# A Spectroscopic Analysis of the California-Kepler Survey Sample: I. Stellar Parameters, Planetary Radii and a Slope in the Radius Gap

## TL;DR

This study uses high-resolution spectroscopic data combined with Gaia parallaxes to precisely determine stellar and planetary radii in the Kepler sample, confirming a bimodal radius distribution and a period-dependent radius gap consistent with photo-evaporation models.

## Contribution

It provides the first high-precision spectroscopic analysis of the CKS sample, revealing a slope in the planetary radius gap related to orbital period, supporting photo-evaporation theories.

## Key findings

- Confirmed bimodal planetary radius distribution with peaks at ~1.47 and ~2.72 R_⊕
- Detected a decreasing trend in the radius gap with orbital period, following a power law R_pl ∝ P^{-0.11}
- Achieved median uncertainties of 2.8% in stellar radii and 3.7% in planetary radii.

## Abstract

We present results from a quantitative spectroscopic analysis conducted on archival Keck/HIRES high-resolution spectra from the California-$Kepler$ Survey (CKS) sample of transiting planetary host stars identified from the $Kepler$ mission. The spectroscopic analysis was based on a carefully selected set of Fe I and Fe II lines, resulting in precise values for the stellar parameters of effective temperature (T$_{\rm eff}$) and surface gravity (log $g$). Combining the stellar parameters with $Gaia$ DR2 parallaxes and precise distances, we derived both stellar and planetary radii for our sample, with a median internal uncertainty of 2.8$\%$ in the stellar radii and 3.7$\%$ in the planetary radii. An investigation into the distribution of planetary radii confirmed the bimodal nature of this distribution for the small radius planets found in previous studies, with peaks at: $\sim$1.47 $\pm$ 0.05 R$_{\oplus}$ and $\sim$2.72 $\pm$ 0.10 R$_{\oplus}$, with a gap at $\sim$ 1.9R$_{\oplus}$. Previous studies that modeled planetary formation that is dominated by photo-evaporation predicted this bimodal radii distribution and the presence of a radius gap, or photo-evaporation valley. Our results are in overall agreement with these models. The high internal precision achieved here in the derived planetary radii clearly reveal the presence of a slope in the photo-evaporation valley for the CKS sample, indicating that the position of the radius gap decreases with orbital period; this decrease was fit by a power law of the form R$_{pl}$ $\propto$ P$^{-0.11}$, which is consistent with photo-evaporation and Earth-like core composition models of planet formation.