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 Mdwarfs 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" (MUSCLES) Treasury Survey to study H${}_2$ fluorescence in Mdwarfs. We detect fluorescence in Hubble Space Telescope spectra of 8/9 planet-hosting and 5/6 non-planet-hosting Mdwarfs. 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$\pm$0.9 using the fluxes of the brightest H${}_2$ feature and two stellar C IV lines. This is compared to 0.9$\pm$0.4 for stars without planets, showing that the planet-hosting Mdwarfs do not have significant excess H${}_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$\times$10${}^{-17}$ ergcm${}^{-2}$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 $\sim$2500-3000K region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H${}_2$ in exoplanets orbiting Mdwarfs.

20 pages, 7 figures, 7 tables; Accepted by ApJ

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