Constraints on properties of the protoplanetary disks around UX Ori and CQ Tau

Abstract

We present Very Large Array observations of the intermediate mass pre-main-sequence stars UX Ori and CQ Tau at 7mm, 3.6cm, and 6cm. These stars are members of the UX Ori variability class, where the origin of optical variability is thought to derive from inhomogeneities in circumstellar disks. Both stars are detected at 7mm but not at longer wavelengths, which confirms that the millimeter emission is dominated by dust. The UX Ori system exhibits a remarkably flat spectral index in the millimeter range, with alpha_mm 2 (F_nu nu^alpha_mm). Two different disk models can reproduce this property: i) a physically small disk with optically thick emission, truncated at a radius about 30 AU, or ii) a massive (~0.3-1 Msun) disk mainly composed of dust particles grown to radii of 10cm (``pebbles''). The observations do not spatially resolve the 7mm emission. We discuss implications of these two models and suggest observational tests that will discriminate between them. The CQ Tau system exhibits a spectral index in the millimeter range of alpha_mm 2.6, consistent with values commonly found for disks around pre-main-sequence stars. The observations marginally resolve the 7mm emission as an elongated structure with full width at half maximum of 2.4"x1.1" (240x110 AU at 100pc distance). The size and inclination of ~63 degrees (implied by circular symmetry) are consistent with flared disk models previously suggested to explain the optical colors and polarization properties.