Circumstellar disks deduced from sub-arcsecond polarization observations of two young stars

Abstract

HERBIG Ae/Be objects are young stars of intermediate mass (3–5 solar masses) surrounded by reflection nebulae composed of dust and gas remaining from the star-forming cloud1–3. Scattering by dust particles strongly polarizes light from the circumstellar nebula. Using imaging polarimetry in excellent conditions of atmospheric seeing (0.4 arcsec) from the La Palma observatory, we have resolved an optically thick disk around the Herbig Ae/Be object V376 Cassiopeiae. Its effective radius increases from 0.8 arc-sec in the near infrared (∼1 μm wavelength) to 1.2 arcsec at optical wavelengths (∼550 nm) because of the higher opacity of the disk material at shorter wavelengths. The corresponding linear disk radius is 500–750 AU, about an order of magnitude larger than our Solar System. The polarization behaviour within a 0.5-arcsec diameter circle around the companion star V633 Cas suggests the presence of a more compact (<0.2 arcsec) unresolved disk, possibly a protoplanetary disk, around this star. The presence of circumstellar disks around these two pre-main-sequence stars in the same star-forming cloud suggests that such disks may be common features of early stellar evolution.