Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars I: Classifying Low-mass Host Stars Observed During Campaigns 1-7

TL;DR

This study provides refined stellar parameters for low-mass stars observed during K2 campaigns, revealing many targets are giants or reddened stars, and improves the accuracy of stellar radii estimates for better exoplanet characterization.

Contribution

The paper offers new spectroscopic classifications and empirical radius measurements for K2 low-mass star candidates, correcting previous catalog estimates and aiding future exoplanet studies.

Findings

49% of targets are giants or reddened stars

Revised stellar radii are on average 39% larger than catalog values

Improved stellar parameters facilitate better exoplanet characterization

Abstract

We present near-infrared spectra for 144 candidate planetary systems identified during Campaigns 1-7 of the NASA K2 Mission. The goal of the survey was to characterize planets orbiting low-mass stars, but our IRTF/SpeX and Palomar/TripleSpec spectroscopic observations revealed that 49% of our targets were actually giant stars or hotter dwarfs reddened by interstellar extinction. For the 72 stars with spectra consistent with classification as cool dwarfs (spectral types K3 - M4), we refined their stellar properties by applying empirical relations based on stars with interferometric radius measurements. Although our revised temperatures are generally consistent with those reported in the Ecliptic Plane Input Catalog (EPIC), our revised stellar radii are typically 0.13 solar radii (39%) larger than the EPIC values, which were based on model isochrones that have been shown to underestimate the radii of cool dwarfs. Our improved stellar characterizations will enable more efficient prioritization of K2 targets for follow-up studies.