What’s kickin’ in partial tidal disruption events?

Stars partially destroyed by a supermassive black hole (SMBH) in a partial tidal disruption event (TDE) can be ejected from the SMBH. Previous investigations attributed this positive-energy/velocity kick to asymmetries in the mass lost by the star near pericentre. We propose that asymmetric mass loss is not predominantly responsible for ‘kicking’ the star, and that these kicks instead arise from the combination of (a) the reformation of the core following an initial phase of quasi-ballistic motion, and (b) the differential shear between the unbound and marginally bound (to the SMBH) material during this phase. We predict that the kick speed vkick is weakly dependent on the stellar properties, and for SMBH masses M• 103 M, vkick is independent of SMBH mass, is not limited to the stellar escape speed vesc, and is related to the surviving core mass Mc approximately as vkick 0.45 (Mc/M) −1/3, where M is the original stellar mass. For M• 103 M, we find that the maximum-attainable kick speed depends on SMBH mass, satisfies vkick,max 0.4 vesc (M•/M) 1/6, and is reached for core masses that satisfy Mc/M 1.7 (M•/M) −1/2. This model predicts that massive stars with M f ew × 10 M could be ejected at speeds 1 − 2 × 103 km s−1 if stripped of 50 per cent of their mass.