A sub-Neptune planet around TOI-1695 discovered and characterized with SPIRou and TESS

Abstract

TOI-1695 is a V-mag=13 M-dwarf star from the northern hemisphere at 45pc from the Sun, around which a 3.134-day periodic transit signal from a super-Earth candidate was identified in TESS photometry. With a transit depth of 1.3mmag, the radius of candidate TOI-1695.01 was estimated by the TESS pipeline to be 1.82R${}_{\oplus }$ with an equilibrium temperature of $\sim 620$K. We successfully detect a reflex motion of the star and establish it is due to a planetary companion at an orbital period consistent with the photometric transit period thanks to a year-long radial-velocity monitoring of TOI-1695 by the SPIRou infrared spectropolarimeter. We use and compare different methods to reduce and analyse those data. We report a 5.5-$\ddot{I}ƒ$ detection of the planetary signal, giving a mass of $5.5\pm 1.0$M${}_{\oplus }$ and a radius of $2.03\pm 0.18$R${}_{\oplus }$. We derive a mean equilibrium planet temperature of $590\pm 90$K. The mean density of this small planet of $3.6\pm 1.1$gcm${}^{-3}$ is similar (1.7-$\ddot{I}ƒ$ lower) than that of the Earth. It leads to a non-negligible fraction of volatiles in its atmosphere with $f_{H,He}=0.28_{-0.23}^{+0.46}$% or $f_{\text{water}}=23\pm 12$%. TOI-1695b is a new sub-Neptune planet at the border of the M-dwarf radius valley that can help test formation scenarios for super-Earth/sub-Neptune-like planets.

15 pages (+9 appendix pages), 19 figures, 8 tables, accepted for publication in A&A