A High Stellar Obliquity in the WASP-7 Exoplanetary System

TL;DR

This study measures a high obliquity of 86+-6 degrees in the WASP-7 system, indicating a significant tilt between the star's rotation axis and the planet's orbit, using the Rossiter-McLaughlin effect.

Contribution

It provides the first precise measurement of stellar obliquity in the WASP-7 system, supporting the pattern of high obliquity in hot-Jupiter hosts without thick convective envelopes.

Findings

WASP-7's stellar obliquity is approximately 86 degrees.

The host star exhibits stellar jitter of about 30 m/s.

Differential rotation was not detected with current data.

Abstract

We measure a tilt of 86+-6 deg between the sky projections of the rotation axis of the WASP-7 star, and the orbital axis of its close-in giant planet. This measurement is based on observations of the Rossiter-McLaughlin (RM) effect with the Planet Finder Spectrograph on the Magellan II telescope. The result conforms with the previously noted pattern among hot-Jupiter hosts, namely, that the hosts lacking thick convective envelopes have high obliquities. Because the planet's trajectory crosses a wide range of stellar latitudes, observations of the RM effect can in principle reveal the stellar differential rotation profile; however, with the present data the signal of differential rotation could not be detected. The host star is found to exhibit radial-velocity noise (``stellar jitter') with an amplitude of ~30m/s over a timescale of days.