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تسجيل مستخدم جديدH$_2$ Fluorescence in M Dwarf Systems: A Stellar Origin
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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 (MUSCLES) 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$,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,$0.4 for stars without planets, showing that the planet-hosting M$;$dwarfs do not have significant excess H$_{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$,times,$10$^{-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 $sim$2500-3000$;$K region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H$_2$ in exoplanets orbiting M$;$dwarfs.
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