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Measuring the temperature and abundance patterns of clouds in the interstellar medium (ISM) provides an observational basis for models of the physical conditions within the clouds, which play an important role in studies of star and planet formation. The Colorado High-resolution Echelle Stellar Spectrograph is a far-ultraviolet rocket-borne instrument designed to study the atomic-to-molecular transitions within diffuse molecular and translucent cloud regions. The final two flights of the instrument observed Scorpii (β Sco) and γ Arae. We present flight results of interstellar molecular hydrogen excitation on the sightlines, including measurements of the column densities and temperatures. These results are compared to previous values that were measured using the damping wings of low J″ H absorption features. For β Sco, we find that the derived column density of the J″ = 1 rotational level differs by a factor of 2–3 when compared to the previous observations. We discuss the discrepancies between the two measurements and show that the source of the difference is due to the opacity of higher rotational levels contributing to the J″ = 1 absorption wing, increasing the inferred column density in the previous work. We extend this analysis to 9 Copernicus and 13 Far-Ultraviolet Spectroscopic Explorer spectra to explore the interdependence of the column densities of different rotational levels and how the H kinetic temperature is influenced by these relationships. We find a revised average gas kinetic temperature of the diffuse molecular ISM of T01 = 68 ± 13 K, 12% lower than the value found previously. [ABSTRACT FROM AUTHOR]

The environment around protoplanetary disks (PPDs) regulates processes that drive the chemical and structural evolution of circumstellar material. We perform a detailed empirical survey of warm molecular hydrogen (H2) absorption observed against H i-Lyα (Lyα: λ1215.67) emission profiles for 22 PPDs, using archival Hubble Space Telescope ultraviolet (UV) spectra to identify H2 absorption signatures and quantify the column densities of H2 ground states in each sightline. We compare thermal equilibrium models of H2 to the observed H2 rovibrational level distributions. We find that, for the majority of targets, there is a clear deviation in high-energy states (Texc ≳ 20,000 K) away from thermal equilibrium populations (T(H2) ≳ 3500 K). We create a metric to estimate the total column density of non-thermal H2 (N(H2)nLTE) and find that the total column densities of thermal (N(H2)) and N(H2)nLTE correlate for transition disks and targets with detectable C iv-pumped H2 fluorescence. We compare N(H2) and N(H2)nLTE to circumstellar observables and find that N(H2)nLTE correlates with X-ray and far-UV luminosities, but no correlations are observed with the luminosities of discrete emission features (e.g., Lyα, C iv). Additionally, N(H2) and N(H2)nLTE are too low to account for the H2 fluorescence observed in PPDs, so we speculate that this H2 may instead be associated with a diffuse, hot, atomic halo surrounding the planet-forming disk. We create a simple photon-pumping model for each target to test this hypothesis and find that Lyα efficiently pumps H2 levels with Texc ≥ 10,000 K out of thermal equilibrium. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]