Starspots and spin-orbit alignment in the WASP-4 exoplanetary system

TL;DR

This study uses high-precision transit photometry of WASP-4b to detect starspot occultations, revealing a near-aligned stellar spin-orbit axis and providing insights into the planet's migration and system evolution.

Contribution

It introduces a method to measure stellar obliquity through starspot occultation analysis, offering more precise constraints than previous techniques.

Findings

Stellar obliquity is approximately zero degrees.

Starspot anomalies recur, indicating aligned stellar rotation and orbital axes.

The system's low obliquity suggests migration preserved initial alignment.

Abstract

We present photometry of 4 transits of the exoplanet WASP-4b, each with a precision of approximately 500 ppm and a time sampling of 40-60s. We have used the data to refine the estimates of the system parameters and ephemerides. During two of the transits we observed a short-lived, low-amplitude anomaly that we interpret as the occultation of a starspot by the planet. We also find evidence for a pair of similar anomalies in previously published photometry. The recurrence of these anomalies suggests that the stellar rotation axis is nearly aligned with the orbital axis, or else the star spot would not have remained on the transit chord. By analyzing the timings of the anomalies we find the sky-projected stellar obliquity to be -1_{-12}^{+14} degrees. This result is consistent with (and more constraining than) a recent observation of the Rossiter-McLaughlin effect. It suggests that the planet migration mechanism preserved the initially low obliquity, or else that tidal evolution has realigned the system. Future applications of this method using data from the Corot and Kepler missions will allow spin-orbit alignment to be probed for many other exoplanets.