HST/STIS Lyman-α observations of the quiet M dwarf GJ 436 Predictions for the exospheric transit signature of the hot Neptune GJ 436b

Lyman-alpha (Ly alpha) emission of neutral hydrogen (lambda 1215.67 angstrom) is the main contributor to the ultraviolet flux of low-mass stars such as M dwarfs. It is also the main light source used in studies of the evaporating upper atmospheres of transiting extrasolar planets with ultraviolet transmission spectroscopy. However, there are very few observations of the Ly alpha emissions of quiet M dwarfs, and none exist for those hosting exoplanets. Here, we present Ly alpha observations of the hot-Neptune host star GJ 436 with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). We detect bright emission in the first resolved and high quality spectrum of a quiet M dwarf at Ly alpha. Using an energy diagram for exoplanets and an N-body particle simulation, this detection enables the possible exospheric signature of the hot Neptune to be estimated as a similar to 11% absorption in the Ly alpha stellar emission, for a typical mass-loss rate of 10(10) g s (1). The atmosphere of the planet GJ 436b is found to be stable to evaporation, and should be readily observable with HST. We also derive a correlation between X-ray and Ly alpha emissions for M dwarfs. This correlation will be useful for predicting the evaporation signatures of planets transiting other quiet M dwarfs.