H 2 Fluorescence in M Dwarf Systems: A Stellar Origin

Abstract

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 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 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 H 2 -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 H 2 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 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 σ upper limit of 4.9 × 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 ∼2500–3000 region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H 2 in exoplanets orbiting dwarfs.