  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
micron. In the course of this program, the detection of a circumstellar annulus
around HD 141569 has already been presented in Augereau et al.(1999b). 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 coronagraphic mask
(~50 AU) to 350-380 AU (3.5-3.8 arcsec) from the star. The axis ratio gives a
disk inclination of 51+/-3 degrees to the line-of-sight and a position angle of
161+/-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 arcsec is nevertheless detected in the
close vicinity of SAO 206462.
