Vortex weighing and dating of planets in protoplanetary discs

Abstract

High-resolution sub-mm observations of some protoplanetary discs reveal non-axisymmetric features, which can often be interpreted as dust concentrations in vortices that form at the edges of gaps carved out by the embedded planets. We use recent results on the time-scale for the planet-driven vortex development in low-viscosity discs to set constraints on the mass and age of a planet producing the vortex. Knowledge of the age of the central star in a vortex-bearing protoplanetary disc system allows one to set a lower limit on the planetary mass at the level of several tens of M⊕. Also, an independent upper limit on the planetary mass would constrain the planetary age, although given the current direct imaging detection limits this constraint is not yet very stringent (it is also sensitively dependent on the disc scale height). These results can be extended to account for the history of planetary mass accretion if it is known. We apply our calculations to several protoplanetary discs harbouring vortex-like features as revealed by ALMA and set limits of (30-50) M⊕ (for disc aspect ratio of 0.1) on the minimum masses of putative planets that could be responsible for these vortices. Our vortex-based method provides an independent way of constraining the properties of embedded planets, complementary to other approaches.