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We report on precise Doppler measurements of L231-32 (TOI-270), a nearby Mdwarf ($d=22$ pc, $M_\star = 0.39$ M$_\odot$, $R_\star = 0.38$ R$_\odot$),which hosts three transiting planets that were recently discovered using datafrom the Transiting Exoplanet Survey Satellite (TESS). The three planets are1.2, 2.4, and 2.1 times the size of Earth and have orbital periods of 3.4, 5.7,and 11.4 days. We obtained 29 high-resolution optical spectra with the newlycommissioned Echelle Spectrograph for Rocky Exoplanet and Stable SpectroscopicObservations (ESPRESSO) and 58 spectra using the High Accuracy Radial velocityPlanet Searcher (HARPS). From these observations, we find the masses of theplanets to be $1.58 \pm 0.26$, $6.15 \pm 0.37$, and $4.78 \pm 0.43$ M$_\oplus$,respectively. The combination of radius and mass measurements suggests that theinnermost planet has a rocky composition similar to that of Earth, while theouter two planets have lower densities. Thus, the inner planet and the outerplanets are on opposite sides of the `radius valley' -- a region in theradius-period diagram with relatively few members, which has been interpretedas a consequence of atmospheric photo-evaporation. We place these findings intothe context of other small close-in planets orbiting M dwarf stars, and usesupport vector machines to determine the location and slope of the M dwarf($T_\mathrm{eff} < 4000$ K) radius valley as a function of orbital period. Wecompare the location of the M dwarf radius valley to the radius valley observedfor FGK stars, and find that its location is a good match to photo-evaporationand core-powered mass loss models. Finally, we show that planets below the Mdwarf radius valley have compositions consistent with stripped rocky cores,whereas most planets above have a lower density consistent with the presence ofa H-He atmosphere.

Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley