The K2 Ecliptic Plane Input Catalog (EPIC) and Stellar Classifications of 138,600 Targets in Campaigns 1-8

TL;DR

This paper presents the construction, classification, and analysis of the EPIC catalog for K2, detailing target properties, classifications, and biases for 138,600 stars observed in Campaigns 1-8, supporting exoplanet and galactic studies.

Contribution

The paper introduces the EPIC catalog, its construction, and stellar classifications, providing a comprehensive resource for K2 target analysis and addressing catalog limitations and biases.

Findings

EPIC provides accurate magnitudes (~0.1 mag) for the Kepler field.

Classified 138,600 targets with typical uncertainties in stellar parameters.

Stars targeted are mainly K-M dwarfs, F-G dwarfs, and K giants, with biases related to galactic latitude.

Abstract

The K2 Mission uses the Kepler spacecraft to obtain high-precision photometry over ~80 day campaigns in the ecliptic plane. The Ecliptic Plane Input Catalog (EPIC) provides coordinates, photometry and kinematics based on a federation of all-sky catalogs to support target selection and target management for the K2 mission. We describe the construction of the EPIC, as well as modifications and shortcomings of the catalog. Kepler magnitudes (Kp) are shown to be accurate to ~0.1 mag for the Kepler field, and the EPIC is typically complete to Kp~17 (Kp~19 for campaigns covered by SDSS). We furthermore classify 138,600 targets in Campaigns 1-8 (~88% of the full target sample) using colors, proper motions, spectroscopy, parallaxes, and galactic population synthesis models, with typical uncertainties for G-type stars of ~3% in Teff, ~0.3 dex in log(g), ~40% in radius, ~10% in mass, and ~40% in distance. Our results show that stars targeted by K2 are dominated by K-M dwarfs (~41% of all selected targets), F-G dwarfs (~36%) and K giants (~21%), consistent with key K2 science programs to search for transiting exoplanets and galactic archeology studies using oscillating red giants. However, we find a significant variation of the fraction of cool dwarfs with galactic latitude, indicating a target selection bias due to interstellar reddening and the increased contamination by giant stars near the galactic plane. We discuss possible systematic errors in the derived stellar properties, and differences to published classifications for K2 exoplanet host stars. The EPIC is hosted at the Mikulski Archive for Space Telescopes (MAST): http://archive.stsci.edu/k2/epic/search.php.