We report the results of the initial Far Ultraviolet Spectroscopic Explorer observations of molecular hydrogen (H 2 ) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H 2 absorption and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (E BϪV = 0.72;). We find a total H 2 column density cm3 times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N( )/N( ) is K. With the aid of ground-based data to calculate an appropriate J = 1 J = 0 ‫ע85‬ 10 multicomponent curve of growth, we have determined column densities for all rotational levels up to . The J = 7 states can be reasonably fitted with a rotational excitation temperature of K. Both the kinetic J ≥ 2 3 0 7 ‫ע‬ 23 and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight.

Neptune-size exoplanets seem particularly sensitive to atmospheric evaporation, making it essential to characterize the stellar high-energy radiation that drives this mechanism. This is particularly important with M dwarfs, which emit a large and variable fraction of their luminosity in the ultraviolet and can display strong flaring behavior. The warm Neptune GJ 3470b, hosted by an M2 dwarf, was found to harbor a giant exosphere of neutral hydrogen thanks to three transits observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). Here we report on three additional transit observations from the Panchromatic Comparative Exoplanet Treasury program, obtained with the HST Cosmic Origin Spectrograph. These data confirm the absorption signature from GJ 3470b’s exosphere in the stellar Lyman-α line and demonstrate its stability over time. No planetary signatures are detected in other stellar lines, setting a 3σ limit on GJ 3470b’s far-ultraviolet (FUV) radius at 1.3 times its Roche lobe radius. We detect three flares from GJ 3470. They show different spectral energy distributions but peak consistently in the Si III line, which traces intermediate-temperature layers in the transition region. These layers appear to play a particular role in GJ 3470’s activity as emission lines that form at lower or higher temperatures than Si III evolved differently over the long term. Based on the measured emission lines, we derive synthetic X-ray and extreme-ultraviolet (X+EUV, or XUV) spectra for the six observed quiescent phases, covering one year, as well as for the three flaring episodes. Our results suggest that most of GJ 3470’s quiescent high-energy emission comes from the EUV domain, with flares amplifying the FUV emission more strongly. The neutral hydrogen photoionization lifetimes and mass loss derived for GJ 3470b show little variation over the epochs, in agreement with the stability of the exosphere. Simulations informed by our XUV spectra are required to understand the atmospheric structure and evolution of GJ 3470b and the role played by evaporation in the formation of the hot-Neptune desert.

Planetary systems are angular momentum reservoirs generated during star formation. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation:The physics of the ISM. Stars form from dense molecular clouds that contain ∼30% of the total interstellar medium (ISM) mass. The structure, properties and lifetimes of molecular clouds are determined by the overall dynamics and evolution of a very complex system -the ISM. Understanding the physics of the ISM is of prime importance not only for Galactic but also for extragalactic and cosmological studies. Most of the ISM volume (∼65%) is filled with diffuse gas at temperatures between 3000 and 300 000 K, representing about 50% of the ISM mass. ing star formation, the so-called accretion-outflow engine. Elementary physical considerations show that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1 AU) where the gravitational field is strong. According to recent numerical simulations, this is also the region where terrestrial planets could form after 1 Myr. One should keep in mind that today the only evidence for life in the Universe comes from a planet located in this inner disk region (at 1 AU) from its parent star. The temperature of the accretion-outflow engine is between 3000 and 10 7 K. After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at ultraviolet wavelengths. The physics of planet formation. Observations of volatiles released by dust, planetesimals and comets provide an extremely powerful tool for determining the relative abundances of the vaporizing species and for studying the photochemical and physical processes acting in the inner parts of young planetary systems. This region is illuminated by the strong UV radiation field produced by the star and the accretion-outflow engine. Absorption spectroscopy provides the most sensitive tool for determining the properties of the circumstellar gas as well as the characteristics of the atmospheres of the inner planets transiting the stellar disk. UV radiation also pumps the electronic transitions of the most abundant molecules (H 2 , CO, etc.) that are observed in the UV.Here we argue that access to the UV spectral range is essential for making progress in this field, since the resonance lines of the most abundant atoms and ions at temperatures between 3000 and 300 000 K, together with the electronic transitions of the most abundant molecules (H 2 , CO, OH, CS, Springer 34 Astrophys Space Sci (2006) 303:33-52 S 2 , CO + 2 , C 2 , O 2 , O 3 , etc.) are at UV wavelengths. A powerful UV-optical instrument would provide an efficient mean for measuring the abundance of ozone in the atmosphere of the thousands of transiting planets expected to be detected by the next space missions (GAIA, Corot, Kepler, etc.). Thus, a follow-up UV mission would be optimal for identifying Earth-like candidates.

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