## 4. ResultsSince 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 parametersGiven a disk inclination with respect to the line of sight which is
45 -
disk position angle on the sky of degrees with respect to east direction. -
ellipse major to minor axis ratio of 1.2 implying a disk inclination of degrees with respect to the line of sight (or from the edge-on case). -
disk extension: 2.0 arcsec from the star corresponding to a distance to the star around 200 AU (assuming a distance to the Earth of 103 parsecs (van den Ancker et al. 1998) and a limiting sensitivity of 5 mJy/arcsec ^{2}.
## 4.2. PhotometryUsing 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/arcsec In the Ks band, we find an emission of
Jy outside the coronograph mask and
a maximum surface brightness of
Jy/arcsec ## 4.3. Model and observation fitIn 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 resultsThe best fit model provides the following parameters (one must keep in mind that they are model-dependent): -
The Henyey-Greenstein phase function parameter found is corresponding to dominant spherical particles with size around 0.1 micron. -
A radial normal optical thickness fitted by a series of broken power laws whose exponents are: 0.6 in the range [10,45]AU, -0.4 in the range [45,70]AU, -1.35 in the range [70,100]AU, -5 outwards from 100 AU, see Fig. 3. -
An exponentially decreasing vertical profile, and an opening angle around 0.1 radian. -
A total scattering area of 10 ^{29}cm^{2}and a dust mass of 0.02 . -
An approximate disk surface density at maximum around particles/m ^{2}assuming single-sized particles of 0.1*µ*m, or an equivalent maximum scattering area of the order of the unity/m^{2}.
© European Southern Observatory (ESO) 2000 Online publication: September 5, 2000 |