17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

June 24-29, 2012
Contribution Abstract

The Occurrence Rate of Habitable Planets Around M Dwarfs: Limits from Kepler
Courtney D Dressing, Harvard-Smithsonian Center for Astrophysics

David Charbonneau Harvard University
David Charbonneau Harvard University

Type: Poster

Topic: Cool Stars as Exoplanet Hosts

Abstract
The Kepler target list contains 5252 stars cooler than 4000K but previous work on the Kepler data has focused on the occurrence rate of planets around FGK stars. Results from Kepler show that the occurrence rate of 2-4 $R_{Earth}$ planets (with periods $< 50$ days) rises toward lower mass stellar hosts, with roughly 1/4 of M0 dwarfs hosting such a planet. Because the stellar mass function peaks at M3-M4 dwarfs, how far this trend extends to later type stars is of paramount importance to the Galactic exoplanet census. To answer this question we investigate the occurrence rate of planets around the coolest Kepler target stars.\\ We use the photometry provided by Brown et al. (2011, ApJ, 142, 112) in the Kepler Input Catalog (KIC) to improve the radius, temperature, and metallicity estimates for the cool stars in the Kepler target list and refine the estimates of the associated planet candidate radii. The original stellar parameters provided in the KIC were determined by comparing the observed optical and infrared colors of the target stars to the expected colors from the ATLAS9 stellar models (Castelli \& Kurcuz 2004, arXiv:astro-ph/0405087). Although the ATLAS9 models accurately represent the spectra of Sun-like stars, they fail to reproduce the observed spectra of cool stars. Accordingly, the stellar parameters for the coolest stars in the Kepler target list are poorly estimated. We refine the stellar parameters by matching the observed photometry to the colors expected from the Dartmouth Stellar Evolutionary Models (Dotter et al. 2008, ApJS, 178, 89), which are better suited to modeling the atmospheres of cool dwarfs. \\ We determine the completeness of the current Kepler Objects of Interest list for cool stars by simulating the likelihood of detecting a transit of a planet with a given radius and period around each of the stars. We investigate the dependence of the occurrence rate on stellar and planetary parameters by comparing the number of planetary candidates detected at a given planetary radius and orbital period to the number of stars around which those planets could have been detected. We find that the occurrence rate of small candidates continues increasing toward lower stellar effective temperatures and hence small planets may be most common around cool stars.