Another super-dense sub-Neptune in K2-182 b and refined mass measurements for K2-199 b and c

Abstract

We combine multiple campaigns of K2 photometry with precision radial velocity measurements from Keck-HIRES to measure the masses of three sub-Neptune-size planets. We confirm the planetary nature of the massive sub-Neptune K2-182 b ($P_{\mathrm{b}}=4.7$ days, $R_{\mathrm{b}}=2.69$ $R_{\oplus }$) and derive refined parameters for K2-199 b and c ($P_{\mathrm{b}}=3.2$ days, $R_{\mathrm{b}}=1.73$ $R_{\oplus }$, and $P_{\mathrm{c}}=7.4$ days, $R_{\mathrm{c}}=2.85$ $R_{\oplus }$). These planets provide valuable data points in the mass-radius plane, especially as TESS continues to reveal an increasingly diverse sample of sub-Neptunes. K2-182 (EPIC 211359660) is a moderately bright ($V=12.0$ mag) early-K dwarf observed during K2 campaigns 5 and 18. We find K2-182 b is potentially one of the densest sub-Neptunes known to date ($20\pm 5$ $M_{\oplus }$ and $5.6\pm 1.4$ g cm${}^{-3}$). K2-199 (EPIC 212779596; $V=12.3$ mag) is a K5V dwarf observed in K2 campaigns 6 and 17 which 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 K2-199 b is likely rocky, at $6.9\pm 1.8$ $M_{\oplus }$ and $7.2_{-2.0}^{+2.1}$ g cm${}^{-3}$. K2-199 c has an intermediate density at $12.4\pm 2.3$ $M_{\oplus }$ and $2.9_{-0.6}^{+0.7}$ g cm${}^{-3}$. We contextualize these planets on the mass-radius plane, discuss a small but intriguing population of "super-dense" sub-Neptunes ($R_{\mathrm{p}}<3$ $R_{\oplus }$, $M_{\mathrm{p}}>20$ $M_{\oplus }$), and consider our prospects for the planets' atmospheric characterization.

42 pages, 10 figures, 9 tables. Accepted for publication in The Astronomical Journal

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