Constraints on Properties of the Protoplanetary Disks around UX Orionis and CQ Tauri

Abstract

We present Very Large Array observations of the intermediate-mass pre-main-sequence stars UX Ori and CQ Tau at 7 mm, 3.6 cm, and 6 cm. 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 7 mm 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 αmm ~ 2 (Fν ∝ ν). Two different disk models can reproduce this property: (1) a physically small disk with optically thick emission, truncated at a radius of about 30 AU, or (2) a massive (~0.3-1 M☉) disk mainly composed of dust particles grown to radii of 10 cm (pebbles). The observations do not spatially resolve the 7 mm 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 αmm ~ 2.6, consistent with values commonly found for disks around pre-main-sequence stars. The observations marginally resolve the 7 mm emission as an elongated structure with full width at half-maximum of 24 × 11 (240 × 110 AU at 100 pc distance). The size and inclination of ~63° (implied by circular symmetry) are consistent with flared disk models that have previously been suggested to explain the optical colors and polarization properties.