Spin-orbit alignment of the TrES-4 transiting planetary system and possible additional radial-velocity variation

We report new radial velocities of the TrES-4 transiting planetary system, including observations of a full planetary transit, with the High Dispersion Spectrograph on the Subaru 8.2 m telescope. Modeling of the Rossiter- McLaughlin effect indicates that the exoplanet TrES-4b has a sky-projected angle of λ - 6.°3 ± 4.°7 between its orbital axis and the host star's spin one. The close spin-orbit alignment of TrES-4b seems to argue against a migration history involving planet-planet scattering or Kozai cycles, although there are two nearby faint stars that might be binary companion candidates. A comparison of our out-of-transit data from four different runs suggests that the star exhibits a radial-velocity variability of ∼20ms ^-1 in excess of a single Keplerian orbit. Although the cause of the excess variability is unknown, we discuss various possibilities such as systematic measurement errors, starspots or other intrinsic motions, and additional companions besides the transiting planet.