They might be giants: luminosity classes, planet frequency, and planet-metallicity relation of the coolest Kepler target stars

Details They might be giants: luminosity classes, planet frequency, and planet-metallicity relation of the coolest Kepler target stars They might be giants: luminosity classes, planet frequency, and planet-metallicity relation of the coolest Kepler target stars

2012-02-24

arxiv.org/abs/1202.5394v1

Astronomy and Astrophysics

Astrophysics

Earth and Planetary Astrophysics

Submitted to ApJ

Scientific paper

We estimate the stellar parameters of late K and early M-type Kepler target stars. We obtained medium resolution visible spectra of 388 stars with Kp-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 95\pm1% of the bright (Kp < 14) red Kepler target stars, and 7\pm2% of dim (Kp < 14) stars, significantly higher than fractions based on the stellar parameters quoted in the Kepler Input Catalog. The KIC effective temperatures are systematically (135 K) higher than temperatures determined from fitting our spectra to PHOENIX stellar models. Through Monte Carlo simulations of the Kepler exoplanet candidate population, we find that there are 0.38\pm0.08 planets per star when giant stars are properly removed, somewhat higher than when a KIC log(g)>4 criteria is used (0.27\pm0.05). Lastly, we show that there is no significant difference in g-r color (a probe of metallicity) between late-type Kepler stars with transiting Earth-to-Neptune sized exoplanet candidates and dwarf stars with no detected transits, in line with what is seen for solar-type stars. We show that a previous claimed offset between these two populations is most likely an artifact of including a large number of misidentified giants.

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