The TESS-Keck Survey. XI. Mass Measurements for Four Transiting sub-Neptunes orbiting K dwarf TOI-1246

Abstract

Multi-planet systems are valuable arenas for investigating exoplanet architectures and comparing planetary siblings. TOI-1246 is one such system, with a moderately bright K dwarf ($\mathrm{V}=11.6,\mathrm{K}=9.9$) and four transiting sub-Neptunes identified by TESS with orbital periods of $4.31\mathrm{d},5.90\mathrm{d},18.66\mathrm{d}$, and $37.92\mathrm{d}$. We collected 130 radial velocity observations with Keck/HIRES and TNG/HARPS-N to measure planet masses. We refit the 14 sectors of TESS photometry to refine planet radii ($2.97\pm 0.06{\mathrm{R}}_{\oplus },2.47\pm 0.08{\mathrm{R}}_{\oplus },3.46\pm 0.09{\mathrm{R}}_{\oplus }$, $3.72\pm 0.16{\mathrm{R}}_{\oplus }$), and confirm the four planets. We find that TOI-1246 e is substantially more massive than the three inner planets ($8.1\pm 1.1{\mathrm{M}}_{\oplus }$, $8.8\pm 1.2{\mathrm{M}}_{\oplus }$, $5.3\pm 1.7{\mathrm{M}}_{\oplus }$, $14.8\pm 2.3{\mathrm{M}}_{\oplus }$). The two outer planets, TOI-1246 d and TOI-1246 e, lie near to the 2:1 resonance (${\mathrm{P}}_{\mathrm{e}}/{\mathrm{P}}_{\mathrm{d}}=2.03$) and exhibit transit timing variations. TOI-1246 is one of the brightest four-planet systems, making it amenable for continued observations. It is one of only six systems with measured masses and radii for all four transiting planets. The planet densities range from $0.70\pm 0.24$ to $3.21\pm 0.44g/c{\mathrm{m}}^3$, implying a range of bulk and atmospheric compositions. We also report a fifth planet candidate found in the RV data with a minimum mass of 25.6 $\pm$ 3.6 ${\mathrm{M}}_{\oplus }$. This planet candidate is exterior to TOI-1246 e with a candidate period of 93.8 d, and we discuss the implications if it is confirmed to be planetary in nature.

Accepted at The Astronomical Journal; 33 pages, 10 figures

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