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Description
We present results from a Keck/HIRES radial velocity campaign to study four sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of understanding their masses, orbits, and heavy-element enrichment. The planets have similar sizes (R_P_=4.5-5.5R_{Earth}_), but have dissimilar masses (M_P_=16-60M_{Earth}_), implying a diversity in their core and envelope masses. K2-32b is the least massive (M_P_=16.5+/-2.7M_{Earth}_) and orbits in close proximity to two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are significantly more massive at M_P_=30.9+/-4.6M_{Earth}_ and M_P_=39.8+/-4.4M_{Earth}_, respectively, and show no signs of additional planets. K2-108b is the most massive at M_P_=59.4+/-4.4M_{Earth}_, implying a large reservoir of heavy elements of about {simeq}50M__. Sub-Saturns as a population have a large diversity in planet mass at a given size. They exhibit remarkably little correlation between mass and size; sub-Saturns range from {simeq}6-60M_{Earth}_, regardless of size. We find a strong correlation between planet mass and host star metallicity, suggesting that metal-rich disks form more massive planet cores. The most massive sub-Saturns tend to lack detected companions and have moderately eccentric orbits, perhaps as a result of a previous epoch of dynamical instability. Finally, we observe only a weak correlation between the planet envelope fraction and present-day equilibrium temperature, suggesting that photo-evaporation does not play a dominant role in determining the amount of gas sub-Saturns accrete from their protoplanetary disks.
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