Dust around Herbig Ae/Be Stars: Porous, Cometary-like Grains?

               Natalia A. Krivova and Vladimir B. Il'in


Abstarct.-- Herbig Ae/Be (HAeBe) stars are pre-main sequence objects of
moderate mass representing the early evolutionary stages of beta Pic-type
stars. The objects of both kinds are surrounded by dust disks/shells, and
the dust is believed to be continually replenished by comet-like bodies
orbiting close to the stars. As the dust particles in the beta Pictoris
disk are likely to be as fluffy as cometary dust is, it is reasonable to
suppose that the grains around HAeBe stars could be porous as well. This
brings up the question, which we try to answer here, of how the grain
porosity affects the observational manifestations of the dust shells of
HAeBe stars: the excess infrared emission, anomalous extinction in the
ultraviolet, and the peculiar behaviour of colour indices and linear
polarization during deep brightness minima observed for about 25% of
HAeBe stars (UX Ori, WW Vul, etc).

 We have performed numerical Monte Carlo simulations of polarized radiation
transfer in spheroidal inhomogeneous shells for three different models of
circumstellar dust grains. In addition to the observational data mentioned
above, intensity scans and polarization maps of the objects have been
constructed in view of possible future observations. It is found that the
porosity of circumstellar grains weakly affects the spectral energy
distribution (when the grain model does not include particles with quite
different optical properties) and does not show pronounced effects in the
intensity and polarization maps for all grain models considered. On the
other hand, the porosity normally reduces single scattering albedo of
particles and, therefore, fluffy grains explain the spectral energy
distributions of UX Ori-like stars and their tracks in the colour-magnitude
diagrams considered in combination much better than compact grains do.
However, highly porous, cometary-like grains are unlikely to be abundant in
the shells as they produce too low linear polarization to account for the
observations of the HAeBe stars.

Key Words: radiative transfer; polarimetry; interplanetary dust;
           comets.