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

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 R ⊕ and ∼2.72 ± 0.10 R ⊕ , with a gap at ∼ 1.9R ⊕ . 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 photoevaporation 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 ∝ P −0.11 , which is consistent with photo-evaporation and Earth-like core composition models of planet formation.

A super-Earth and a mini-Neptune around Kepler-59

“…As we found in Section 2.3 and taking into account the periods obtained here, planets b and c are near the Fulton gap, just around the minimum occurring at ∼1.8 R ⊕ in the radius distribution (Fulton et al 2017;Martinez et al 2019). Planets around this gap are thought to be either gas dwarfs consisting of rocky cores embedded in H2-rich gas envelopes, or water worlds containing significant amounts of H2O-dominated fluid/ice in addition to rock and gas.…”

Signatures of the core-powered mass-loss mechanism in the exoplanet population: dependence on stellar properties and observational predictions

“…In addition, transit-timing variations (e.g., Carter et al 2012;Hadden & Lithwick 2017) and follow-up radial velocity (e.g., Marcy et al 2014b;Weiss & Marcy 2014) measurements have revealed that planets smaller than about 1.6 R ⊕ have higher densities suggesting rocky 'Earth-like' E-mail: akashgpt@ucla.edu compositions while larger planets have lower densities consistent with significant H/He envelopes (e.g., Marcy et al 2014a;Rogers 2015). Intriguingly, analyses of early photometry data from the Kepler Input Catalog (e.g., Brown et al 2011), the recent spectroscopic follow-up of planet-hosting stars by the California-Kepler Survey (CKS; Petigura et al 2017), and the latest studies incorporating Gaia astrometry and asteroseismology-based data have all revealed a 'radius valley' in the distribution of these small, close-in (orbital period < 100 days) exoplanets (e.g., Owen & Wu 2013;Lopez & Fortney 2013;Fulton et al 2017;Van Eylen et al 2018;Berger et al 2018;Martinez et al 2019).…”

“…This radius valley marks the transition from a population of small, rocky 'super-Earths' to a population of large 'sub-Neptunes' with significant H/He envelopes (e.g., Owen & Wu 2013;Lopez & Fortney 2013Rogers 2015;Ginzburg et al 2016). Furthermore, studies involving Gaia astrometry (Martinez et al 2019) and asteroseismologybased high-precision measurements (Van Eylen et al 2018) of stellar parameters have measured the slope of the radius valley and obtained values of d logR p /d logM * = −0.11 +0.03 −0.03 and −0.09 +0.04 −0.02 , respectively. The bimodality in the radius distribution has been attributed to photoevaporation of H/He envelopes by highenergy radiation (e.g., XUV) from the host stars (e.g., Owen & Wu 2013;Lopez & Fortney 2013;Jin et al 2014;Chen & Rogers 2016;Owen & Wu 2017;Jin & Mordasini 2018).…”

Current Population Statistics Do Not Favor Photoevaporation over Core-powered Mass Loss as the Dominant Cause of the Exoplanet Radius Gap

“…The two groups are separated by a sharp drop in the occurrence rate of planets with radii near 1.8 R ⊕ (Fulton et al 2017), often termed the radius gap. As the precision of planetary radius measurements has improved, now reaching 10% or better for around 1700<4 R ⊕ planets, the radius gap has become increasingly clear (Owen & Wu 2013;Fulton et al 2017;Fulton & Petigura 2018;Van Eylen et al 2018;MacDonald 2019;Martinez et al 2019).…”