Stellar characterization, Magnesium Abundances and Chromospheric Activity Analysis of Stars with Confirmed Exoplanets from the K2 mission

TL;DR

This paper presents a uniform, large-scale spectroscopic analysis of 301 confirmed K2 exoplanet host stars, deriving precise stellar parameters ($T_{ m eff}$, $\log g$), Fe/Mg abundances, and chromospheric activity from high-resolution spectra. It combines Stefan–Boltzmann and isochrone-based methods (PARAM and isochrones) to infer stellar radii and masses, propagating these to planetary radii via transit depths and a comprehensive error budget, and it robustly detects the planetary radius gap near $R_{pl} \approx 1.9\,R_\oplus$ across multiple radius estimations. The study also characterizes the chemical and dynamical context, showing a predominantly thin-disk population with [Fe/H] and [Mg/H] enrichment for hosts of larger planets, while [Mg/Fe] shows no strong dependence on planet size or multiplicity; activity indicators reveal a bimodal distribution and a trend of decreasing activity for larger planets, with possible implications for atmospheric erosion. Overall, the work provides a high-quality, homogeneous dataset that links stellar chemistry and activity to exoplanet properties, enabling robust population-level inferences and informing planet formation and evolution theories.

Abstract

We present a homogeneous spectroscopic analysis of confirmed K2 mission exoplanet-hosting stars, comprising 301 targets with high-resolution optical spectra from HIRES and TRES taken from ExoFOP. We derived effective temperatures, surface gravities, and iron and magnesium abundances in LTE by measuring the equivalent widths of Fe I, Fe II, and Mg I lines. Three estimates of stellar masses and radii were obtained via Stefan-Boltzmann and isochrone methods using the codes PARAM and isochrones. These were used to derive exoplanetary radii reaching internal precisions of 2.5%, 2.6%, and 6.6%, respectively, and the radius gap being consistently detected near 1.9 R$_{\oplus}$. We measured chromospheric activity from the Ca II H & K and H$α$ lines. Within the low-activity range ($\log R^{\prime}_{HK} < -4.75$), stellar activity appears to decrease with increasing planetary radius from super-Earths, sub-Neptunes, sub-Saturns, into the Jupiter regime. According to the [Mg/Fe] measurements, most of our K2 planet hosts belong to the Galactic thin disk, but our sample has a population from the thick disk (high-alpha sequence). Most stars show consistent chemo-dynamical behavior. We find that the [Mg/Fe] ratios are indistinguishable between systems containing Large or Small exoplanets, as well as Single- or Multi-exoplanetary systems. Both the [Fe/H] and [Mg/H] distributions reveal that stars hosting large planets are more iron- and magnesium-enhanced than those having only small planets, further confirming the link between stellar abundances and exoplanetary size, but no significant differences are found between the Single- versus Multi-exoplanetary systems.

Figures (10)

• Figure 1: Kiel diagram of the sample stars. Gray dashed lines show solar-metallicity evolutionary tracks for 4.6 and 10 Gyr from the Yonsei–Yale models. The solar proxy result is indicated by a black star.
• Figure 2: Comparison between the stellar radii determined in this work using the Stefan-Boltzmann law (SB) and using the isochrone method implemented in PARAM and isochrones (left panel). Comparisons of the stellar radii and masses in this study with those determined through asteroseismology by huber2016ApJS..224....2H (middle and right panels).
• Figure 3: Distribution of planetary radii of our exoplanets sample, derived using stellar radii estimated from the Stefan-Boltzmann equation and the isochrones method using PARAM and isochrones packages. The vertical line highlights the radius gap region.
• Figure 4: Iron abundance distributions (top left panel) are shown for the K2 host star sample from this study in comparison with the metallicity distributions for the CKS sample from Ghezzi2021 and red-giants from the APOGEE survey. The median uncertainties in the iron abundances for each sample are shown on the left. In the second row panel, we show the cumulative distribution functions (CDFs) of metallicities of host stars of single planets (red) versus multiple planets (blue), Small planets (black) versus and Large planets (orange), and Single Small (purple) versus Single Large (green). In the third and bottom panels, we show the same cases as for metallicity, but now for [Mg/H] and [Mg/Fe], respectively. The [Mg/H] distribution is noticeably shifted toward higher values for stars hosting Large planets, but this is not shown for [Mg/Fe]. The shaded regions around each CDF correspond to the 95% confidence intervals calculated via bootstrap.
• Figure 5: [Mg/Fe] ratio as a function of [Fe/H] derived in this work (blue circles) compared with the APOGEE data (gray symbols) is shown in the left panel. The dashed line represents the separation between the low-alpha and high-alpha sequences (thin and thick disks according to the division from weinberg2022ApJS..260...32W. There is a larger fraction of Single detected Small planets compared to Single detected Large planets in the low-alpha sequence (3.6 times more small planets) when compared to the high-alpha sequence (1.5 times more small planets).