Stellar Characterization and a Chromospheric Activity Analysis of a K2 Sample of Planet-Hosting Stars

TL;DR

This study characterizes 109 K2 planet-hosting stars by deriving stellar parameters, analyzing chromospheric activity, and examining the impact of activity on stellar parameter estimation, revealing a radius gap and activity trends.

Contribution

It provides detailed stellar parameters and chromospheric activity analysis for a K2 sample, including the detection of the radius gap and activity-rotation relationship.

Findings

Median stellar and planetary radii precisions are 1.8% and 2.3%.

The radius gap near 1.9 R_earth was detected.

Chromospheric activity decreases with increasing stellar rotation period.

Abstract

Effective temperatures, surface gravities, and iron abundances were derived for 109 stars observed by the K2 mission using equivalent width measurements of Fe I and Fe II lines. Calculations were carried out in LTE using Kurucz model atmospheres. Stellar masses and radii were derived by combining the stellar parameters with Gaia DR3 parallaxes, V-magnitudes, and isochrones. The derived stellar and planetary radii have median internal precision of 1.8%, and 2.3%, respectively. The radius gap near $\rm R_{planet}\sim 1.9 R_\oplus$ was detected in this K2 sample. Chromospheric activity was measured from the Ca II H and K lines using the Values of $\log R^\prime_{\rm HK}$ were investigated as a function of stellar rotational period (P$_{rot}$) and we found that chromospheric activity decreases with increasing P$_{rot}$, although there is a large scatter in $\log R^\prime_{\rm HK}$ ($\sim$0.5) for a given P$_{rot}$. Activity levels in this sample reveal a paucity of F & G dwarfs with intermediate activity levels (Vaughan-Preston gap). The effect that stellar activity might have on the derivation of stellar parameters was investigated by including magnetically-sensitive Fe I lines in the analysis and we find no significant differences between parameters with and without magnetically-sensitive lines, although the more active stars ($\log R^\prime _{\rm HK}>-5.0$) exhibit a larger scatter in the differences in $T_{\rm eff}$ and [Fe/H].