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EUV radiation is primarily responsible for driving hydrodynamic mass loss and thus determining whether an exoplanet can retain its atmosphere and water. The Lyman- α flux is primarily responsible for photo-dissociating water and methane, and therefore plays a major role in determining the chemistry in the upper atmospheres of exoplanets. Since interstellar hydrogen absorbs much of the EUV and Lyman- α radiation, reconstruction and theoretical techniques are needed to determine the intrinsic flux levels received by exoplanets. We describe the techniques and their limitations for estimating the extreme ultraviolet (EUV) spectral energy distribution (10–91.2 nm) and hydrogen Lyman- α flux (121.6 nm) emitted by host stars and incident on exoplanet atmospheres. We evaluate how each reconstruction technique can match the observed solar EUV spectral energy distribution and Lyman- α flux. Each technique has its limitations, but the techniques that can reconstruct the observed solar emission and are based on stellar activity observables that are not affected by interstellar absorption should best explain the intrinsic EUV and Lyman- α stellar emission. [ABSTRACT FROM AUTHOR]

Extreme-ultraviolet and X-ray emission from stellar coronae drives mass loss from exoplanet atmospheres, and ultraviolet emission from stellar chromospheres drives photochemistry in exoplanet atmospheres. Comparisons of the spectral energy distributions of host stars are, therefore, essential for understanding the evolution and habitability of exoplanets. The large number of stars observed with the MUSCLES, Mega-MUSCLES, and other recent Hubble Space Telescope observing programs has provided for the first time a large sample (79 stars) of reconstructed Lyα fluxes that we compare with X-ray fluxes to identify significant patterns in the relative emission from these two atmospheric regions as a function of stellar age and effective temperature. We find that as stars age on the main sequence, the emissions from their chromospheres and coronae follow a pattern in response to the amount of magnetic heating in these atmospheric layers. A single trend-line slope describes the pattern of X-ray versus Lyα emission for G and K dwarfs, but the different trend lines for M dwarf stars show that the Lyα fluxes of M stars are significantly smaller than those of warmer stars with the same X-ray flux. The X-ray and Lyα luminosities divided by the stellar bolometric luminosities show different patterns depending on stellar age. The L(Lyα)/L(bol) ratios increase smoothly to cooler stars of all ages, but the L(X)/L(bol) ratios show different trends. For older stars, the increase in coronal emission with decreasing is much steeper than that of chromospheric emission. We suggest a fundamental link between atmospheric properties and trend lines relating coronal and chromospheric heating, [ABSTRACT FROM AUTHOR]

We describe the interface between the outer heliosphere and the local interstellar medium (LISM) surrounding the Sun. The components of the inner LISM are the four partially ionized clouds (the Local Interstellar Cloud (LIC), G cloud, Blue cloud, and Aql cloud) that are in contact with the outer heliosphere, and ionized gas produced by EUV radiation primarily from ϵ CMa. We construct the three-dimensional shape of the LIC based on interstellar line absorption along 62 sightlines and show that in the directions of ϵ CMa, β CMa, and Sirius B the neutral hydrogen column density from the center of the LIC is a minimum. We call this region the "hydrogen hole." In this direction, the presence of Blue cloud absorption and the absence of LIC absorption can be simply explained by the Blue cloud lying just outside the heliosphere. We propose that the outer edge of the Blue cloud is a Strömgren shell driven toward the heliosphere by high pressures in the H ii region. We find that the vectors of neutral and ionized helium flowing through the heliosphere are inconsistent with the LIC flow vector, and that the nearby intercloud gas is consistent with ionization by ϵ CMa and other stellar sources without requiring additional sources of ionization or million-degree plasma. In the upwind direction, the heliosphere is passing through an environment of several LISM clouds, which may explain the recent influx of interstellar grains containing 60Fe from supernova ejecta measured in Antarctic snow. [ABSTRACT FROM AUTHOR]

The discovery of exoplanets and the desire to understand their atmospheric chemical composition and habitability provides a new rationale for understanding the radiation from X-rays to radio wavelengths emitted by their host stars. Semiempirical models of stellar atmospheres that include accurate treatment of radiative transfer of all important atoms, ions, and molecules provide the essential basis for understanding a star's emitted radiation that is our main data source for characterizing a star and the radiation environment of its exoplanets. In Solar-type and cooler stars, the ultraviolet and extreme ultraviolet radiation formed in their chromospheres and transition regions drive the photochemistry in exoplanet atmospheres. In this review, I describe and critique the development of semiempirical static and time-dependent models of the chromospheres and transition regions of the Sun and cooler stars as well as the spectroscopic diagnostics upon which these models are based. The related topics of stellar coronae and winds and their theoretical bases are beyond the scope of this review. [ABSTRACT FROM AUTHOR]

Exoplanets are born and evolve in the radiation and particle environment created by their host star. The host star's optical and infrared radiation heats the exoplanet's lower atmosphere and surface, while the ultraviolet, extreme ultraviolet and X-radiation control the photochemistry and mass loss from the exoplanet's upper atmosphere. Stellar radiation, especially at the shorter wavelengths, changes dramatically as a host star evolves leading to changes in the planet's atmosphere and habitability. This paper reviews the present state of our knowledge concerning the time-dependent radiation emitted by stars with convective zones, that is stars with spectral types F, G, K, and M, which comprise nearly all of the host stars of detected exoplanets. [ABSTRACT FROM AUTHOR]

Extreme ultraviolet (EUV) radiations (10-117 nm) from host stars play important roles in the ionization, heating, and mass loss from exoplanet atmospheres. Together with the host star's Lyα and far-UV (117-170 nm) radiation, EUV radiation photodissociates important molecules, thereby changing the chemistry in exoplanet atmospheres. Since stellar EUV fluxes cannot now be measured and interstellar neutral hydrogen completely obscures stellar radiation between 40 and 91.2 nm, even for the nearest stars, we must estimate the unobservable EUV flux by indirect methods. New non-LTE semiempirical models of the solar chromosphere and corona and solar irradiance measurements show that the ratio of EUV flux in a variety of wavelength bands to the Lyα flux varies slowly with the Lyα flux and thus with the magnetic heating rate. This suggests and we confirm that solar EUV/Lyα flux ratios based on the models and observations are similar to the available 10-40 nm flux ratios observed with the Extreme Ultraviolet Explorer (EUVE) satellite and the 91.2-117 nm flux observed with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite for F5 V-M5 V stars. We provide formulae for predicting EUV flux ratios based on the EUVE and FUSE stellar data and on the solar models, which are essential input for modeling the atmospheres of exoplanets. [ABSTRACT FROM AUTHOR]

The Lyα emission line dominates the far-ultraviolet spectra of late-type stars and is a major source for photodissociation of important molecules including H2O, CH4, and CO2 in exoplanet atmospheres. The incident flux in this line illuminating an exoplanet's atmosphere cannot be measured directly as neutral hydrogen in the interstellar medium (ISM) attenuates most of the flux reaching the Earth. Reconstruction of the intrinsic Lyα line has been accomplished for a limited number of nearby stars, but is not feasible for distant or faint host stars. We identify correlations connecting the intrinsic Lyα flux with the flux in other emission lines formed in the stellar chromosphere, and find that these correlations depend only gradually on the flux in the other lines. These correlations, which are based on Hubble Space Telescope spectra, reconstructed Lyα line fluxes, and irradiance spectra of the quiet and active Sun, are required for photochemical models of exoplanet atmospheres when intrinsic Lyα fluxes are not available. We find a tight correlation of the intrinsic Lyα flux with stellar X-ray flux for F5 V to K5 V stars, but much larger dispersion for Mstars. We also show that knowledge of the stellar effective temperature and rotation rate can provide reasonably accurate estimates of the Lyα flux for G and K stars, and less accurate estimates for cooler stars. [ABSTRACT FROM AUTHOR]

The tar-ultraviolet (FUV) continuum flux is recognized as a very sensitive diagnostic of the temperature structure of the Sun's lower chromosphere. Until now analysis of the available stellar FUV data has shown that solar-type stars must also have chromospheres, but quantitative analyses of stellar FUV continua require far higher quality spectra and comparison with new non-LTE chromosphere models. We present accurate far-ultraviolet (FUV, 1150-1500 Å) continuum flux measurements for solar-mass stars, made feasible by the high throughput and very low detector background of the Cosmic Origins Spectrograph on the Hubbble Space Telescope. We show that the continuum flux can be measured above the detector background even for the faintest star in our sample. We find a clear trend of increasing continuum brightness temperature at all FUV wavelengths with decreasing rotational period, which provides an important measure of magnetic heating rates in stellar chromospheres. Comparison with semiempirical solar flux models shows that the most rapidly rotating solar-mass stars have FUV continuum brightness temperatures similar to the brightest faculae seen on the Sun. The thermal structure of the brightest solar faculae therefore provides a first-order estimate of the thermal structure and heating rate for the most rapidly rotating solar-mass stars in our sample. [ABSTRACT FROM AUTHOR]

Understanding the complex structure, dynamics, and ionization of gas in the nearby interstellar medium is required before one can realistically model interstellar gas in other galaxies. High-resolution ultraviolet spectra provide the essential data for such studies because the resonance lines of most important atoms, ions, and molecules are located in the ultraviolet, and high spectral resolution is needed to resolve line profiles and determine the velocity structure along a line of sight. I list ten important physical questions concerning interstellar gas that require a more sensitive spectrometer than STIS and the desired spectral resolution to answer these questions. [ABSTRACT FROM AUTHOR]

In this summary of the conference Space Astronomy: the UV Window to the Universe, held in El Escorial, Spain, May 28 to June 1, 2007, I identify the important scientific questions posed by the speakers and the corresponding discoveries that future ultraviolet space instruments should enable. The science objectives described by the various speakers naturally fall into groups according to the needed instrumental requirements: wavelength coverage, spectral resolution, sensitivity, rapid access to targets, monitoring, and signal/noise. Although most of the science objectives presented during the conference require UV spectra in the 1,170–3,200 Å range, there are important science objectives that require spectra in the 912–1,170 Å range and at shorter wavelengths. I identify the limitations of present instruments for meeting these requirements. To avoid the upcoming UV dark age, important work must be done to properly build the World Space Observatory (WSO) and to plan future space missions. [ABSTRACT FROM AUTHOR]

I summarize and comment upon studies of stellar coronae using the remarkable spectroscopic and imaging capabilities of the Chandra X-ray Observatory. Nearly all types of stars are detected as X-ray sources, except for the late-B to early-A stars and the cool giants and supergiants. Chandra''s high resolution spectroscopy provides emission measure distributions, electron densities, coronal abundances, and tantilizing hints about the structure and evolution of stellar coronae. Chandra''s high resolution imaging is resolving the crowded fields of young clusters into their constituent stars from massive O stars to brown dwarfs. [Copyright &y& Elsevier]

Stellar ultraviolet (UV) radiation drives photochemistry, and extreme-ultraviolet (EUV) radiation drives mass loss in exoplanet atmospheres. However, the UV flux is partly unobservable due to interstellar absorption, particularly in the EUV range (100–912 Å). It is therefore necessary to reconstruct the unobservable spectra in order to characterize the radiation environment of exoplanets. In the present work, we use a radiative transfer code SSRPM to build one-dimensional semiempirical models of two M dwarf exoplanet hosts, GJ 832 and GJ 581, and synthesize their spectra. SSRPM is equipped with an extensive atomic and molecular database and full-NLTE capabilities. We use observations in the visible, ultraviolet, and X-ray ranges to constrain atmospheric structures of the modeled stars. The synthesized integrated EUV fluxes are found to be in good agreement with other reconstruction techniques, but the spectral energy distributions disagree significantly across the EUV range. More than two-thirds of the EUV flux is formed above 105 K. We find that the far-ultraviolet (FUV) continuum contributes 42%–54% of the entire FUV flux between 1450 and 1700 Å. The comparison of stellar structures of GJ 832 and GJ 581 suggests that GJ 832 is a more magnetically active star, which is corroborated by other activity indicators. [ABSTRACT FROM AUTHOR]

Astronomers have long recognized the critical need for ultraviolet imaging, photometry and spectroscopy of stars, planets, and galaxies, but this need could not be satisfied without access to space and the development of efficient instrumentation. When UV measurements became feasible, first with rockets and then with satellites, major discoveries came rapidly. It is true in the UV spectral region as in all others, that significant increases in sensitivity, spectral resolution, and time domain coverage have led to significant new understanding of astrophysical phenomena. I will describe a selection of these discoveries made in each of three eras: (1) the early history of rocket instrumentation and Copernicus, the first UV satellite, (2) the discovery phase pioneered by the IUE, FUSE and EUVE satellites, and (3) the full flowering of UV astronomy with the successful operation of HST and its many instruments. I will also mention a few areas where future UV instrumentation could lead to new discoveries. This review concentrates on developments in stellar and interstellar UV spectroscopy; the major discoveries in galactic, extragalactic, and solar system research are beyond the scope of this review. The important topic of UV technologies and detectors, which enable the remarkable advances in UV astronomy are also not included in this review. [ABSTRACT FROM AUTHOR]

Accretion plays a central role in the physics that governs the evolution and dispersal of protoplanetary disks. The primary goal of this paper is to analyze the stability over time of the mass accretion rate onto TW Hya, the nearest accreting solar-mass young star. We measure veiling across the optical spectrum in 1169 archival high-resolution spectra of TW Hya, obtained from 1998–2022. The veiling is then converted to accretion rate using 26 flux-calibrated spectra that cover the Balmer jump. The accretion rate measured from the excess continuum has an average of 2.51 × 10−9 M ⊙ yr−1 and a Gaussian distribution with an FWHM of 0.22 dex. This accretion rate may be underestimated by a factor of up to 1.5 because of uncertainty in the bolometric correction and another factor of 1.7 because of excluding the fraction of accretion energy that escapes in lines, especially Ly α. The accretion luminosities are well correlated with He line luminosities but poorly correlated with H α and H β luminosity. The accretion rate is always flickering over hours but on longer timescales has been stable over 25 years. This level of variability is consistent with previous measurements for most, but not all, accreting young stars. [ABSTRACT FROM AUTHOR]

The article discusses a report published in the issue by R. Lallement and colleagues of the observation of the Lyman-alpha (Lyα) atomic hydrogen emission line in the Milky Way by the U.S. National Aeronautics & Space Administration (NASA) Voyager spacecraft. Lyα emissions are a key indicator of star formation, but are undetectable by spacecraft close to Earth due to interference from the heliospheric background glow of hydrogen gas at the same wavelength. Ultraviolet spectrometers on the Voyager were able to collect data on the galactic Lyα emission line once the spacecraft passed far enough from the sun for the heliospheric glow to decrease.

We compare high-resolution ultraviolet spectra of the Sun and thirteen solar-mass main-sequence stars with different rotational periods that serve as proxies for their different ages and magnetic field structures. In this, the second paper in the series, we study the dependence of ultraviolet emission-line centroid velocities on stellar rotation period, as rotation rates decrease from that of the Pleiades star HII314 (Prot = 1.47 days) to α Cen A (Prot = 28 days). Our stellar sample of F9 V to G5 V stars consists of six stars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope (HST) and eight stars observed with the Space Telescope Imaging Spectrograph on HST. We find a systematic trend of increasing redshift with more rapid rotation (decreasing rotation period) that is similar to the increase in line redshift between quiet and plage regions on the Sun. The fastest-rotating solar-mass star in our study, HII314, shows significantly enhanced redshifts at all temperatures above log T = 4.6, including the corona, which is very different from the redshift pattern observed in the more slowly rotating stars. This difference in the redshift pattern suggests that a qualitative change in the magnetic-heating process occurs near Prot = 2 days. We propose that HII314 is an example of a solar-mass star with a magnetic heating rate too large for the physical processes responsible for the redshift pattern to operate in the same way as for the more slowly rotating stars. HII314 may therefore lie above the high activity end of the set of solar-like phenomena that is often called the “solar-stellar connection.” [ABSTRACT FROM AUTHOR]

Thanks to dedicated long-term missions like Voyager and GOES over the past 50 years, much insight has been gained on the activity of our Sun, the solar wind, its interaction with the interstellar medium, and, thus, about the formation, the evolution, and the structure of the heliosphere. Additionally, with the help of multi-wavelength observations by the Hubble Space Telescope, Kepler, and TESS, we not only were able to detect a variety of extrasolar planets and exomoons but also to study the characteristics of their host stars, and thus became aware that other stars drive bow shocks and astrospheres. Although features like, e.g., stellar winds, could not be measured directly, over the past years several techniques have been developed allowing us to indirectly derive properties like stellar mass-loss rates and stellar wind speeds, information that can be used as direct input to existing astrospheric modeling codes. In this review, the astrospheric modeling efforts of various stars will be presented. Starting with the heliosphere as a benchmark of astrospheric studies, investigating the paleo-heliospheric changes and the Balmer H α projections to 1 pc , we investigate the surroundings of cool and hot stars, but also of more exotic objects like neutron stars. While pulsar wind nebulae (PWNs) might be a source of high-energy galactic cosmic rays (GCRs), the astrospheric environments of cool and hot stars form a natural shield against GCRs. Their modulation within these astrospheres, and the possible impact of turbulence, are also addressed. This review shows that all of the presented modeling efforts are in excellent agreement with currently available observations. [ABSTRACT FROM AUTHOR]

Mini-Neptunes orbiting M stars are a growing population of known exoplanets. Some of them are located very close to their host star, receiving large amounts of UV radiation. Many M stars emit strong chromospheric emission in the H i Lyman α line (Lyα) at 1215.67 Å, the brightest far-UV emission line. We show that the effect of incoming Lyα flux can significantly change the photochemistry of mini-Neptunes’ atmospheres. We use GJ 436b as an example, considering different metallicities for its atmospheric composition. For solar composition, H2O-mixing ratios show the largest change because of Lyα radiation. H2O absorbs most of this radiation, thereby shielding CH4, whose dissociation is driven mainly by radiation at other far-UV wavelengths (∼1300 Å). H2O photolysis also affects other species in the atmosphere, including H, H2, CO2, CO, OH and O. For an atmosphere with high metallicity, H2O- and CO2-mixing ratios show the biggest change, thereby shielding CH4. Direct measurements of the UV flux of the host stars are important for understanding the photochemistry in exoplanets’ atmospheres. This is crucial, especially in the region between 1 and 10−6 bars, which is the part of the atmosphere that generates most of the observable spectral features. [ABSTRACT FROM AUTHOR]

Observations of H i Lyman α, the brightest UV emission line of late-type stars, are critical for understanding stellar chromospheres and transition regions, modeling photochemistry in exoplanet atmospheres, and measuring the abundances of neutral hydrogen and deuterium in the interstellar medium. Yet Lyα observations are notoriously challenging owing to severe attenuation from interstellar gas, hindering our understanding of this important emission line's basic morphology. We present high-resolution far- and near-UV spectroscopy of five G, K, and M dwarfs with radial velocities large enough to Doppler-shift the stellar Lyα emission line away from much of the interstellar attenuation, allowing the line core to be directly observed. We detect self-reversal in the Lyα emission-line core for all targets, and we show that the self-reversal depth decreases with increasing surface gravity. Mg ii self-reversed emission-line profiles provide some useful information to constrain the Lyα line core, but the differences are significant enough that Mg ii cannot be used directly as an intrinsic Lyα template during reconstructions. We show that reconstructions that neglect self-reversal could overestimate intrinsic Lyα fluxes by as much as 60%–100% for G and K dwarfs and 40%–170% for M dwarfs. The five stars of our sample have low magnetic activity and subsolar metallicity; a larger sample size is needed to determine how sensitive these results are to these factors. [ABSTRACT FROM AUTHOR]

Abstract: Recent observations of several planet-hosting M dwarfs show that most have FUV/NUV flux ratios 1000 times greater than that of the Sun. Here we show that the atmospheric oxygen contents (O2 and O3) of potentially habitable planets in this type of UV environment could be 2–3 orders of magnitude greater than those of their counterparts around Sun-like stars as a result of decreased photolysis of O3, H2O2, and HO2. Thus detectable levels of atmospheric oxygen, in combination with the existence of H2O and CO2, may not be the most promising biosignatures on planets around stars with high FUV/NUV ratios such as the observed M dwarfs. [Copyright &y& Elsevier]

We present a far-ultraviolet (FUV) spectral atlas consisting of spectra of 91 pre-main-sequence stars. Most stars in this sample were observed with the Space Telescope Imaging Spectrograph and Advanced Camera for Surveys on the Hubble Space Telescope (HST). A few archival spectra from the International Ultraviolet Explorer and the Goddard High Resolution Spectrograph on HST are included for completeness. We find strong correlations among the O I λ 1304 triplet, the Si IV λλ1394/1403 doublet, the C IV λ 1549 doublet, and the He II λ 1640 line luminosities. For classical T Tauri stars (CTTSs), we also find strong correlations between these lines and the accretion luminosity, suggesting that these lines form in processes related to accretion. These FUV line fluxes and X-ray luminosity correlate loosely with large scatters. The FUV emission also correlates well with Hα, Hβ, and Can K line luminosities. These correlations between FUV and optical diagnostics can be used to obtain rough estimates of FUV line fluxes from optical observations. Molecular hydrogen (H2) emission is generally present in the spectra of actively accreting CTTSs but not the weak-lined T Tauri stars that are not accreting. The presence of H2 emission in the spectrum of HD 98800 N suggests that the disk should be classified as actively accreting rather than a debris disk. We discuss the importance of FUV radiation, including the hydrogen Lyα line, on the photoevaporation of exoplanet atmospheres. We find that the Ca II/C IV flux ratios for more evolved stars are lower than those for less evolved accretors, indicating preferential depletion of refractory metals into dust grains. [ABSTRACT FROM AUTHOR]

High-resolution UV spectra of stellar H i Lyα lines from the Hubble Space Telescope (HST) provide observational constraints on the winds of coronal main-sequence stars, thanks to an astrospheric absorption signature created by the interaction between the stellar winds and the interstellar medium. We report the results of a new HST survey of M dwarf stars, yielding six new detections of astrospheric absorption. We estimate mass-loss rates for these detections and upper limits for nondetections. These new constraints allow us to characterize the nature of M dwarf winds and their dependence on coronal activity for the first time. For a clear majority of the M dwarfs, we find winds that are weaker than or comparable in strength to that of the Sun, i.e.,. However, two of the M dwarfs have much stronger winds: YZ CMi (M4 Ve;) and GJ 15AB (M2 V+M3.5 V;). Even these winds are much weaker than expectations if the solar relation between flare energy and coronal mass ejection (CME) mass extended to M dwarfs. Thus, the solar flare/CME relation does not appear to apply to M dwarfs, with important ramifications for the habitability of exoplanets around M dwarfs. There is evidence for some increase in with coronal activity as quantified by X-ray flux, but with much scatter. One or more other factors must be involved in determining wind strength besides spectral type and coronal activity, with magnetic topology being one clear possibility. [ABSTRACT FROM AUTHOR]

We analyze high-resolution spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope toward 34 nearby stars (⩽100 pc) to record Mg II, Fe II, and Mn II absorption due to the local interstellar medium (LISM). Observations span the entire sky, probing previously unobserved regions of the LISM. The heavy ions studied in this survey produce narrow absorption features that facilitate the identification of multiple interstellar components. We detected one to six individual absorption components along any given sight line, and the number of absorbers roughly correlates with the pathlength. This high-resolution near-ultraviolet (NUV) spectroscopic survey was specifically designed for sight lines with existing far-UV (FUV) observations. The FUV spectra include many intrinsically broad absorption lines (i.e., of low atomic mass ions) and are often observed at medium resolution. The LISM NUV narrow-line absorption component structure presented here can be used to more accurately interpret the archival FUV observations. As an example of this synergy, we present a new analysis of the temperature and turbulence along the line of sight toward ε Ind. The new observations of LISM velocity structure are also critical in the interpretation of astrospheric absorption derived from fitting the saturated H I Lyα profile. As an example, we reanalyze the spectrum of λ And and find that this star likely does have an astrosphere. Two stars in the sample that have circumstellar disks (49 Cet and HD141569) show evidence for absorption due to disk gas. Finally, the substantially increased number of sight lines is used to test and refine the three-dimensional kinematic model of the LISM and search for previously unidentified clouds within the Local Bubble. We find that every prediction made by the Redfield & Linsky kinematic model of the LISM is confirmed by an observed component in the new lines of sight. [ABSTRACT FROM AUTHOR]

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux Fλ ≈ 1.0 x 10-14 erg cm-2 s-1 Å-1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Lyα absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy. [ABSTRACT FROM AUTHOR]

The direct study of molecular gas in inner protoplanetary disks is complicated by uncertainties in the spatial distribution of the gas, the time variability of the source, and the comparison of observations across a wide range of wavelengths. Some of these challenges can be mitigated with far-ultraviolet spectroscopy. Using new observations obtained with the Hubble Space Telescope Cosmic Origins Spectrograph, we measure column densities and rovibrational temperatures for CO and H2 observed on the line of sight through the AA Tauri circumstellar disk. CO A - X absorption bands are observed against the far-UV continuum. The CO absorption is characterized by log10 (N(12CO)) = 17.5 ± 0.5 cm-2 and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed K, although this rotational temperature may underestimate the local kinetic temperature of the CO-bearing gas. We also detect 13CO in absorption with an isotopic ratio of ~20. We do not observe H2 absorption against the continuum; however, hot H2 (v > 0) is detected in absorption against the Lyα emission line. We measure the column densities in eight individual rovibrational states, determining a total log10 (N(H2)) = Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed with a thermal temperature of Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed K. The high temperature of the molecules, the relatively small H2 column density, and the high inclination of the AA Tauri disk suggest that the absorbing gas resides in an inner disk atmosphere. If the H2 and CO are cospatial within a molecular layer ~0.6 AU thick, this region is characterized by Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed with an observed (CO/H2) ratio of ~0.4. We also find evidence for a departure from a purely thermal H2 distribution, suggesting that excitation by continuum photons and H2 formation may be altering the level populations in the molecular gas. [ABSTRACT FROM AUTHOR]

We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX- 11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 1017 cm2 and Trot(CO) 500 ± 200 K for the absorbing gas. We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H I Lyα. All three objects show emission from CO bands at λ > 1560 #197;, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H2, and photo-excited H2, all of which appeared as a "continuum" whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) 1018-1019 cm2, Trot(CO) ≳ 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H2 emission, concluding that the observations of UV-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio (N(CO)IN(H2)) in the inner disk is 1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm. [ABSTRACT FROM AUTHOR]

The formation timescale and final architecture of exoplanetary systems are closely related to the properties of the molecular disks from which they form. Observations of the spatial distribution and lifetime of the molecular gas at planet-forming radii (a < 10 AU) are important for understanding the formation and evolution of exoplanetary systems. Toward this end, we present the largest spectrally resolved survey of H2 emission around low-mass pre-main-sequence stars compiled to date. We use a combination of new and archival far-ultraviolet spectra from the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph instruments on the Hubble Space Telescope to sample 34 T Tauri stars (27 actively accreting Classical T Tauri Stars and 7 non-accreting Weak-lined T Tauri Stars) with ages ranging from ∼1 to 10 Myr. We observe fluorescent H2 emission, excited by Lyα photons, in 100% of the accreting sources, including all of the transitional disks in our sample (CS Cha, DM Tau, GM Aur, UX Tau A, LkCa 15, HD 135344B, and TW Hya). The spatial distribution of the emitting gas is inferred from spectrally resolved H2 line profiles. Some of the emitting gas is produced in outflowing material, but the majority of H2 emission appears to originate in a rotating disk. For the disk-dominated targets, the H2 emission originates predominately at a ≲ 3 AU. The emission line widths and inner molecular radii are found to be roughly consistent with those measured from mid-IR CO spectra. [ABSTRACT FROM AUTHOR]