Since the dataset in J band is more accurate (because of a larger integration time, better observing conditions, and a better sensitivity of the whole system) than in Ks band, the geometrical parameters and the disk model are based on the J band image. We checked in a second step that the results are compatible with the Ks band data.
4.1. Disk geometrical parameters
Given a disk inclination with respect to the line of sight which is 45o, we expect to observe some deviations from the perfect ellipse case. Indeed, particles relatively large with respect to the wavelength produce non-isotropic scattering. One simple way to reduce this effect is to symmetrize the disk i.e. construct an isotropic-like virtual disk by averaging the original disk image and its symmetrical image with respect to the major axis. Best ellipse fitting on this symmetrized images gives the following results:
Using the stars HD 101713 (B9V) and HD 97218 (G8) as references whose V magnitude is known and using appropriate V-J and V-Ks color indices, we derived the following photometry for the disk in J and Ks band: a disk flux in J band of Jy outside the coronograph mask and a maximum surface brightness of Jy/arcsec2. From images of HD 100546 and corresponding reference star images (HD 101713 and HD97218) obtained when shifting the targets outside of the mask, we can deduce a total of scattered flux in the J band around 1 Jy; note that we get also a disk-shaped emission when removing a scaled reference to the HD 100546 image obtained outside the mask, but its signal to noise is very low. This result is compatible with the infrared excess computed considering a Kurucz model of the star (B9V spectral type). Consequently, the disk flux outside the coronograph mask represents roughly 30% of the total disk emission.
In the Ks band, we find an emission of Jy outside the coronograph mask and a maximum surface brightness of Jy/arcsec2
4.3. Model and observation fit
In order to derive physical parameters of the disk, we built a numerical model of starlight scattering by an optically thin disk made of dust particles. Assuming that no multiple scattering is occurring in the disk, and since the exact physical properties of the particles are poorly known (composition, shape, size distribution), we chose to use a global, single-parameter, scattering phase function. The most practical one, although not realistic, is the Henyey-Greenstein phase function (Henyey & Greenstein 1941). The disk is assumed to be seen with an inclination derived above, and to have a proper thickness similar to the one of Pic parameterized following Artymowicz et al. 1989. The disk midplane density is described by a scattering area which follows a series of broken power laws. We start with a first guess deduced from the radial surface brightness, and we iterate on the parameters until we obtain a satisfactory fit along the four directions defined on Fig. 2.
4.4. Fit results
The best fit model provides the following parameters (one must keep in mind that they are model-dependent):
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000