摘要：We combine multiple campaigns of K2 photometry with precision radial velocity measurements from Keck-HIRES to measure the masses of three sub-Neptune-sized planets. We confirm the planetary nature of the massive sub-Neptune K2-182 b (Pb = 4.7 days, Rb = 2.69 R⊕) and derive refined parameters for K2-199 b and c (Pb = 3.2 days, Rb = 1.73 R⊕ and Pc = 7.4 days, Rc = 2.85 R⊕). These planets provide valuable data points in the mass–radius plane, especially as TESS continues to reveal an increasingly diverse sample of sub-Neptunes. The moderately bright (V = 12.0 mag) early K dwarf K2-182 (EPIC 211359660) was observed during K2 campaigns 5 and 18. We find that K2-182 b is potentially one of the densest sub-Neptunes known to date (20 ± 5 M⊕ and 5.6 ± 1.4 g cm−3). The K5V dwarf K2-199 (EPIC 212779596; V = 12.3 mag), observed in K2 campaigns 6 and 17, hosts two recently confirmed planets. We refine the orbital and planetary parameters for K2-199 b and c by modeling both campaigns of K2 photometry and adding 12 Keck-HIRES measurements to the existing radial velocity data set (N = 33). We find that K2-199 b is likely rocky, at 6.9 ± 1.8 M⊕ and g cm−3, and that K2-199 c has an intermediate density at 12.4 ± 2.3 M⊕ and g cm−3. We contextualize these planets on the mass–radius plane, discuss a small but intriguing population of "superdense" sub-Neptunes (Rp < 3 R⊕, Mp >20 M⊕), and consider our prospects for the planets' atmospheric characterization.
