HST/NICMOS2 coronagraphic observations of the circumstellar environment ofthree 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 1415 69 was detected and has already been presented in Augereau et al. (1999b). In this paper, we report the detection of art elliptical structure around t he Herbig Be star HD 100546 extending from the very close edge of the coron agraphic mask (similar to 50 AU) to 350-380 AU (3.5-3.8 ") from the star. T he axis ratio gives a disk inclination of 51 degrees +/- 3 degrees to the l ine-of-sight and a position angle of 161 degrees +/- 5 degrees, measured ea st of north. At 50 AU, the disk has a surface brightness between 10.5 and 1 1 mag/arcsec(2), then follows a -2.92 +/- 0.04 radial power law up to 250-2 70 AU and finally falls as r(-5.5+/-0.2). The inferred optical thickness su ggests that the disk is at least marginally optically thick inside 80 AU an d optically thin further out. Combined with anisotropic scattering properti es, this could explain the shape of a brightness asymmetry observed along t he minor axis of the disk. This asymmetry needs to be confirmed. The circum stellar disks around SAO 206462 and MWC 480 are not resolved, leading to co nstraints on the dust, distribution. A tight binary system separated by onl y 0.32 " +/- 0.04 " is nevertheless detected in the close vicinity of SAO 2 06-162.