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) indicates the rotation axis is
tilted towards (away from) the line of sight.Comment: Revised for ApJL, submitted 8 May 201
