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The close environment of four old Pre-Main Sequence stars has been observedthanks to the coronagraphic mode of the HST/NICMOS2 camera at lambda=1.6micron. In the course of this program, the detection of a circumstellar annulusaround HD 141569 has already been presented in Augereau et al.(1999b). In thispaper, we report the detection of an elliptical structure around the Herbig Bestar HD 100546 extending from the very close edge of the coronagraphic mask(~50 AU) to 350-380 AU (3.5-3.8 arcsec) from the star. The axis ratio gives adisk inclination of 51+/-3 degrees to the line-of-sight and a position angle of161+/-5 degrees, measured east of north. At 50 AU, the disk has a surfacebrightness between 10.5 and 11 mag/arcsec^2, then follows a -2.92+/-0.04 radialpower law up to 250-270 AU and finally falls as r^{-5.5+/-0.2}. The inferredoptical thickness suggests that the disk is at least marginally optically thickinside 80 AU and optically thin further out. Combined with anisotropicscattering properties, this could explain the shape of a brightness asymmetryobserved along the minor axis of the disk. This asymmetry needs to beconfirmed. The circumstellar disks around SAO 206462 and MWC 480 are notresolved, leading to constraints on the dust distribution. A tight binarysystem separated by only 0.32+/-0.04 arcsec is nevertheless detected in theclose vicinity of SAO 206462.Comment: 13 pages, accepted for publication in Astronomy & Astrophysic

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