The Formation and Erosion of Small Planet Cores and Envelopes: Exploring Planet Occurrence as a Function of Stellar Mass and Metallicity

Objectives: A major and lasting scientific contribution of the prime Kepler mission (2009-2013) is the sample of over 4000 planet candidates having orbital periods extending out to <1 year. This distribution encodes key aspects of planet formation physics including the growth of solid cores, accretion of gas, photoevaporation of gaseous envelopes, and orbital migration. To bring the demographics of planets into sharper focus, our group conducted the California-Kepler Survey (CKS), a Keck/HIRES survey of 1300 planet-hosting stars (mostly FG-type). CKS revealed that small planets have a bimodal size distribution, with a paucity of planets between 1.5 and 2.0 Earth-radii. Moreover, CKS provided the clearest evidence yet of a Sub-Neptune Desert, a dearth of highly-irradiated 2 4 Earth-radius planets. Previous theoretical work predicted the gap and desert due to photoevaporative sculpting of planet envelopes by XUV radiation. These theories make the clear and testable prediction that the planet distribution in the radius-flux plane should vary as a function of stellar mass and metallicity. To test these predictions, we have supplemented CKS with spectroscopic and imaging survey of 400 KM-type stars, CKS-Cool. We request ADAP support for the analysis and interpretation of planet properties as a function of stellar mass that synergistically combines the Kepler, Gaia, and the CKS spectroscopic datasets. Stellar mass and metallicity are known to be astrophysically correlated, but with our enhanced leverage in stellar mass, we will disentangle the effects of mass and metallicity on the planet formation process. Finally, our science involves the identification and removal of SB2s from our sample, and we propose to develop an open-source SB2 detector, TwoSpec for our needs but also for the broader astronomical community. This code will be particularly relevant to the ongoing global follow-up effort to confirm and characterize planet candidates detected by NASA s TESS mission. Methods: We will construct a high-precision, high-purity sample of ~2600 Kepler planets, supplementing the Kepler dataset with our Keck spectroscopy/imaging. Our catalog will sample stellar hosts ranging from 0.5 to 1.5 solar masses. We will measure stellar masses, radii, metallicities, and other properties through a joint analysis of Keck/HIRES spectra, Gaia parallaxes, and 2MASS photometry, and we will update derived planet properties. False positives and/or heavily diluted transits will be identified and removed through SB2 searches and Keck/NIRC2 imaging/NRM. We will then compute planet occurrence as a function of stellar mass (folding in the DR25 completeness products) and measure how important features in the planet population vary with stellar mass and metallicity. These variations in planet occurrence will be compared directly to predictions from photoevaporation theory and other models Relevance to the ADAP: The Kepler candidates and completeness products are in the public domain and archived at MAST. The CKS dataset is publicly available at the KOA/ExoFOP. While our analysis incorporates Gaia data, our focus is on the Kepler dataset. Our proposal supports NASA s strategic goals to characterize the diverse population of exoplanets and directly supports the scientific yield of NASA s investment in the Kepler mission. CKS-Cool probes the formation and erosion of exoplanet atmospheres, which will be studied in detailed through transmission/eclipse spectroscopy with JWST. Finally, by measuring planet occurrence and its variation with stellar mass and metallicity, CKS-Cool will inform instrument and survey design of future planned/proposed NASA direct imaging missions such as WFIRST, HabEx, and LUVOIR.