# Summary of K2 Program GO12085

Title: Measuring Stellar Activity of Exoplanet Candidate Host Stars

PI: Rinehart, Stephen (NASA -- Goddard Space Flight Center)
CoIs: Walkowicz, Lucianne M; Boyd, Patricia T.; Wakeford, Hannah R; Angerhausen, Daniel; Saxena, Prabal; Kostov, Veselin; Placek, Ben

Direct measurements of the fundamental properties of stars (e.g. radius, rotation, mass, age) are critical for understanding stellar astrophysics. Rotation periods, in particular, provide important clues to the physics behind the formation and evolution of stars and subsequent planetary systems. When the latter are detected, studying the relation between the orbital period of the planet(s) and the rotation period of the parent star can also shed light on the spin-orbit alignment and on the angular momentum history of the system (e.g. Bouvier et al. 2013). In addition, stellar activity due to rotation and/or pulsations affects the measured radial velocity signature of the host star, and can even mimic inferred Keplerian motion. 

Prior to NASAs Kepler mission, stellar rotation was typically deduced from spectroscopy using the rotationally-broadened absorption line profiles. This technique, however, is both expensive in terms of telescope time, and more importantly, suffers from degeneracies stemming from its dependency on the a-priori unknown stellar radius and obliquity; it is also tailored to fast rotators. The mission has truly revolutionized the field, offering, for the first time, continuous monitoring of stellar flux allowing precise measurements of the rotation periods of tens of thousands of stars (e.g. McQuillan, Mazeh & Aigrain 2014). K2 builds up on this legacy, offering new opportunities to study stellar variability in a variety of settings  stars at the bottom of the main sequence, white dwarfs, binaries, stars in open clusters, etc (e.g. Kawaler 2014, Scholz et al. 2015, Rebel et al. 2016). 

Here we propose to observe 9 exoplanet-candidate host stars (1 transiting and 8 RV planets), study their long-term stellar activity and measure their photometrically-modulated rotation periods. Our targets span a range of stellar types (from K giants to M dwarfs), of planetary periods (from 3 days to 5 years) and masses/sizes (from ~2.5 REarth to ~10MJup). The rotation periods for six targets have been inferred from spectroscopically-measured vsin i (with various degree of confidence) and model-dependent radius. These periods range from 20 to 60 days  ideally suited for robust detection and confirmed by K2. For three of the targets the rotation periods are unknown. In addition, the nature of some of these candidates suffers from degeneracy between the measured radial velocities and inferred Keplerian motions. 

Our program will a) provide direct measurements for the rotation periods; b) break degeneracies between stellar rotation and exoplanetary orbits; and c) allow for comprehensive characterization of these systems. In combination with v sin i measurements (where available), our results will also allow model-independent measurements of the stellar radii. In addition, the observations will be sensitive to transits caused by additional exoplanets, and in the case of the one transiting planet  to transit timing variations. While there are ground-based photometric measurements for some of our targets, none of the available data compares to the precision, quality and uninterrupted monitoring of K2, making the observatory uniquely-suited for the proposed investigation. Thus our program will add to the growing legacy of stellar activity research enabled by the mission, provide input for gyrochronological studies, and offer new insight into the stellar astrophysics of these planet-hosting stars. The proposed study will directly support NASAs goal to Discover and study planets around other stars, in order to understand the underlying physical processes of exoplanets and to improve understanding of their origins.

# Targets requested by this program that have been observed (2)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
246340370, 347.294699, -2.260744, 8.617, GO12085_LC|GO12083_LC|GO12039_LC|GO12062_LC|GO12122_LC|GO12071_LC|GO12049_LC|GO12904_LC
246375295, 353.616168, -1.580036, 11.933, GO12085_LC|GO12062_LC|GO12099_LC|GO12122_LC|GO12124_LC|GO12071_LC|GO12902_LC|GO12903_LC|GO12062_SC|GO12099_SC|GO12124_SC