Using Star Spots to Measure the Spin-orbit Alignment of Transiting Planets

TL;DR

This paper introduces a photometric method using star spot crossings during planetary transits to measure the spin-orbit alignment of exoplanets, eliminating the need for spectroscopic follow-up and leveraging space telescope data.

Contribution

The authors develop a novel technique to determine stellar spin-orbit alignment from light curves of spotted stars, applicable to data from space telescopes like CoRoT and Kepler.

Findings

Applied method to CoRoT-2, finding lambda= 4.7 deg +/- 12.3 deg.

Confirmed the method's consistency with spectroscopic measurements.

Estimated stellar inclination with large uncertainty due to star spots.

Abstract

Spectroscopic follow-up of dozens of transiting planets has revealed the degree of alignment between the equators of stars and the orbits of the planets they host. Here we determine a method, applicable to spotted stars, that can reveal the same information from the photometric discovery data, with no need for follow-up. A spot model fit to the global light curve, parametrized by the spin orientation of the star, predicts when the planet will transit the spots. Observing several spot crossings during different transits then leads to constraints on the spin-orbit alignment. In cases where stellar spots are small, the stellar inclination, and hence the true alignment, rather than just the sky projection, can be obtained. This method has become possible with the advent of space telescopes such as CoRoT and Kepler, which photometrically monitor transiting planets over a nearly continuous, long time baseline. We apply our method to CoRoT-2, and find the projected spin-orbit alignment angle, lambda= 4.7 deg +/- 12.3 deg, in excellent agreement with a previous determination that employed the Rossiter-McLaughlin effect. The large spots of the parent star, CoRoT-2, limit our precision on the stellar inclination: i_s = 84 deg +/- 36 deg, where i_s < 90 deg (> 90 deg) indicates the rotation axis is tilted towards (away from) the line of sight.