Optical polarised phase function of the HR4796A dust ring (Milli et al.)

Vol. 626
In section 10. Planets and planetary systems

Optical polarized phase function of the HR4796A dust ring

by J. Milli, N.Engler, H.M. Schmid, et al. 2019, A&A, 626, A54 alt

The analysis of light scattered from debris disks is a powerful way to characterize the physical properties of dust particles that are orbiting other stars. The 10-Myr old HR4796A A-star harbors one of the brightest debris disks, having a fractional luminosity as high as ~0.5 %. Previous near-infrared observations have allowed measurement of the scattering phase function over a wide range of phase angles, indicating the existence of particles of a few tens of microns in size, which are likely in the form of aggregates. Using SPHERE/ZIMPOL on this system, Milli et al. now report the first observation of the phase function of a debris disk in linearly polarized light. They find that the polarized phase function is roughly constant below 80° phase angle, and sharply decreases at larger angles. Although the polarization inversion is not directly observed, the trend suggests that the polarized fraction would cancel at ~160°, as is the case for solar system comets. As expected based on previous studies, the observations are inconsistent with Mie scattering by compact spheres. Instead, they point to large fluffy particles with small monomers, or large compact particles with irregular surface roughness.