2017AJ....153..142P

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 = 4.5-5.5 R_⊕), but have dissimilar masses (M_P = 16-60 M_⊕), implying a diversity in their core and envelope masses. K2-32b is the least massive (M_P=16.5±2.7 M_⊕) 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±4.6 M_⊕ and M_P=39.8±4.4 M_⊕, respectively, and show no signs of additional planets. K2-108b is the most massive at M_P=59.4±4.4 M_⊕, implying a large reservoir of heavy elements of about ≃50 M_⊕. 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 ≃6-60 M_⊕, 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.