Speaker
Description
Short-period white dwarf (WD) binaries are prime targets for LISA, where both gravitational-wave (GW) emission and dynamical tides drive the spin-orbital evolution. Previous treatments of resonance locking — in which the tidal torque and GW-driven decay evolve in concert to hold the forcing frequency constant — rely on a quasi-adiabatic description in which the tidal torque is dissipative by construction. We couple the non-adiabatic oscillation codes MAD and MAD_tides with Peters' equations to study this evolution in DA white dwarfs before and into the ZZ Ceti instability strip.
We find that the sign of the tidal torque at a given resonance varies strongly with forcing frequency and stellar spin, a behaviour inaccessible to quasi-adiabatic formalisms. Systems undergo successive locking episodes rather than remaining locked on a single mode, each episode ending when the torque reverses sign as the primary spins up. Tidal effects leave the merging timescale essentially unchanged, while spinning the primary up significantly, reaching 0.3–0.4 of critical rotation at Roche-lobe filling.