The Spin-Orbit Alignment of the HD17156 Transiting Eccentric Planetary System

TL;DR

This study measures the spin-orbit alignment of the HD17156 system using high-precision radial velocity data, finding the planetary orbit is closely aligned with the star's rotation axis, contrary to previous reports of misalignment.

Contribution

The paper provides a refined measurement of the spin-orbit angle for HD17156, demonstrating the orbit's alignment with the stellar rotation axis through combined data analysis.

Findings

The combined data indicates a lambda of approximately 9.4 degrees.

The planetary orbit is well aligned with the stellar rotation axis.

Previous reports of misalignment are contradicted by this analysis.

Abstract

We present high precision radial velocity observations of HD17156 during a transit of its eccentric Jovian planet. In these data, we detect the Rossiter-McLaughlin effect, which is an apparent perturbation in the velocity of the star due to the progressive occultation of part of the rotating stellar photosphere by the transiting planet. This system had previously been reported by Narita et al. (2008) to exhibit a lambda = 62 +/- 25 degree misalignment of the projected planetary orbital axis and the stellar rotation axis. We model our data, along with the Narita et al. data, and obtain lambda = 9.4 +/- 9.3 degrees for the combined data set. We thus conclude that the planetary orbital axis is actually very well aligned with the stellar rotation axis.