2017ApJ...845....3K

Observations of molecular hydrogen (H₂) fluorescence are a potentially useful tool for measuring the H₂ abundance in exoplanet atmospheres. This emission was previously observed in M  dwarfs with planetary systems. However, low signal-to-noise prevented a conclusive determination of its origin. Possible sources include exoplanetary atmospheres, circumstellar gas disks, and the stellar surface. We use observations from the "Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanet Host Stars" Treasury Survey to study H₂ fluorescence in M  dwarfs. We detect fluorescence in Hubble Space Telescope spectra of 8/9 planet-hosting and 5/6 non-planet-hosting M  dwarfs. The detection statistics, velocity centroids, and line widths of the emission suggest a stellar origin. We calculate H₂-to-stellar-ion flux ratios to compare flux levels between stars. For stars with planets, we find an average ratio of 1.7±0.9, using the fluxes of the brightest H₂ feature and two stellar C IV lines. This is compared to 0.9±0.4 for stars without planets, showing that the planet-hosting M  dwarfs do not have significant excess H₂ emission. This claim is supported by the direct FUV imaging of GJ 832, where no fluorescence is observed at the expected star-planet separation. Additionally, the 3σ upper limit of 4.9 x 10^–17 erg cm^–2 s^–1 from these observations is two orders of magnitude below the spectroscopically observed H₂ flux. We constrain the location of the fluorescing H₂ using 1D radiative transfer models, and find that it could reside in starspots or a ∼2500-3000  K region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H₂ in exoplanets orbiting M  dwarfs.