The TESS-Keck Survey. A Warm, Superdense Sub-Neptune Orbiting TOI-1824

We confirm a massive sub-Neptune planet in a 22.8 d orbit around TOI-1824 (T_eff = 5200 K, V = 9.72 mag). At 2.7 ± 0.1 R_⊕ and 19.1 ± 3.2 M_⊕, TOI-1824 b lies near a small group of superdense sub-Neptunes in the mass-radius diagram (planets with R_p < 3 R_⊕ and M_p > 20 M_⊕). TESS first identified TOI-1824 b as an object of interest in April 2020 after two transits in Sector 22 were matched with a single transit in Sector 21. TOI-1824 was subsequently targeted for ground-based Doppler monitoring with Keck-HIRES and Lick-APF. We use a total of 48 radial velocity measurements from Keck-HIRES and 94 radial velocity measurements from Lick-APF to constrain TOI-1824 b’s mass to better than 5-σ precision. TOI-1824 b has a bulk density of 5.3 ± 1.0 g cm^-3, situating it near a small group of superdense Sub-Neptunes with R_p < 3 R_⊕ and M_p > 20 M_⊕. We discuss the possible formation and evolution scenarios for TOI-1824 b and whether or not the superdense sub-Neptunes represent a distinct population in the mass-radius plane.