Stellar obliquities and magnetic activities from Transit Chord Correlation

The light curve of a transiting planet shows anomalies  whenever the planet passes in front of active regions on the host star.  We present a method named “Transit Chord Correlation” that statistically extracts information about the stellar obliquity as well as surface magnetic activity of the host star from these anomalies. After applying this method to the highest SNR CoRoT, Kepler and K2 system, we found 10 planetary systems and 8 eclipsing binaries that have well-aligned orbits. In contrast, we found no systems with retrograde orbits among our sample; this provides further evidence that hot Jupiters around cool stars (below Kraft break) generally have well-aligned orbits. Our new method is also able to reveal the stellar obliquity of M dwarf planets. We show that  Kepler-45b is aligned to within 10 degrees; this is only the second M dwarf with a measured obliquity. Finally, we use the set of well-aligned planets to probe the surface magnetic activity of their host stars. We show that there are some common patterns of stellar activity for solar-type stars including persistently active longitudes (lifetime >100 days), emerging magnetic flux tubes and low active latitudes (<30 degrees).

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