Continuous and Burst-like Accretion onto Substellar Companions in Mira Winds

We present numerical hydrodynamical modeling of the effects of a giant planet or brown dwarf companion orbiting within the extended atmosphere and wind formation zone of an approximately solar-mass Mira variable star. The large-scale, timedependent accretion flows within the radially oscillating and outflowing circumstellar gas around Miras are related to Bondi-Hoyle-Lyttleton flows, but have not, to our knowledge, been previously modelled. The new models presented in this paper illustrate the changes in accretion and wake dynamics as the companion mass is varied over a range from 10 to 50 Jupiter masses (MJ), and generalize the results of the single model we presented in an earlier paper. The character of the accretion onto the companion changes greatly as the companion mass is increased. At the lowest companion masses considered here, a low continuous rate of mass accretion is punctuated by large, nearly periodic bursts of accretion. When the companion mass is large, the mass accretion has both a continuous part, and a rapidly varying, nearly stochastic part. Surprisingly, the angular momentum of the accreted gas shows an opposite trend with mass, varying nearly periodically at large companion masses, and stochastically at low masses. These trends can be understood as the result of the interplay between the shocks and radial oscillations in the circumstellar gas, and the wake flow behind the companion. Boundary conditions also affect the character of the accretion. The equation of state, however, is found to have little effect, at least for gamma-law gases, with gamma in the range from 1 to 5/3. Models with accretion bursts may produce observable optical brightenings, and may affect SiO maser emission, as we suggested in previous papers. Interruptions of continuous accretion, or shadowing effects, could give rise to bursts of dimming in the optical. Such dimming effects are likely to be correlated with bursts and optical flashes, helping to explain some rather mysterious Hipparcos observations.