TOI-674b: an oasis in the desert of exo-Neptunes transiting a nearby M dwarf

Abstract

We use TESS, Spitzer, ground-based light curves and HARPS spectrograph radial velocity measurements to establish the physical properties of the transiting exoplanet candidate TOI-674b. We perform a joint fit of the light curves and radial velocity time series to measure the mass, radius, and orbital parameters of the candidate. We confirm and characterize TOI-674b, a low-density super-Neptune transiting a nearby M dwarf. The host star (TIC 158588995, $V=14.2$ mag, $J=10.3$ mag) is characterized by its M2V spectral type with ${\mathrm{M}}_{\star }=0.420\pm 0.010$ M${}_{\odot }$, ${\mathrm{R}}_{\star }=0.420\pm 0.013$ R${}_{\odot }$, and ${\mathrm{T}}_{\mathrm{eff}}=3514\pm 57$ K, and is located at a distance $d=46.16\pm 0.03$ pc. Combining the available transit light curves plus radial velocity measurements and jointly fitting a circular orbit model, we find an orbital period of $1.977143\pm 3\times 10^{-6}$ days, a planetary radius of $5.25\pm 0.17$ ${\mathrm{R}}_{\oplus }$, and a mass of $23.6\pm 3.3$ ${\mathrm{M}}_{\oplus }$ implying a mean density of ${\rho }_{\mathrm{p}}=0.91\pm 0.15$ [g cm${}^{-3}$]. A non-circular orbit model fit delivers similar planetary mass and radius values within the uncertainties. Given the measured planetary radius and mass, TOI-674b is one of the largest and most massive super-Neptune class planets discovered around an M type star to date. It is also a resident of the so-called Neptunian desert and a promising candidate for atmospheric characterisation using the James Webb Space Telescope.

22 pages, 16 figures. Accepted for publication in A&A

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