Spin-Orbit Alignment of the TrES-4 Transiting Planetary System and Possible Additional Radial Velocity Variation

TL;DR

This study presents new radial velocity measurements of the TrES-4 system, revealing a well-aligned orbit and potential additional stellar companions, while discussing possible causes for observed velocity variability.

Contribution

It provides the first detailed Rossiter-McLaughlin effect measurement for TrES-4 and explores the implications for planetary migration and additional stellar companions.

Findings

TrES-4b has a closely aligned spin-orbit angle of approximately 6.3 degrees.

Radial velocity variability of about 20 m/s suggests possible additional companions or stellar activity.

The system's orbital alignment challenges migration scenarios involving planet-planet scattering or Kozai cycles.

Abstract

We report new radial velocities of the TrES-4 transiting planetary system, including observations of a full transit, with the High Dispersion Spectrograph of the Subaru 8.2m telescope. Modeling of the Rossiter-McLaughlin effect indicates that TrES-4b has closely aligned orbital and stellar spin axes, with $\lambda = 6.3^{\circ} \pm 4.7^{\circ}$. The close spin-orbit alignment angle 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 could be binary companion candidates. Comparison of our out-of-transit data from 4 different runs suggest that the star exhibits radial velocity variability of $\sim$20 ms^-1 in excess of a single Keplerian orbit. Although the cause of the excess radial velocity variability is unknown, we discuss various possibilities including systematic measurement errors, starspots or other intrinsic motions, and additional companions besides the transiting planet.