Evolution of Planetary Orbits with Stellar Mass Loss and Tidal Dissipation

Abstract

Intermediate mass stars and stellar remnants often host planets, and these dynamical systems evolve because of mass loss and tides. This paper considers the combined action of stellar mass loss and tidal dissipation on planetary orbits in order to determine the conditions required for planetary survival. Stellar mass loss is included using a so-called Jeans model, described by a dimensionless mass loss rate γand an index β. We use an analogous prescription to model tidal effects, described here by a dimensionless dissipation rate Γand two indices (q,p). The initial conditions are determined by the starting value of angular momentum parameter η(equivalently, the initial eccentricity) and the phase θof the orbit. Within the context of this model, we derive an analytic formula for the critical dissipation rate Γ, which marks the boundary between orbits that spiral outward due to stellar mass loss and those that spiral inward due to tidal dissipation. This analytic result Γ=Γ(γ,β,q,p,η,θ) is essentially exact for initially circular orbits and holds to within an accuracy of 50% over the entire multi-dimensional parameter space, where the individual parameters vary by several orders of magnitude. For stars that experience mass loss, the stellar radius often displays quasi-periodic variations, which produce corresponding variations in tidal forcing; we generalize the calculation to include such pulsations using a semi-analytic treatment that holds to the same accuracy as the non-pulsating case. These results can be used in many applications, e.g., to predict/constrain properties of planetary systems orbiting white dwarfs.