Two Giant Planets Orbiting Rapidly Rotating Stars
KELT-7b and HAT-P-56b are two hot-Jupitets that orbit rapidly rotating F-type host stars. KELT-7b has 1.28 ± 0.18 times the mass and 1.53 times the radius of Jupiter, while HAT-P-56b has 2.18 ± 0.25 times the mass and 1.47 times the radius of Jupiter. KELT-7b orbits its host star every 2.73 days and the maximum rotation period of its host star is 1.08 ± 0.03 days. HAT-P-56 orbits its host star every 2.79 days and the maximum rotation period of its host star is 1.8 ± 0.2 days. Both KELT-7b and HAT-P-56b belong to a rare class of super-synchronous hot-Jupiters whose orbital periods are longer than the rotation periods of their host stars.
A recent study by Zhou et al. (2016) show that the spin-orbit alignments of KELT-7b and HAT-P-56b are 2.8 ± 0.6 degeees and 8 ± 2 degrees, respectively. This means that the orbital planes of both planets are closely aligned with the equatorial planes of their host stars. Planets found in such well-aligned orbits are believed to have migrated towards their host star in the protoplanetary disk via planet-gas interactions. On the contrary, planets found in highly inclined orbits experienced dynamical interactions such as planet-planet scattering events.
In the study by Zhou et al. (2016), there appears to be no evidence that the rotation rates of the host stars of KELT-7b and HAT-P-56b have been modified by star-planet tidal interactions. This indicates that the current spin-orbit angles of KELT-7b and HAT-P-56b most likely represent their initial configurations right after they have migrated towards their host stars via planet-gas interactions in the protoplanetary disk.
Zhou et al. (2016), "Spin orbit alignment for KELT-7b and HAT-P-56b via Doppler tomography with TRES", arXiv:1605.01991 [astro-ph.EP]