H2 Fluorescence in M Dwarf Systems: A Stellar Origin

Observations of molecular hydrogen (H-2) fluorescence are a potentially useful tool for measuring the H-2 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 H-2 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 H-2-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 H-2 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 H-2 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 3 sigma 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 H-2 flux. We constrain the location of the fluorescing H-2 using 1D radiative transfer models, and find that it could reside in starspots or a similar to 2500-3000 K region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H-2 in exoplanets orbiting M dwarfs.