White Dwarf Kicks via Episodic Mass Ejection from Red Giant Stars

Recent observations have found evidence that white dwarf (WD) stars receive a kick of about 1 km/s as the envelopes of their red giant progenitors are expelled. We show that these kicks can arise from asymmetric and episodic mass loss of red giant stars. Based on simple hydrodynamic models of red giant mass loss, each mass ejection event likely expels of order about ! 10-4, M, imparting a small and randomly oriented kick to the star, with the net kick to the WD arising from a combination of many (N about 10 4) events. Each kick is of order v k about 5, m/s, with a total kick of order v k,tot about sqrt N v k about 0.5, km/s. We predict substantially larger net kicks for higher mass WDs. We also model the orbital evolution of binary stars experiencing a stochastic series of kicks, developing analytic models to explain numerical integrations. For nominal mass ejection parameters, widely separated binaries (a 10 cubed, AU) can become unbound, with larger fractions of unbound systems at larger separations and for higher WD masses. The orbital eccentricities can also approach unity, causing stellar collisions that may result in luminous transients and eccentric common envelope events.