Small Planets in the Galactic Context: Host Star Kinematics, Iron, and $\alpha$ Element Enhancement

Abstract

We explored the occurrence rate of small close-in planets among $\textit{Kepler}$ target stars as a function of the iron abundance and the stellar total velocity $V_\mathrm{tot}$. We estimated the occurrence rate of those planets by combining information from LAMOST and the California-$\textit{Kepler}$ Survey (CKS) and found that iron-poor stars exhibit an increase in the occurrence with $V_\mathrm{tot}$ from $f < 0.2$ planets per star at $ V_\mathrm{tot} < 30\ \mathrm{km~s}^{-1}$ to $f \sim 1.2$ at $V_\mathrm{tot} > 90\ \mathrm{km~s}^{-1}$. We suggest this planetary profusion may be a result of a higher abundance of $\alpha$ elements associated with iron-poor, high-velocity stars. Furthermore, we have identified an increase in small planet occurrence with iron abundance, particularly for the slower stars ($V_\mathrm{tot} < 30\ \mathrm{km~s}^{-1}$), where the occurrence increased to $f \sim 1.1$ planets per star in the iron-rich domain. Our results suggest there are two regions in the $([\mathrm{Fe}/\mathrm{H}],[\alpha/\mathrm{Fe}])$ plane in which stars tend to form and maintain small planets. We argue that analysis of the effect of overall metal content on planet occurrence is incomplete without including information on both iron and $\alpha$ element enhancement.