A Spectroscopic Analysis of a Sample of K2 Planet-Host Stars: Stellar Parameters, Metallicities and Planetary Radii

TL;DR

This study provides a homogeneous spectroscopic analysis of K2 and Kepler host stars, deriving stellar parameters and planetary radii, confirming the radius gap and exploring planet-star property relations.

Contribution

It introduces a consistent spectroscopic methodology for stellar parameter determination and applies it to K2 and Kepler stars, improving planetary radius estimates and confirming known exoplanet trends.

Findings

Median internal uncertainty of 4.4% in planetary radii.

Detection of the radius gap near 1.9 R_⊕.

Confirmed correlations between planetary radius, orbital period, and metallicity.

Abstract

The physical properties of transiting exoplanets are connected with the physical properties of their host stars. We present a homogeneous spectroscopic analysis based on spectra of FGK-type stars observed with the Hydra spectrograph on the WIYN telescope. We derived effective temperatures, surface gravities, and metallicities, for 81 stars observed by K2 and 33 from Kepler 1. We constructed an Fe I and II line list that is adequate for the analysis of R$\sim$18,000 spectra covering 6050-6350 \r{A} and adopted the spectroscopic technique based on equivalent width measurements. The calculations were done in LTE using Kurucz model atmospheres and the qoyllur-quipu (q$^2$) package. We validated our methodology via analysis of a benchmark solar twin and solar proxies, which are used as the solar reference. We estimated the effects that including Zeeman sensitive Fe I lines have on the derived stellar parameters for young and possibly active stars in our sample and found it not to be significant. Stellar masses and radii were derived by combining the stellar parameters with Gaia EDR3 and V magnitudes and isochrones. The measured stellar radii have 4.2\% median internal precision, leading to a median internal uncertainty of 4.4\% in the derived planetary radii. With our sample of 83 confirmed planets orbiting K2 host stars, the radius gap near R$_{planet}1.9R{_\plus}$ is detected, in agreement with previous findings. Relations between the planetary radius, orbital period and metallicity are explored and these also confirm previous findings for Kepler 1 systems.