and finally falls as r(-5.5+/-0.2). The inferred optical thickness suggests that the disk is at least marginally optically thick inside 80 AU 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 206-162.