H2 Fluorescence in M Dwarf Systems: A Stellar Origin

Abstract

Abstract 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 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 dwarfs. We detect fluorescence in Hubble Space Telescope spectra of 8/9 planet-hosting and 5/6 non-planet-hosting 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 , using the fluxes of the brightest H2 feature and two stellar C iv lines. This is compared to for stars without planets, showing that the planet-hosting 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 × 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 region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H2 in exoplanets orbiting dwarfs.