Spectroscopically accessible terrestrial exoplanets are those orbiting nearby (< 15 pc), small (< 0.3 MSun) stars. There exists a growing sample of such worlds whose atmospheres we can interrogate. However, M dwarfs spend more time in the highly energetic pre-main-sequence phase than do their G star counterparts, and it is unclear if terrestrial worlds in orbit around M dwarfs can retain atmospheres. During its first year of observations TESS discovered LHS 3844b, a highly irradiated terrestrial exoplanet (Rp = 1.3 REarth; Teq = 805 K) orbiting a mid-M dwarf (Ms = 0.15 MSun; Rs = 0.19 RSun) 14.9 pc away. Spitzer phase curve data demonstrated that this planet is devoid of atmospheres with surface pressures greater than 10 bar. Combining 13 optical ground-based transit observations with the Magellan Clay Telescope and the LDSS3C spectrograph, we show that clear, low mean molecular weight atmospheres down to 0.1 bar are disfavored (5.2-sigma), as is a low-altitude cloud deck for surface pressures greater than 1 bar (5.3-sigma). To complete the picture of the LHS 3844 system, we combine these planetary atmospheric results with an ultraviolet spectrum of the host M4.5 star LHS 3844 from 10 HST/COS orbits. The time series of this UV data set reveals a far-ultraviolet short-duration flare during which count rates in prominent FUV transition lines increase by an order of magnitude. The absolute flare energy in the FUV is 9.0 ± 4.5×1028 erg with an equivalent duration of 356 ± 177 s. We place an upper limit on the FUV flare frequency of 4.9 ± 2.2 flares/day. We separate out the flare spectrum from the quiescent spectrum in order to estimate flaring and quiescent levels of high-energy output from LHS 3844. The Lyα line is obscured by geocoronal contamination from Earth’s atmosphere, but we use known correlations with observed transition line fluxes to estimate the Lyα flux. Archival Swift-XRT observations of the LHS 3844 field are consistent with no X-ray flux at the location of LHS 3844, but a preliminary scaling of the extreme ultraviolet spectrum of Proxima Centauri to LHS 3844 suggests that the planet LHS 3844b receives 2.1×104 erg/cm2/s in X-ray flux, corresponding to a hydrodynamic mass loss rate of 2×1010 g/s. Though an atmosphere around LHS 3844b is disfavored, this is the first high-energy spectrum of an inactive (Prot = 128 days; no Hα emission) mid-M star. As such we can use this spectrum as an input to mass-loss rate calculations and photochemical models of the atmospheres of terrestrial exoplanets orbiting similar M dwarfs.
This work was supported by grants from the NSF GRFP and the John Templeton Foundation.