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

Abstract

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 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 ∼1.47 ± 0.05 and ∼2.72 ± 0.10 R ⊕ with a gap at ∼1.9 R ⊕ . Previous studies that modeled planetary formation that is dominated by photoevaporation predicted this bimodal radii distribution and the presence of a radius gap, or photoevaporation valley. Our results are in overall agreement with these models, as well as core powered mass-loss models. The high internal precision achieved here in the derived planetary radii clearly reveal the presence of a slope in the photoevaporation 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 ∝ P −0.11 , which is consistent with both photoevaporation and core powered mass-loss models of planet formation, with Earth-like core compositions.