The Discovery and Follow-up of Four Transiting Short-period Sub-Neptunes Orbiting M dwarfs

Abstract

Sub-Neptunes with $2-3R_{\oplus }$ are intermediate in size between rocky planets and Neptune-sized planets. The orbital properties and bulk compositions of transiting sub-Neptunes provide clues to the formation and evolution of close-in small planets. In this paper, we present the discovery and follow-up of four sub-Neptunes orbiting M dwarfs (TOI-782, TOI-1448, TOI-2120, and TOI-2406), three of which were newly validated by ground-based follow-up observations and statistical analyses. TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b have radii of $R_{\mathrm{p}}=2.740_{-0.079}^{+0.082}{R}_{\oplus }$, $2.769_{-0.068}^{+0.073}{R}_{\oplus }$, $2.120\pm 0.067R_{\oplus }$, and $2.830_{-0.066}^{+0.068}{R}_{\oplus }$ and orbital periods of $P=8.02$, $8.11$, $5.80$, and $3.08$\,days, respectively. Doppler monitoring with Subaru/InfraRed Doppler instrument led to 2$\ddot{I}ƒ$ upper limits on the masses of $<19.1M_{\oplus }$, $<19.5M_{\oplus }$, $<6.8M_{\oplus }$, and $<15.6M_{\oplus }$ for TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b, respectively. The mass-radius relationship of these four sub-Neptunes testifies to the existence of volatile material in their interiors. These four sub-Neptunes, which are located above the so-called ``radius valley'', are likely to retain a significant atmosphere and/or an icy mantle on the core, such as a water world. We find that at least three of the four sub-Neptunes (TOI-782 b, TOI-2120 b, and TOI-2406 b) orbiting M dwarfs older than 1 Gyr, are likely to have eccentricities of $e\sim 0.2-0.3$. The fact that tidal circularization of their orbits is not achieved over 1 Gyr suggests inefficient tidal dissipation in their interiors.

Accepted for publication in AJ, 32 pages, 17 figures, 6 tables