A key goal of the Kepler Mission is to derive planet occurrence rates, and in particular the occurrence rate of Earth-like planets around Sun-like stars. In addition to a catalog of planet candidates and the assessment of false-positive rates and completeness, this task requires accurate fundamental properties (temperatures, radii, masses, etc) of stars which were observed. These properties are not only required for the host stars to characterize the radii and orbital periods of planets, but also for the parent sample since occurrence rates must account for planets that were not detected. For example, a significant number of subgiant or giant stars have been misclassified as dwarfs would bias occurrence rates since small planets are harder to detect around larger stars.
As co-chair of the Kepler Star Properties Working Group I have led the first complete reclassification of over 190,000 stars in the Kepler target catalog, based on the collection of literature values derived using different observational methods (photometry, spectroscopy, asteroseismology, etc). The study yielded the detection of over ~2000 new oscillating giants stars (which were previously unclassified) and has since been used for a variety of Kepler science, including the study of terrestrial planet occurrence studies around Sun-like stars by the Kepler team. Together with my collaborator Savita Mathur we recently published an update to this Kepler Stellar Properties Catalog.
After the end of the Kepler mission in 2013, the spacecraft was repurposed to the K2 Mission to observe different fields along the ecliptic plane. To support target selection for the K2 Mission and the generation of apertures by the K2 team at NASA Ames I created the Ecliptic Plane Input Catalog, which also included the classification of over 160,000 stars observed by K2. The plot above compares the populations of stellar populations observed by the Kepler and K2 Missions based on these classifications. It demonstrates that K2 has observed a significantly higher fraction of cool dwarfs, a result largely due to the fact that we expect to find more planets in the habitable zone around those stars.
The release of Gaia DR2 parallaxes has led to a dramatic improvements in our understanding of the properties of stellar populations observed by transit surveys. I am the PI of an NSF-funded project to improve the properties of Kepler stars and planets using Gaia DR2. Led by my graduate student Travis Berger, we have recently published the first re-characterization of radii of Kepler stars and planets using Gaia parallaxes.