Kavli Affiliate: Andrew Vanderburg
| First 5 Authors: Michael Greklek-McKeon, Shreyas Vissapragada, Heather A. Knutson, Akihiko Fukui, Morgan Saidel
| Summary:
TOI-1266 is a benchmark system of two temperate ($<$ 450 K) sub-Neptune-sized
planets orbiting a nearby M dwarf exhibiting a rare inverted architecture with
a larger interior planet. In this study, we characterize transit timing
variations (TTVs) in the TOI-1266 system using high-precision ground-based
follow-up and new TESS data. We confirm the presence of a third exterior
non-transiting planet, TOI-1266 d (P = 32.5 d, $M_d$ = 3.68$^{+1.05}_{-1.11}
M_{oplus}$), and combine the TTVs with archival radial velocity (RV)
measurements to improve our knowledge of the planetary masses and radii. We
find that, consistent with previous studies, TOI-1266 b ($R_b$ = 2.52 $pm$
0.08 $R_{oplus}$, $M_b$ = 4.46 $pm$ 0.69 $M_{oplus}$) has a low bulk density
requiring the presence of a hydrogen-rich envelope, while TOI-1266 c ($R_c$ =
1.98 $pm$ 0.10 $R_{oplus}$, $M_c$ = 3.17 $pm$ 0.76 $M_{oplus}$) has a
higher bulk density that can be matched by either a hydrogen-rich or water-rich
envelope. Our new dynamical model reveals that this system is arranged in a
rare configuration with the inner and outer planets located near the 3:1 period
ratio with a non-resonant planet in between them. Our dynamical fits indicate
that the inner and outer planet have significantly nonzero eccentricities ($e_b
+ e_d = 0.076^{+0.029}_{-0.019}$), suggesting that TOI-1266 b may have an
inflated envelope due to tidal heating. Finally, we explore the corresponding
implications for the formation and long-term evolution of the system, which
contains two of the most favorable cool ($<$ 500 K) sub-Neptunes for
atmospheric characterization with JWST.
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