Quantitative correlation of refractory elemental abundances between rocky exoplanets and their host stars

Abstract

Context. Stellar elemental abundances are generally used to constrain the interiors of rocky exoplanets by assuming planet’s relative abundances of major refractory elements (Fe, Mg, and Si) are similar to those of their host stars. Very recently, a non-one-to-one correlation was found among the compositions of low-mass planets and their host stars. It is therefore of great interest to further explore this correlation for larger samples of rocky exoplanets. Aims. We focus on a large sample of rocky exoplanets and compute their bulk elemental abundance ratios. We analyze the quantitative correlation between rocky exoplanets and their host stars by comparing the abundance ratios of these refractory elements. Methods. The interior of rocky exoplanets is assumed to be an iron-rich core overlaid with a silicate mantle. We constrained the bulk composition of rocky exoplanets from their measured mass and radius, using Bayesian statistical approaches. Then we used orthogonal distance regression (ODR) to characterize the compositional correlation between rocky exoplanets and their host stars. Results. Some rocky exoplanets are shown to have high iron-mass fractions and are thus likely to be iron-enriched super-Mercuries. We find the iron content of rocky exoplanets is dependent on the metallicity [Fe/H] of their host stars. The planets formed around a higher metallicity star generally span a wider range of iron masses, allowing for a higher iron content. Moreover, we directly compared the iron-mass fractions of rocky exoplanets with those deduced from the refractory elemental abundance ratios of their host stars. The results suggest that most rocky planets are more iron-enriched with respect to the initial protoplanetary disk.