17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

Type: Oral

Topic: Cool Stars as Exoplanet Hosts

Abstract
When a transiting planet passes in front of a starspot on the photosphere of its host star, an anomaly is observed in the light curve. We have been developing a new method that exploits these spot-crossing anomalies to measure the obliquity of the star with respect to the orbital plane of the planet. Obliquities are important because they are fundamental geometric parameters, and because they bear clues about the formation, migration, and tidal evolution of close-in planets. If the spot-crossing anomalies are observed to recur with a progressively increasing phase during consecutive transits, we may conclude the trajectory of the spots is parallel to the orbital motion of the planet, and the host star has a low obliquity. In some cases, if spots do not recur, we have a very clear indication of a high obliquity system. When a host star has a very low obliquity, the transiting planet can only pass in front of spots within a narrow range of latitudes. However, there is a very interesting subgroup of systems with large obliquities where a transiting object can occult spots over a very wide range of latitudes. Thanks to the appearance of active latitudes on the host star we identified HAT-P-11b as part of this subgroup. Knowledge of the orientation of the star allowed us to measure the latitude of the spots transited by the planet, information that can be used to construct an analog of the solar butterfly diagram. Independently of the obliquity, if the planet occults a spot then the amplitude and the duration of the anomaly will depend on the size and brightness contrast of the spot. If that spot is also causing significant flux variations in the total starlight (due to rotation) then a combined model of the transit anomalies and the out-of-transit flux variations can lead to unique determinations of the size and brightness contrast of the spot. For the first time, we can study individual spots, with sizes, temperatures and latitudes. This technique is mostly applicable to cool stars, where the stars are generally more active. Even for faint stars, large spots can be easily detected, especially if the have a high brightness contrast respect to the photosphere. In this contribution we will explain the methodology to find and study spot-crossing events on Kepler transiting systems, and to measure their obliquities. In addition to the already published systems, tens of systems show clear anomalies that will help us study obliquities for new and exciting systems such as multiple planetary systems, small planets and binary stars. We will show preliminary results for different systems that belong to all these categories. Their hosts stars also show a wide variety of temperatures, but are mostly cool stars, which can help us to understand the physical differences of spots for different type of stars.