Stellar parameters of K5 and later type Kepler stars

We obtain medium-resolution visible spectra of 382 stars with K_P_-J>2 (=~K5 and later spectral type). We determine luminosity class by comparing the strength of gravity-sensitive indices (CaH, K I, Ca II, and Na I) to their strength in a sample of stars of known luminosity class.

We find that giants constitute 96%+/-1% of the bright (K_P_<14) Kepler target stars, and 7%+/-3% of dim (K_P_>14) stars, significantly higher than fractions based on the stellar parameters quoted in the Kepler Input Catalog (KIC). The KIC effective temperatures are systematically (110^+15^_-35_K) higher than temperatures we determine from fitting our spectra to PHOENIX stellar models. Through Monte Carlo simulations of the Kepler exoplanet candidate population, we find a planet occurrence of 0.36+/-0.08 when giant stars are properly removed, somewhat higher than when a KIC log g>4 criterion is used (0.27+/-0.05).