Continuous and burst-like accretion on to substellar companions in Mira winds

Abstract

ABSTRACT We present numerical hydrodynamical modeling of the effects of a giant planetor brown dwarf companion orbiting within the extended atmosphere and wind for-mation zone of an approximately solar-mass Mira variable star. The large-scale, time-dependent accretion flows within the radially oscillating and outflowing circumstellargas around Miras are related to Bondi-Hoyle-Lyttleton flows, but have not, to ourknowledge, been previously modelled. The new models presented in this paper illus-trate the changes in accretion and wake dynamics as the companion mass is variedover a range from 10 to 50 Jupiter masses (M J ), and generalize the results of thesingle model we presented in an earlier paper.The character of the accretion onto the companion changes greatly as the com-panion mass is increased. At the lowest companion masses considered here, a lowcontinuous rate of mass accretion is punctuated by large, nearly periodic bursts of ac-cretion. When the companion mass is large, the mass accretion has both a continuouspart, and a rapidly varying, nearly stochastic part. Surprisingly, the angular momen-tum of the accreted gas shows an opposite trend with mass, varying nearly periodicallyat large companion masses, and stochastically at low masses. These trends can be un-derstood as the result of the interplay between the shocks and radial oscillations inthe circumstellar gas, and the wake flow behind the companion. Boundary conditionsalso affect the character of the accretion. The equation of state, however, is found tohave little effect, at least for gamma-law gases, with gamma in the range from 1 to5/3.Models with accretion bursts may produce observable optical brightenings, andmay affect SiO maser emission, as we suggested in previous papers. Interruptions ofcontinuous accretion, or shadowing effects, could give rise to bursts of dimming inthe optical. Such dimming effects are likely to be correlated with bursts and opticalflashes, helping to explain some rather mysterious Hipparcos observations.Key words:stars: variables: stars – AGB and Post-AGB – accretion, accretion discs – masers –circumstellar matter – planetary systems: stars – winds.