Revised Masses for Low Density Planets Orbiting the Disordered M-dwarf System TOI-1266

TL;DR

This study refines the mass and density measurements of two low-density sub-Neptune planets orbiting an M-dwarf, revealing their likely rocky cores with gaseous envelopes and improving measurement precision by combining new and existing data.

Contribution

It provides updated, more precise mass and density estimates for the TOI-1266 planets using combined radial velocity data, highlighting their low densities and potential compositions.

Findings

Masses of TOI-1266 b and c are approximately 4.1 and 2.4 Earth masses.

Planets have low densities around 1.3 g/cm³, indicating gaseous envelopes.

Combined data improved mass measurement precision by about 30%.

Abstract

We present an analysis of 126 new radial velocity measurements from the MAROON-X spectrograph to investigate the TOI-1266 system, which hosts two transiting sub-Neptunes at 10.8 and 18.8 days. We measure masses of $M_{b}=4.01~\pm~0.55~M_{\oplus}$ for TOI-1266 b and $M_{c}=2.00~\pm~0.72~M_{\oplus}$ for TOI-1266 c. Our mass measurements agree with existing HARPS-N observations which we combined using a weighted average yielding masses for TOI-1266 b, and c of $M_{b}=4.10~\pm~0.43~M_{\oplus}$, $M_{c}=2.4~\pm~0.54~M_{\oplus}$ respectively. The combined dataset enabled a $\approx30\%$ improvement in mass precision. With bulk densities of $\rho_{b}$ = 1.25 $\pm$ 0.36 g cm$^{-3}$ and $\rho_{c}$ = 1.36 $\pm$ 0.31 g cm$^{-3}$, the planets are among the lowest density sub-Neptunes orbiting an M dwarf. They are both consistent with rocky cores surrounded by hydrogen helium envelopes. TOI-1266 c may also be consistent with a water-rich composition, but we disfavor that interpretation from an Occam's razor perspective.