TOI-1259Ab: A warm Jupiter orbiting a K-dwarf white-dwarf binary on a low-obliquity orbit

The evolution of one member of a stellar binary into a white dwarf has been proposed as a mechanism that triggers the formation of close-in gas giant planets. The star's asymmetric mass loss during the Asymptotic Giant Branch stage gives it a velocity recoil or “kick” that can initiate eccentric Lidov-Kozai oscillations, potentially causing a planet around the secondary star to migrate inward and perturb the eccentricity and inclination of its orbit. Here we present a measurement of the stellar obliquity of TOI-1259Ab, a gas giant in a close-in orbit around a K star with a white dwarf companion about 1650,au away. By using the NEID spectrograph to detect the Rossiter-McLaughlin effect during the planetary transit, we find the sky-projected obliquity to be łambda = 7^ +20 _ -21 , _ When combined with estimates of the stellar rotation period, radius, and projected rotation velocity, we find the true 3D obliquity to be ψ = 24^ +14 -12 , (ψ < 47 at 95% confidence), revealing that the orbit of TOI-1259Ab is on a low-obliquity orbit with respect to the star's equatorial plane. Because the planet's orbit is too wide for tidal realignment to be expected, TOI-1259Ab might have formed quiescently in this low-obliquity configuration. Alternatively, as we show with dynamical simulations, eccentric Lidov-Kozai oscillations triggered by the evolution of the binary companion expect to lead to a low-obliquity configuration with a probability of sim14%.