Engulfment of a hot Jupiter as a possible origin of the rapid spin and internal spin misalignment of the planet-hosting red giant Kepler-56

ABSTRACT A recent asteroseismic analysis suggests that Kepler-56 – a planet-hosting red giant – exhibits a unique spin structure: (1) the spin axes of the core and envelope are misaligned; and (2) the envelope rotates approximately an order of magnitude faster than typical red giants. In this paper, we investigate a feasible scenario to reproduce this spin structure by estimating the amount of the angular momentum (AM) supply from the planets through the simplified calculation of the time evolution of AM. As a result, unless the tidal efficiency is extremely high, we show that the tidal interactions between the known close-in planets (Kepler-56 b and c) are insufficient to supply the AM required to accelerate Kepler-56 from the spin rate observed in typical red giants. We also show that the engulfment of a hot Jupiter can be expected to provide sufficient AM supply for the acceleration and that the mass and orbit of the engulfed hot Jupiter are constrained by a mass of 0.5–2 Jupiter masses and an orbital period of 1–6 days. On the other hand, if Kepler 56 was already rapidly spinning before entering the RG stage and requires no acceleration, the obliquity damping by the known close-in planets can reproduce the spin structure of Kepler-56. Even in such cases, planetary engulfment during the MS stage might be involved in achieving rapid spin before the tidal alignment. These discussions demonstrate the importance of Kepler-56 as a candidate for planetary engulfment that may leave traces of its spin structure.