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The star [Formula: see text] Pictoris harbors a young planetary system of about 20 million years old, which is characterized by the presence of a gaseous and dusty debris disk, at least two massive planets and many minor bodies. For more than thirty years, exocomets transiting the star have been detected using spectroscopy, probing the gaseous part of the cometary comas and tails. The detection of the dusty component of the tails can be performed through photometric observations of the transits. Since 2018, the Transiting Exoplanet Survey Satellite has observed [Formula: see text] Pic for a total of 156 days. Here we report an analysis of the TESS photometric data set with the identification of a total of 30 transits of exocomets. Our statistical analysis shows that the number of transiting exocomet events (N) as a function of the absorption depth (AD) in the light curve follows a power law in the form [Formula: see text], where [Formula: see text]. This distribution of absorption depth leads to a differential comet size distribution proportional to [Formula: see text], where [Formula: see text], showing a striking similarity to the size distribution of comets in the Solar system and the distribution of a collisionally relaxed population ([Formula: see text]).

(Abridged) Short-period gas giant exoplanets are susceptible to intense atmospheric escape due to their large scale heights and strong high-energy irradiation. This process is thought to occur ubiquitously, but to date we have only detected direct evidence of atmospheric escape in hot Jupiters and warm Neptunes. The paucity of cases for intermediate, Saturn-sized exoplanets at varying levels of irradiation precludes a detailed understanding of the underlying physics in atmospheric escape of hot gas giants. Our objectives here are to assess the high-energy environment of the warm ($T_\mathrm{eq} = 970$ K) Saturn WASP-29 b and search for signatures of atmospheric escape. We used far-ultraviolet (FUV) observations from the Hubble Space Telescope to analyze the flux time series of H I, C II, Si III, Si IV, and N V during the transit of WASP-29 b. At 3$\sigma$ confidence, we rule out any in-transit absorption of H Ilarger than 92% in the Lyman-$\alpha$ blue wing and 19% in the red wing. We found an in-transit flux decrease of $39\%^{+12\%}_{-11\%}$ in the ground-state C II emission line at 133.45 nm. But due to this signal being significantly present in only one visit, it is difficult to attribute a planetary or stellar origin for the ground-state C II signal. We place 3$\sigma$ absorption upper limits of 40%, 49% and 24% for Si III, Si IV, and for excited-state C II at 133.57 nm, respectively. Low activity levels and the faint X-ray luminosity suggest that WASP-29 is an old, inactive star. An energy-limited approximation combined with the reconstructed EUV spectrum of the host suggests that the planet is losing its atmosphere at a rate of $4 \times 10^9$ g s$^{-1}$. The non-detection at Lyman-$\alpha$ could be partly explained by a low fraction of escaping neutral hydrogen, or by the state of fast radiative blow-out we infer from the reconstructed stellar Lyman-$\alpha$ line., Comment: 10 pages, 8 figures, accepted for publication in Astronomy & Astrophysics. v2 with a few text changes for consistency

Mass is one of the most important parameters for determining the true nature of an astronomical object. Yet, many published exoplanets in on-line database, such as exoplanet.eu or the NASA exoplanet archive, still lacks a measurement of their true mass, in particular those detected thanks to radial velocity (RV) variations of their host star. For those, only the minimum mass, or m sin(i), is known, owing to the insensitivity of RVs to the inclination of the detected orbit compared to the plane-of-the-sky. The mass that is given in database is generally that of an assumed edge-on system (90 degrees), but many other inclinations are likely, even extreme values closer to 0 degree (face-on configuration). In such case, the mass of the published object could be strongly underestimated, even by 1 or 2 orders of magnitude. We used a recently developed tool, called GASTON (Kiefer et al. 2019b & 2019c), to take advantage of the voluminous Gaia astrometric database, in order to constrain the inclination and true mass of several hundreds of published exoplanet candidates (Kiefer et al. 2020, submitted). In this presentation, we will present the method and report on several exoplanet candidates reclassified in the stellar domain, among which unknown brown/M-dwarf. We also confirm the planetary nature of a few tens of candidates.

The young planetary system β Pictoris is surrounded by a debris disk of dust and gas. The gas source of this disk could be exocomets (or “falling and evaporating bodies”, FEBs), which produce refractory elements (Mg, Ca, Fe) through sublimation of dust grains at several tens of stellar radii. Nearly 1700 high resolution spectra of β Pictoris were obtained between 2003 and 2017 using the HARPS spectrograph. In Paper I, we showed that a high signal to noise ratio allows the detection of many weak Fe I lines in more than ten excited levels, and we derived the physical characteristics of the iron gas in the disk. The measured temperature of the gas (~1300 K) suggests that it is produced by evaporation of grains at about 0.3 au (38 R⋆) from the star. Here, we describe the yearly variations of the column densities of all Fe I components (from both ground and excited levels). The drop in the Fe I ground level column density after 2011 coincides with a drop in Fe I excited levels column density, as well as in the Ca II doublet and a ground level Ca I line at the same epoch. All drops are compatible with photoionisation-recombination equilibrium and β Pic like relative abundances, in a medium at 1300 K and at 0.3 au from β Pictoris. Interestingly, this warm medium does not correlate with the numerous exocomets in the circumstellar environnement of this young star.

The debris disk surrounding $\beta$ Pictoris has a gas composition rich in carbon and oxygen, relative to solar abundances. Two possible scenarios have been proposed to explain this enrichment. The preferential production scenario suggests that the gas produced may be naturally rich in C and O, while the alternative preferential depletion scenario states that the enrichment has evolved to the current state from a gas with solar-like abundances. In the latter case, the radiation pressure from the star expels the gas outwards, leaving behind species less sensitive to stellar radiation such as C and O. Nitrogen is also not sensitive to radiation pressure due to its low oscillator strength, which would make it also overabundant under the preferential depletion scenario. As such, the abundance of N in the disk may provide clues to why C and O are overabundant. We aim to measure the N column density in the direction of $\beta$ Pic, and use this information to disentangle these different scenarios explaining the C and O overabundance. Using far-UV spectroscopic data collected by the HST's Cosmic Origins Spectrograph (COS) instrument, we analyse the spectrum and characterise the NI triplet by modelling the absorption lines. We measure the N column density in the direction of $\beta$ Pic for the first time, and find it to be $\log(N_{\mathrm{NI}}/1\,\mathrm{cm}^2) = 14.9\pm0.7$. The N gas is found to be consistent with solar abundances and Halley dust. The solar N abundance supports the preferential production hypothesis, in which the composition of gas in $\beta$\,Pic is the result of photodesorption from icy grains rich in C and O or collisional vaporisation of C and O rich dust in the disk. It does not support the hypothesis that C and O are overabundant due to the insensitivity of C and O to radiation pressure thereby leaving them to accumulate in the disk., Comment: 8 pages, 3 figures. Accepted for publication in A&A

Mass is one of the most important parameters for determining the true nature of an astronomical object. Yet, many published exoplanets lack a measurement of their true mass, in particular those detected as a result of radial-velocity (RV) variations of their host star. For those examples, only the minimum mass, or m sin i, is known, owing to the insensitivity of RVs to the inclination of the detected orbit compared to the plane of the sky. The mass that is given in databases is generally that of an assumed edge-on system (~90°), but many other inclinations are possible, even extreme values closer to 0° (face-on). In such a case, the mass of the published object could be strongly underestimated by up to two orders of magnitude. In the present study, we use GASTON, a recently developed tool taking advantage of the voluminous Gaia astrometric database to constrain the inclination and true mass of several hundreds of published exoplanet candidates. We find nine exoplanet candidates in the stellar or brown dwarf (BD) domain, among which six were never characterized. We show that 30 Ari B b, HD 141937 b, HD 148427 b, HD 6718 b, HIP 65891 b, and HD 16760 b have masses larger than 13.5 MJ at 3σ. We also confirm the planetary nature of 27 exoplanets, including HD 10180 c, d and g. Studying the orbital periods, eccentricities, and host-star metallicities in the BD domain, we found distributions with respect to true masses consistent with other publications. The distribution of orbital periods shows of a void of BD detections below ~100 d, while eccentricity and metallicity distributions agree with a transition between BDs similar to planets and BDs similar to stars in the range 40–50 MJ.

The impact of the stellar mass on the giant planet properties is still to be fully understood. Main-Sequence (MS) stars more massive than the Sun remain relatively unexplored in radial velocity (RV) surveys, due to their characteristics that hinder classical RV measurements. Our aim is to characterize the close (up to 2.5 au) giant planet (GP) and brown dwarf (BD) population around AF MS stars and compare this population to stars with different masses. We used the SOPHIE spectrograph located on the 1.93m telescope at Observatoire de Haute-Provence to observe 125 northern, MS AF dwarfs. We used our dedicated SAFIR software to compute the RV and other spectroscopic observables. We characterized the detected sub-stellar companions and computed the GP and BD occurrence rates combining the present SOPHIE survey and a similar HARPS survey. We present new data on two known planetary systems around the F5-6V dwarfs HD16232 and HD113337. For the latter, we report an additional RV variation that might be induced by a second GP on a wider orbit. We also report the detection of fifteen binaries or massive sub-stellar companions with high-amplitude RV variations or long-term RV trends. Based on 225 targets observed with SOPHIE or HARPS, we constraint the BD frequency within 2-3 au around AF stars to be below 4 percents (1-sigma). For Jupiter-mass GP within 2-3 au (periods below 1000 days), we found the occurrence rate to be 3.7 (+3/-1) percents around AF stars with masses below 1.5 solar masses, and to be below 6 percents around AF stars with masses above 1.5 solar masses. For periods smaller than 10 days, we find the GP occurrence rate to be below 3 or 4.5 percents, respectively. Our results are compatible with the GP frequency reported around FGK dwarfs and are compatible with a possible increase of GP orbital periods with the stellar mass as predicted by formation models., Comment: 32 pages, 21 figures. Accepted to Astronomy & Astrophysics on August 30, 2018

We present the detection of potassium in the atmosphere of HAT-P-1b using optical transit narrowband photometry. The results are obtained using the 10.4 m Gran Telescopio Canarias (GTC) together with the OSIRIS instrument in tunable filter imaging mode. We observed four transits, two at continuum wavelengths outside the potassium feature, at 6792 {\AA} and 8844 {\AA}, and two probing the potassium feature in the line wing at 7582.0 {\AA} and the line core at 7664.9 {\AA} using a 12 {\AA} filter width (R~650). The planet-to-star radius ratios in the continuum are found to be $R_{\rm{pl}}/R_{\star}$ = 0.1176 $\pm$ 0.0013 at 6792 {\AA} and $R_{\rm{pl}}/R_{\star}$ = 0.1168 $\pm$ 0.0022 at 8844 {\AA}, significantly lower than the two observations in the potassium line: $R_{\rm{pl}}/R_{\star}$ = 0.1248 $\pm$ 0.0014 in the line wing at 7582.0 {\AA} and $R_{\rm{pl}}/R_{\star}$ = 0.1268 $\pm$ 0.0012 in the line core at 7664.9 {\AA}. With a weighted mean of the observations outside the potassium feature $R_{\rm{pl}}/R_{\star}$ = 0.1174 $\pm$ 0.0010, the potassium is detected as an increase in the radius ratio of {\Delta}$R_{\rm{pl}}/R_{\star}$ = 0.0073 $\pm$ 0.0017 at 7582.0 {\AA} and {\Delta}$R_{\rm{pl}}/R_{\star}$ = 0.0094 $\pm$ 0.0016 at 7664.9 {\AA} (a significance of 4.3 and 6.1 $\sigma$ respectively). We hypothesise that the strong detection of potassium is caused by a large scale height, which can be explained by a high-temperature at the base of the upper atmosphere. A lower mean molecular mass caused by the dissociation of molecular hydrogen into atomic hydrogen by the EUV flux from the host star may also partly explain the amplitude of our detection., Comment: 11 pages, 7 figures

As revealed by its peculiar Kepler light curve, the enigmatic star KIC 8462852 undergoes short and deep flux dimmings at a priori unrelated epochs. This star presents nonetheless all other characteristics of a quiet 1 Gyr old F3V star. These dimmings resemble the absorption features expected for the transit of dust cometary tails. The exocomet scenario is therefore most commonly advocated. We reanalysed the Kepler data and extracted a new high-quality light curve to allow for the search of shallow signatures of single or a few exocomets. We discovered that among the 22 flux dimming events that we identified, two events present a striking similarity. These events occurred 928.25 days apart and lasted for 4.4 days with a drop in the star brightness by 1000 ppm. We show that the light curve of these events is well explained by the occultation of the star by a giant ring system or by the transit of a string of half a dozen exocomets with a typical dust production rate of 105–106 kg s-1. Assuming that these two similar events are related to the transit of the same object, we derive a period of 928.25 days. The following transit was expected in March 2017 but bad weather prohibited us from detecting it from ground-based spectroscopy. We predict that the next event will occur between 3−8 October 2019.

The young planetary system {\beta} Pictoris is surrounded by a circumstellar disk of dust and gas. Because both dust and gas have a lifetime shorter than the system age, they need to be replenished continuously. The gas composition is partly known, but its location and its origin are still a puzzle. The gas source could be the exocomets (or so-called falling and evaporating bodies, FEBs), which are observed as transient features in absorption lines of refractory elements (Mg, Ca, and Fe) when they transit in front of the star at several tens of stellar radii. Nearly 1700 high-resolution spectra of {\beta} Pictoris have been obtained from 2003 to 2015 using the HARPS spectrograph. In these spectra, the circumstellar disk is always detected as a stable component among the numerous variable absorption signatures of transiting exocomets. Summing all the 1700 spectra allowed us to reach a signal-to-noise ratio higher than 1000, which is an unprecedentedly high number for a {\beta} Pictoris spectrum. It revealed many weak Fe I absorption lines of the circumstellar gas in more than ten excited states. These weak lines bring new information on the physical properties of the neutral iron gas in the circumstellar disk. The population of the first excited levels follows a Boltzmann distribution with a slope consistent with a gas temperature of about 1300 K; this temperature corresponds to a distance to the star of ~ 38 RStar and implies a turbulence of {\xi} ~ 0.8 km/s., Comment: 11 pages, 8 figures. To be published in Astronomy & Astrophysics

Using Spitzer photometry at 4.5 microns, we search for rings and satellites around the long period transiting planet CoRoT-9b. We observed two transits in 2010 and 2011. From their non-detection, we derive upper limits on the plausible physical characteristics of these objects in the planet environment. We show that a satellite larger than about 2.5 Earth radii is excluded at 3-$\sigma$ for a wide range of elongations at the two epochs of observations. Combining the two observations, we conclude that rings are excluded for a wide variety of sizes and inclination. We find that for a ring extending up to the Roche limit, its inclination angle from the edge-on configuration as seen from the Earth must be lower than 13{\deg} in the case of silicate composition and lower than 3{\deg} in the case of material with water ice density., Comment: Accepted for publication in A&A

The M dwarf GJ 436 hosts a transiting warm Neptune known to experience atmospheric escape. Previous observations revealed the presence of a giant hydrogen exosphere transiting the star for more than 5 h, and absorbing up to 56% of the flux in the blue wing of the stellar Lyman-{\alpha} line of neutral hydrogen (H i Ly{\alpha}). The unexpected size of this comet-like exosphere prevented observing the full transit of its tail. In this Letter, we present new Ly{\alpha} observations of GJ 436 obtained with the Space Telescope Imaging Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The stability of the Ly{\alpha} line over six years allowed us to combine these new observations with archival data sets, substantially expanding the coverage of the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep occulting the star for 10-25 h after the transit of the planet, remarkably confirming a previous prediction based on 3D numerical simulations with the EVaporating Exoplanet code (EVE). This result strengthens the interpretation that the exosphere of GJ 436b is shaped by both radiative braking and charge exchanges with the stellar wind. We further report flux decreases of 15 +/- 2% and 47 +/- 10% in the red wing of the Ly{\alpha} line and in the line of ionised silicon (Si iii). Despite some temporal variability possibly linked with stellar activity, these two signals occur during the exospheric transit and could be of planetary origin. Follow-up observations will be required to assess the possibility that the redshifted Ly{\alpha} and Si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star., Comment: 10 pages, 7 figures, published in A&A

The young and nearby star \beta\ Pictoris (\beta\ Pic) is surrounded by a debris disk composed of dust and gas known to host a myriad evaporating exocomets, planetesimals and at least one planet. At an edge-on inclination, as seen from Earth, this system is ideal for debris disk studies providing an excellent opportunity to use absorption spectroscopy to study the planet forming environment. Using the Cosmic Origins Spectrograph (COS) instrument on the Hubble Space Telescope (HST) we observe the most abundant element in the disk, hydrogen, through the HI Lyman \alpha\ (Ly-\alpha\) line. We present a new technique to decrease the contamination of the Ly-\alpha\ line by geocoronal airglow in COS spectra. This Airglow Virtual Motion (AVM) technique allows us to shift the Ly-\alpha\ line of the astrophysical target away from the contaminating airglow emission revealing more of the astrophysical line profile. The column density of hydrogen in the \beta\ Pic stable gas disk at the stellar radial velocity is measured to be $\log(N_{\mathrm{H}}/1 \mathrm{cm}^2) \ll 18.5$. The Ly-\alpha\ emission line profile is found to be asymmetric and we propose that this is caused by HI falling in towards the star with a bulk radial velocity of $41\pm6$ km/s relative to \beta\ Pic and a column density of $\log(N_{\mathrm{H}}/1 \mathrm{cm}^2) = 18.6\pm0.1$. The high column density of hydrogen relative to the hydrogen content of CI chondrite meteorites indicates that the bulk of the hydrogen gas does not come from the dust in the disk. This column density reveals a hydrogen abundance much lower than solar, which excludes the possibility that the detected hydrogen could be a remnant of the protoplanetary disk or gas expelled by the star. We hypothesise that the hydrogen gas observed falling towards the star arises from the dissociation of water originating from evaporating exocomets., Comment: 11 pages including 10 figures

Context. Brown dwarfs (BD) are substellar objects intermediate between planets and stars with masses of ~13–80 MJ. While isolated BDs are most likely produced by gravitational collapse in molecular clouds down to masses of a few MJ, a non-negligible fraction of low-mass companions might be formed through the planet-formation channel in protoplanetary discs. The upper mass limit of objects formed within discs is still observationally unknown, the main reason being the strong dearth of BD companions at orbital periods shorter than 10 yr, also known as the BD desert. Aims. To address this question, we aim at determining the best statistics of companions within the 10–100 MJ mass regime and located closer than ~10 au to the primary star, while minimising observation and selection bias. Methods. We made extensive use of the radial velocity (RV) surveys of northern hemisphere FGK stars within 60 pc of the Sun, performed with the SOPHIE spectrograph at the Observatoire de Haute-Provence. We derived the Keplerian solutions of the RV variations of 54 sources. Public astrometric data of the HIPPARCOS and Gaia missions allowed us to constrain the masses of the companions for most sources. We introduce GASTON, a new method to derive inclination combining RVs and Keplerian and astrometric excess noise from Gaia DR1. Results. We report the discovery of 12 new BD candidates. For five of them, additional astrometric data led to a revision of their mass in the M-dwarf regime. Among the seven remaining objects, four are confirmed BD companions, and three others are likely also in this mass regime. Moreover, we report the detection of 42 M-dwarfs within the range of 90 MJ–0.52 M⊙. The resulting M sin i-P distribution of BD candidates shows a clear drop in the detection rate below 80-day orbital period. Above that limit, the BD desert appears rather wet, with a uniform distribution of the M sin i. We derive a minimum BD-detection frequency around Solar-like stars of 2.0 ± 0.5%.

Exoplanets orbiting close to their parent stars could lose some fraction of their atmospheres because of the extreme irradiation. Atmospheric mass loss primarily affects low-mass exoplanets, leading to suggest that hot rocky planets might have begun as Neptune-like, but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum. Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 +/- 3.5% (1 sigma), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start ~2 h before, and end >3 h after the ~1 h optical transit, which is substantially different from one previous claim (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of ~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past., Published in Nature on 25 June 2015. Preprint is 28 pages, 12 figures, 2 tables

The exoplanet HD97658b provides a rare opportunity to probe the atmospheric composition and evolution of moderately irradiated super-Earths. It transits a bright K star at a moderate orbital distance of 0.08 au. Its low density is compatible with a massive steam envelope that could photodissociate at high altitudes and become observable as escaping hydrogen. Our analysis of 3 transits with HST/STIS at Ly-alpha reveals no such signature, suggesting that the thermosphere is not hydrodynamically expanding and is subjected to a low escape of neutral hydrogen (, 15 pages, 8 figures

We present an optical to near-infrared transmission spectrum of the hot JupiterHAT-P-1b, based on Hubble Space Telescope observations, covering the spectral regime from 0.29 to 1.027μm with Space Telescope Imaging Spectrograph (STIS), which is coupled with a recent Wide Field Camera 3 (WFC3) transit (1.087 to 1.687μm). We derive refined physical parameters of the HAT-P-1 system, including an improved orbital ephemeris. The transmission spectrum shows a strong absorption signature shortward of 0.55μm, with a strong blueward slope into the near-ultraviolet. We detect atmospheric sodium absorption at a 3.3σ significance level, but find no evidence for the potassium feature. The red data imply a marginally flat spectrum with a tentative absorption enhancement at wavelength longer than ~0.85μm. The STIS and WFC3 spectra differ significantly in absolute radius level (4.3 ± 1.6 pressure scaleheights), implying strong optical absorption in the atmosphere of HAT-P-1b. The optical to near-infrared difference cannot be explained by stellar activity, as simultaneous stellar activity monitoring of the G0V HAT-P-1b host star and its identical companion show no significant activity that could explain the result. We compare the complete STIS and WFC3 transmission spectrum with theoretical atmospheric models which include haze, sodium and an extra optical absorber. We find that both an optical absorber and a supersolar sodium to water abundance ratio might be a scenario explaining the HAT-P-1b observations. Our results suggest that strong optical absorbers may be a dominant atmospheric feature in some hot Jupiter exoplanets. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

In 1981, Beta Pictoris showed strong and rapid photometric variations possibly due to a transiting giant planet. Later, a planetary mass companion to the star, Beta Pic b, was identified using imagery. Observations at different epochs (2003 and 2009-2015) detected the planet at a projected distance of 6 to 9 AU from the star and showed that the planet is on an edge-on orbit. The observed motion is consistent with an inferior conjunction in 1981, and Beta Pic b can be the transiting planet proposed to explain the photometric event observed at that time. Assuming that the 1981 event is related to the transit or the inferior conjunction of Beta Pic b on an edge-on orbit, we search for the planetary orbit in agreement with all the measurements of the planet position published so far. We find two different orbits that are compatible with all these constraints: (i) an orbit with a period of 17.97$\pm$0.08 years along with an eccentricity of around 0.12 and (ii) an orbit with a period of 36.38$\pm$0.13 years and a larger eccentricity of about 0.32. In the near future, new imaging observations should allow us to discriminate between these two different orbits. We also estimate the possible dates for the next transits, which could take place as early as 2017 or 2018, even for a long-period orbit., Accepted for publication in A&A

We present the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) optical transmission spectroscopy of the cool Saturn-mass exoplanet WASP-39b from 0.29-1.025 μm, along with complementary transit observations from Spitzer IRAC at 3.6 and 4.5 μm. The low density and large atmospheric pressure scale height of WASP-39b make it particularly amenable to atmospheric characterization using this technique. We detect a Rayleigh scattering slope as well as sodium and potassium absorption features; this is the first exoplanet in which both alkali features are clearly detected with the extended wings predicted by cloud-free atmosphere models. The full transmission spectrum is well matched by a clear H 2 -dominated atmosphere, or one containing a weak contribution from haze, in good agreement with the preliminary reduction of these data presented in Sing et al. WASP-39b is predicted to have a pressure-temperature profile comparable to that of HD 189733b and WASP-6b, making it one of the coolest transiting gas giants observed in our HST STIS survey. Despite this similarity, WASP-39b appears to be largely cloud-free, while the transmission spectra of HD 189733b and WASP-6b both indicate the presence of high altitude clouds or hazes. These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres.

We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran Telescopio Canarias (GTC). The time series observations were performed using long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS instrument, enabling differential specrophotometric transit lightcurves capable of measuring the exoplanet's transmission spectrum. Two optical low-resolution grisms were used to cover the optical wavelength range from 3800 to 9300{\AA}. We find that sub-mmag level slit losses between the target and reference star prevent full optical transmission spectra from being constructed, limiting our analysis to differential absorption depths over ~1000{\AA} regions. Wider long slits or multi-object grism spectroscopy with wide masks will likely prove effective in minimising the observed slit-loss trends. During both transits, we detect significant absorption in the planetary atmosphere of XO-2b using a 50{\AA} bandpass centred on the Na I doublet, with absorption depths of Delta(R_pl/R_star)^2=0.049+/-0.017 % using the R500R grism and 0.047+/-0.011 % using the R500B grism (combined 5.2-sigma significance from both transits). The sodium feature is unresolved in our low-resolution spectra, with detailed modelling also likely ruling out significant line-wing absorption over an ~800{\AA} region surrounding the doublet. Combined with narrowband photometric measurements, XO-2b is the first hot Jupiter with evidence for both sodium and potassium present in the planet's atmosphere.

We present transmission spectra of the hot Jupiter HD 189733b taken with the Space Telescope Imaging Spectrograph aboard HST. The spectra cover the wavelength range 5808-6380 Ang with a resolving power of R=5000. We detect absorption from the NaI doublet within the exoplanet's atmosphere at the 9 sigma confidence level within a 5 Ang band (absorption depth 0.09 +/- 0.01%) and use the data to measure the doublet's spectral absorption profile. We detect only the narrow cores of the doublet. The narrowness of the feature could be due to an obscuring high-altitude haze of an unknown composition or a significantly sub-solar NaI abundance hiding the line wings beneath a H2 Rayleigh signature. We compare the spectral absorption profile over 5.5 scale heights with model spectral absorption profiles and constrain the temperature at different atmospheric regions, allowing us to construct a vertical temperature profile. We identify two temperature regimes; a 1280 +/- 240 K region derived from the NaI doublet line wings corresponding to altitudes below ~ 500 km, and a 2800 +/- 400 K region derived from the NaI doublet line cores corresponding to altitudes from ~ 500-4000 km. The zero altitude is defined by the white-light radius of Rp/Rstar=0.15628 +/- 0.00009. The temperature rises with altitude, which is likely evidence of a thermosphere. The absolute pressure scale depends on the species responsible for the Rayleigh signature and its abundance. We discuss a plausible scenario for this species, a high-altitude silicate haze, and the atmospheric temperature-pressure profile that results. In this case, the high altitude temperature rise for HD 189733b occurs at pressures of 10^-5 to 10^-8 bar.

Statistical analysis of over a thousand spectra of the star β Pictoris reveals that it has two kinds of exocomets circling it: old exhausted comets trapped in mean-motion resonance with a massive planet, and fragments of comets. The nearby star β Pictoris possesses a young planetary system that appears much like our own would have been few million years after its formation. This analysis of more a thousand archival spectra recorded between 2003 and 2011 reveals variable dust absorption signatures arising from transiting exocomets belonging to two distinct families of comets. First, an old volatile-exhausted population displaying signs of orbital evolution due to interactions with the host planet and second, a volatile-rich population presumably originating from the break-up of a few parent bodies. The young planetary system surrounding the star β Pictoris harbours active minor bodies1,2,3,4,5,6. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System7. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies8,9,10,11, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.

The irradiation of close-in planets by their star influences their evolution and might be responsible for a population of ultra-short period planets eroded to their bare core. In orbit around a bright, nearby G-type star, the super-Earth 55 Cnc e offers the possibility to address these issues through UV transit observations. We used the Hubble Space Telescope to observe the transit in the FUV over 3 epochs in Apr. 2016, Jan. 2017, and Feb. 2017. These observations reveal significant short- and long-term variability in 55 Cnc chromospheric emission lines. In the last 2 epochs, we detected a larger flux in the C III, Si III, and Si IV lines after the planet passed the approaching quadrature, followed by a flux decrease in the Si IV doublet. In the second epoch these variations are contemporaneous with flux decreases in the Si II and C II doublet. All epochs show flux decreases in the N V doublet as well, albeit at different orbital phases. These flux decreases are consistent with absorption from optically thin clouds of gas, are mostly localized at low and redshifted radial velocities in the star rest frame, and occur preferentially before and during the transit. These 3 points make it unlikely that the variations are purely stellar, yet we show that the occulting material is also unlikely to originate from the planet. We tentatively propose that the motion of 55 Cnc e at the fringes of the stellar corona leads to the formation of a cool coronal rain. The inhomogeneity and temporal evolution of the stellar corona would be responsible for the differences between the visits. Additional variations are detected in the C II doublet in the first epoch and in the O I triplet in all epochs with a different behavior that points toward intrinsic stellar variability. Further observations at FUV wavelengths are required to disentangle between star-planet interactions and the activity of the star, Comment: 22 pages, 20 figures, accepted for publication in A&A

Context. The search for extrasolar planets has been limited so far to close orbit (typ. ≤5 au) planets around mature solar-type stars on the one hand, and to planets on wide orbits (≥10 au) around young stars on the other hand. To get a better view of the full giant planet population, we have started a survey to search for giant planets around a sample of carefully selected young stars. Aims: This paper aims at exploring the giant planet population around one of our targets, β Pictoris, over a wide range of separations. With a disk and a planet already known, the β Pictoris system is indeed a very precious system for studies of planetary formation and evolution, as well as of planet- disk interactions. Methods: We analyse more than 2000 HARPS high- resolution spectra taken over 13 years as well as NaCo images recorded between 2003 and 2016. We combine these data to compute the detection probabilities of planets throughout the disk, from a fraction of au to a few dozen au. Results: We exclude the presence of planets more massive than 3 MJup closer than 1 au and further than 10 au, with a 90% probability. 15+ MJup companions are excluded throughout the disk except between 3 and 5 au with a 90% probability. In this region, we exclude companions with masses larger than 18 (resp. 30) MJup with probabilities of 60 (resp. 90) %. Based on data obtained with the ESO3.6 m/HARPS spectrograph at La Silla, and with NaCO on the VLT.The RV data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz- bin/qcat?J/A+A/612/A108

Circumstellar disks surrounding young forming stars, are likely the location where planets form. While the gaseous phase represents up to ~99% of the disk mass and control the dynamics, most of disk properties relies on dust analyses. The main constituent of the gaseous component, molecular hydrogen (H 2 ), remains nearly out of reach and the gas disk is probed through emission lines of minor tracers, such as CO. In this lecture, we will first recall how H 2 symmetric molecular structure makes its detection difficult. We will then review the most significant results achieved so far, thanks to new generation of ground and space-based telescopes, with a special emphasize given to Herbig Ae/be, which are pre-main sequence stars of intermediate mass. Though the first direct estimates of circumstellar disk mass have been reported, observation of H 2 is still challenging detection.

International audience; We report on the discovery of a substellar companion or a massive Jupiter orbiting the G5V star HD 16760 using the spectrograph SOPHIE installed on the OHP 1.93-m telescope. Characteristics and performances of the spectrograph are presented, as well as the SOPHIE exoplanet consortium program. With a minimum mass of 14.3 {M}_Jup, an orbital period of 465 days and an eccentricity of 0.067, HD 16760b seems to be located just at the end of the mass distribution of giant planets, close to the planet/brown-dwarf transition. Its quite circular orbit supports a formation in a gaseous protoplanetary disk. Based on observations made with SOPHIE spectrograph on the 1.93-m telescope at Observatoire de Haute-Provence (CNRS/OAMP), France (program 07A.PNP.CONS). Table 2 is also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/853

Water, methane and carbon-monoxide are expected to be among the most abundant molecules besides molecular hydrogen in the hot atmosphere of close-in EGPs. Transit observations in the mid-IR allow the atmospheric content of transiting planets to be determined. We present new primary transit observations of the hot-jupiter HD189733b, obtained simultaneously at 4.5 and 8 micron with IRAC instrument onboard Spitzer. Together with a new refined analysis of previous observations at 3.6 and 5.8 micron using the same instrument, we are able to derive the system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from fits of the transit light curves at these four wavelengths. We measure the four planet-to-star radius ratios, to be (R_p/R_*)= 0.1545 +/- 0.0003, 0.1557 +/- 0.0003, 0.1547 +/- 0.0005, 0.1544 +/- 0.0004 at 3.6, 4.5, 5.8, and 8 micron respectively. The high accuracy of the measurement allows the search for atmospheric molecular absorbers. Contrary to a previous analysis of the same dataset, our study is robust against systematics and reveals that water vapor absorption at 5.8 micron is not detected in this photometric dataset. Furthermore, in the band centered around 4.5 micron we find a hint of excess absorption with an apparent planetary radius Delta(R_p/R_*) = 0.00128 +/- 0.00056 larger (2.3 sigma) than the one measured simultaneously at 8 micron. This value is 4 sigma above what would be expected for an atmosphere where water vapor is the only absorbing species in the near infrared. This shows that an additional species absorbing around 4.5 micron could be present in the atmosphere. CO being a strong absorber at this wavelength is a possible candidate and this may suggest a large CO/H2O ratio between 5 and 60., Comment: Accepted for publication in ApJ, 11 pages, 4 figures, 2 tables

In 1981, β Pictoris showed strong and rapid photometric variations that were attributed to the transit of a giant comet or a planet orbiting at several AUs. Recently, a candidate planet has been identified by imagery in the circumstellar disk of β Pictoris. This planet, named β Pic b, is observed at a projected distance of 8 AU from the central star. It is therefore a plausible candidate for the photometric event observed in 1981. The coincidence of the observed position of the planet in November 2003 and the calculated position assuming that the 1981 transit is due to a planet orbiting at 8 AU is intriguing. Assuming that the planet that is detected on the image is the same as the object transiting in November 1981, we estimate ranges of possible orbital distances and periods. In the favored scenario, the planet orbits at ~8 AU and was seen close to its quadrature position in the 2003 images. In this case, most of the uncertainties are related to error bars on the position in 2003. Uncertainties related to the stellar mass and orbital eccentricity are also discussed. We find a semi-major axis in the range [7.6–8.7] AU and an orbital period in the range [15.9–19.5] years. We give predictions for imaging observations at quadrature in the southwest branch of the disk in future years (2011–2015). We also estimate possible dates for the next transits and anti-transits.

We present the transmission spectra of the hot-Jupiter HD209458b taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Our analysis combines data at two resolutions and applies a complete pixel-by-pixel limb-darkening correction to fully reveal the spectral line shapes of atmospheric absorption features. Terrestrial-based Na I and H I contamination are identified which mask the strong exoplanetary absorption signature in the Na core, which we find reaches total absorption levels of ~0.11% in a 4.4 Ang band. The Na spectral line profile is characterized by a wide absorption profile at the lowest absorption depths, and a sharp transition to a narrow absorption profile at higher absorption values. The transmission spectra also shows the presence of an additional absorber at ~6,250 Ang, observed at both medium and low resolutions. We performed various limb-darkening tests, including using high precision limb-darkening measurements of the sun to characterize a general trend of Atlas models to slightly overestimate the amount of limb-darkening at all wavelengths, likely due to the limitations of the model's one-dimensional nature. We conclude that, despite these limitations, Atlas models can still successfully model limb-darkening in high signal-to-noise transits of solar-type stars, like HD209458, to a high level of precision over the entire optical regime (3,000-10,000 Ang) at transit phases between 2nd and 3rd contact.

Transiting planets, such as HD209458b, offer a unique opportunity to scrutinize the planetary atmospheric content. Although molecular hydrogen is expected to be the main atmospheric constituent, H2 remains uncovered because of the lack of strong transition from near-ultraviolet to near-infrared. Here we analyse the absorption spectrum of HD209458b obtained by Sing et al. (2008a) which provides a measurement of the absorption depth in the 3000-6200 AA wavelength range. We show that the rise in absorption depth at short wavelengths can be interpreted as Rayleigh scattering within the atmosphere of HD209458b. Since Rayleigh scattering traces the entire atmosphere, this detection enables a direct determination of the pressure-altitude relationship, which is required to determine the absolute fraction of other elements such as sodium. At the zero altitude defined by the absorption depth of 1.453%, which corresponds to a planetary radius of 0.1205 times the stellar radius, we find a pressure of 33+/-5 mbar. Using the variation of the Rayleigh scattering cross-section as a function of wavelength, we determine the temperature to be 2200+/-260 K at 33 mbar pressure., To be published in A&A

Exoplanet search programs need to study how to disentangle radial-velocity (RV) variations due to Doppler motion and the noise induced by stellar activity. We monitored the active K2V HD 189733 with the high-resolution SOPHIE spectrograph (OHP, France). We refined the orbital parameters of HD 189733b and put limitations on the eccentricity and on a long-term velocity gradient. We subtracted the orbital motion of the planet and compared the variability of activity spectroscopic indices (HeI, Hα, Ca II H&K lines) to the evolution of the RV residuals and the shape of spectral lines. All are in agreement with an active stellar surface in rotation. We used such correlations to correct for the RV jitter due to stellar activity. This results in achieving a high precision on the orbital parameters, with a semi-amplitude: K=200.56±0.88m⋅s−1 and a derived planet mass of MP=1.13±0.03 MJup.

The two prototype hot-Jupiter exoplanets HD209458b and HD189733b are currently offering an unprecedented view of their atmospheres. As discussed here, primary transit transmission spectra provide the opportunity to identify specific atomic and molecular species, determine their abundances, and recover temperature-pressure-altitude information. We present a reanalysis of existing HST/STIS data on HD209458b, providing a complete optical transmission spectrum. Analysis of this spectrum have revealed: (1) the planetary abundance of sodium which is ~2X solar (2) a depletion of sodium at high altitudes due to condensation or ionization (3) Rayleigh scattering by H2 (3) a high temperature at pressures of 10's mbar consistent with the dayside inversion (4) a separate high-altitude hot temperature from the planet's thermosphere and (5) likely absorption by TiO/VO. While HD209458b and HD189733b are currently the best candidates for these studies, another ~10 exoplanets are good targets with today's instruments for future transmission-based atmospheric detections.

Following re-analysis of Hubble Space Telescope observations of primary transits of the extrasolar planet HD209458b at Lyman-alpha, Ben-Jaffel (2007, BJ007) claims that no sign of evaporation is observed. Here we show that, in fact, this new analysis is consistent with the one of Vidal-Madjar et al. (2003, VM003) and supports the detection of evaporation. The apparent disagreement is mainly due to the disparate wavelength ranges that are used to derive the transit absorption depth. VM003 derives a (15+/-4)% absorption depth during transit over the core of the stellar Lyman-alpha line (from -130 km/s to +100 km/s), and this result agrees with the (8.9+/-2.1)% absorption depth reported by BJ007 from a slightly expanded dataset but over a larger wavelength range (+/-200 km/s). These measurements agree also with the (5+/-2)% absorption reported by Vidal-Madjar et al. (2004) over the whole Lyman-alpha line from independent, lower-resolution data. We show that stellar Lyman-alpha variability is unlikely to significantly affect those detections. The HI atoms must necessarily have velocities above the escape velocities and/or be outside the Roche lobe, given the lobe shape and orientation. Absorption by HI in HD209458b's atmosphere has thus been detected with different datasets, and now with independent analyses. All these results strengthen the concept of evaporating hot-Jupiters, as well as the modelization of this phenomenon.

The transit spectrum of the exoplanet HD 189733b has recently been obtained between 0.55 and 1.05 microns. Here we present an analysis of this spectrum. We develop first-order equations to interpret absorption spectra. In the case of HD 189733b, we show that the observed slope of the absorption as a function of wavelength is characteristic of extinction proportional to the inverse of the fourth power of the wavelength (lambda^-4). Assuming an extinction dominated by Rayleigh scattering, we derive an atmospheric temperature of 1340+/-150 K. If molecular hydrogen is responsible for the Rayleigh scattering, the atmospheric pressure at the planetary characteristic radius of 0.1564 stellar radius must be 410+/-30 mbar. However the preferred scenario is scattering by condensate particles. Using the Mie approximation, we find that the particles must have a low value for the imaginary part of the refraction index. We identify MgSiO3 as a possible abundant condensate whose particle size must be between 0.01 and 0.1 microns. For this condensate, assuming solar abundance, the pressure at 0.1564 stellar radius is found to be between a few microbars and few millibars, and the temperature is found to be in the range 1340-1540 K, and both depend on the particle size., Comment: Accepted for publication in A&A Letter

We present Spitzer Space Telescope observations of the extrasolar planet HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with the Infrared Array Camera. The system parameters, including planetary radius, stellar radius, and impact parameter are derived from fits to the transit light curves at both wavelengths. We measure two consistent planet-to-star radius ratios, (Rp/Rs)[3.6$\mu$m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and (Rp/Rs)[5.8$\mu$m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include both the random and systematic errors in the transit baseline. Although planet radii are determined at 1%-accuracy, if all uncertainties are taken into account the resulting error bars are still too large to allow for the detection of atmospheric constituants like water vapour. This illustrates the need to observe multiple transits with the longest possible out-of-transit baseline, in order to achieve the precision required by transmission spectroscopy of giant extrasolar planets., Comment: Accepted in The Astrophysical Journal Letters

We report Hubble Space Telescope (HST) optical to near-infrared transmission spectroscopy of the hot Jupiter WASP-6b, measured with the Space Telescope Imaging Spectrograph (STIS) and Spitzer's InfraRed Array Camera (IRAC). The resulting spectrum covers the range $0.29-4.5\,\mu$m. We find evidence for modest stellar activity of WASP-6b and take it into account in the transmission spectrum. The overall main characteristic of the spectrum is an increasing radius as a function of decreasing wavelength corresponding to a change of $\Delta (R_p/R_{\ast})=0.0071$ from 0.33 to $4.5\,\mu$m. The spectrum suggests an effective extinction cross-section with a power law of index consistent with Rayleigh scattering, with temperatures of $973\pm144$ K at the planetary terminator. We compare the transmission spectrum with hot-Jupiter atmospheric models including condensate-free and aerosol-dominated models incorporating Mie theory. While none of the clear-atmosphere models is found to be in good agreement with the data, we find that the complete spectrum can be described by models that include significant opacity from aerosols including Fe-poor Mg$_2$SiO$_4$, MgSiO$_3$, KCl and Na$_2$S dust condensates. WASP-6b is the second planet after HD189733b which has equilibrium temperatures near $\sim1200$ K and shows prominent atmospheric scattering in the optical., Comment: 18 pages, 15 figures, 7 tables

Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1 to 1.7 {\mu}m). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted. The low amplitude of water signatures could be explained by very low water abundances, which may be a sign that water was depleted in the protoplanetary disk at the planet's formation location, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes, as found in some optical spectra. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3-5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures., Comment: This is the authors version of the manuscript, 18 pages including Methods. Published in Nature, available at http://nature.com/articles/doi:10.1038/nature16068 spectra also available at http://www.astro.ex.ac.uk/people/sing/David_Sing/Spectra.html

The Working Group on Extrasolar Planets (hereafter the WGESP) was created at a meeting of the IAU Executive Council in 1999 as a Working Group of IAU Division III and was renewed for three more years at the IAU General Assembly in 2003. The charge of the WGESP is to act as a focal point for international research on extrasolar planets. The membership of the WGESP has remained unchanged for the last three years.

We present the result of a follow-up Stromgren photometric survey of sixteen southern bright stars with circumstellar material, in order to detect possible weak photometric variations. We found new variations of the $\beta~\:$Pictoris brightness from 1999 to 2002 with a weak long-term variation of ~$-0.8\times 10^{-3}$ mag per year, over about 3 years. These variations look similar to those seen from 1975 to 1981 and from 1995 to 1998 (Nitschelm et al. 2000, A&AS, 145, 275). They can be due to differential occultation by dust inhomogeneities transiting the star through the years. We detected new periodic variations for HD 256 (HR 10) with periods ranging from 0.35 day to 6.69 days during several months. These variations may also be interpreted in terms of variable obscuration due to structures in the circumstellar disk suspected to be surrounding this star.

Since the discovery of the first Hot Jupiter around 51 Pegasi in 1995, the field of exoplanet detection has been driven by the improvements in the precision of radial velocity measurements. Reaching accuracies lower than 10 m s -1 on Doppler measurements has allowed to explore the domain of gas giants like Jupiter and Saturn. Now, the domain of 1 m s -1 accuracy has been reached, opening the realm of Neptune-mass planets and lower. In parallel, the transit method has come of age. with six detections in the past two years. Transiting planets offer a direct view into planet structure and composition. Planetary size, density, and even the composition of their atmosphere can be measured during the transit. Several elements have already been detected in the transiting planet HD209458, which seems to be slowly evaporating. In the coming few years, the combination of space-based transit surveys and dedicated radial-velocity spectrograph will open the domain of terrestrial exoplanets.

High-resolution ( ∼ 0.22 A ) spectra of the north jovian aurora were obtained in the 905–1180 A window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H2 spectra and the spectral range allows the study of self-absorption. Below 1100 A, transitions connecting to the v ″ ⩽ 2 levels of the H2 ground state are partially or totally absorbed by the overlying H2 molecules. The FUSE spectra provide information on the overlying H2 column and on the vibrational distribution of H2. Transitions from high-energy H2 Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920–970, 1030–1080, and 1090–1180 A spectral windows. In a first approach (single-layer model ), the synthetic spectra are generated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H2 gas temperature and 1.3 × 10 18 , 1.5 × 10 20 , and 1.3 × 10 20 cm − 2 for the H2 self-absorbing vertical column respectively. Comparison between the H2 column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H2 rovibrational temperatures, close to those deduced from spectral analyses of H+3 auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model ). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 μbar, that is ∼ 100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H2 self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations.

We present the analysis of FUSE observations of two Herbig AeBe stars known to harbor young circumstellar disks: HD 100546 and HD 163296. In both cases we detect absorption lines from warm and dense H2. The thermalization of the rotational levels up to J~4 allows evaluation of the temperature, density and typical size of the absorbing layer. These quantities are consistent with absorption of the light of the central star by a thin layer of a circumstellar disk seen at an intermediate inclination., Comment: 5 pages, 4 figures, to be published in A&A

The first detection and identification of deuterium Balmer lines are reported in five H II regions of the Galaxy and the Small Magellanic Cloud. The D I lines appear as faint, narrow emission features in the blue wings of the H I Balmer lines and can be distinguished from high-velocity H I emission. The identification with deuterium and the excitation mechanism by fluorescence are both established beyond doubt. Measurements of deuterium Balmer series might lead to a new, optical method of deuterium abundance measurement in the interstellar medium and beyond.

Since HI, OI, and DI have nearly the same ionization potential, the deuterium-to-oxygen ratio (D/O) is an important tracer of the D/H ratio and its putative spatial variations. D/O is indeed very sensitive to astration, both because of deuterium destruction and oxygen production. Here, we present DI, OI, and NI interstellar column density measurements performed with the Far Ultraviolet Spectroscopic Explorer (FUSE) on eight nearby lines of sight. The first results of this survey show that D/O is a better D/H proxy than D/N, and that D/O is constant in the local interstellar medium. The mean value is D / O =3.81(±0.18)×10 −2 (1σ) . This result supports both D/H and O/H stability in the LISM.

High-resolution far-UV spectra of the O-type subdwarf BD +28°4211 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the interstellar deuterium, nitrogen, and oxygen abundances in this direction. The interstellar D I transitions are analyzed down to Lyι at 920.7 A. The star was observed several times at different target offsets in the direction of spectral dispersion. The aligned and co-added spectra have high signal-to-noise ratios (S/N = 50-100). D I, N I, and O I transitions were analyzed with curve-of-growth and profile fitting techniques. A model of interstellar molecular hydrogen on the line of sight was derived from H2 lines in the FUSE spectra and used to help analyze some features where blending with H2 was significant. The H I column density was determined from high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of Lyα to be log N(H ) = 19.846 ± 0.035 (2 σ), which is higher than is typical for sight lines in the local interstellar medium (ISM) studied for D/H. We found that D/H = (1.39 ± 0.21) × 10-5 (2 σ) and O/H = (2.37 ± 0.55) × 10-4 (2 σ). O/H toward BD +28°4211 appears to be significantly below the mean O/H ratio for the ISM and the Local Bubble.

We present a deuterium abundance analysis of the line of sight toward the white dwarf WD 2211-495 observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Numerous interstellar lines are detected on the continuum of the stellar spectrum. A thorough analysis was performed through the simultaneous fit of interstellar absorption lines detected in the four FUSE channels of multiple observations with different slits. We excluded all saturated lines in order to reduce possible systematic errors on the column density measurements. We report the determination of the average interstellar D/O and D/N ratios along this line of sight at the 95% confidence level: D/O = (4.0 ? 1.2) ? 10-2 and D/N = (4.4 ? 1.3) ? 10-1. In conjunction with FUSE observations of other nearby sight lines, the results of this study will allow a deeper understanding of the present-day abundance of deuterium in the local interstellar medium and its evolution with time.

High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187 A ˚ , were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). These data were used in conjunction with existing high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) obser- vations to evaluate the total H i ,D i ,O i, and N i column densities along the line of sight. Previous determina- tions of N(D i) based upon GHRS and STIS observations were controversial as a result of the saturated strength of the D i Lyline. In the present analysis the column density of D i has been measured using only the unsaturated Lyand Lylines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are log NðD i Þ¼ 13:40 � 0:07, log NðO i Þ¼ 14:86 � 0:07, and log NðN i Þ¼ 13:87 � 0:07, quoted with 2 � uncertainties. The measurement of the H i column density by profile fitting of the Lyline has been found to be uncertain. If additional weak, hot interstellar components are added to the three detected clouds along the line of sight, the H i column den- sity can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Ly� are excellent. The new estimate of N(H i) toward G191-B2B reads log NðH i Þ¼ 18:18 � 0:18 (2 � ), so that the average D/H ratio on the line of sight is D=H ¼ 1:66 þ0:9 � 0:6 � 10 � 5 (2 � ). Subject headings: ISM: abundances — ISM: clouds — stars: individual (G191-B2B) — ultraviolet: ISM

We present measurements of the column densities of interstellar D I and O I made with the Far Ultraviolet Spectroscopic Explorer (FUSE) and of H I made with the International Ultraviolet Explorer (IUE) toward the sdOB star Feige 110 [(l,b) = (7409, - 5907); d = 179 pc; z = -154 pc]. Our determination of the D I column density made use of curve-of-growth fitting and profile fitting analyses, while our O I column density determination used only curve-of-growth techniques. The H I column density was estimated by fitting the damping wings of the interstellar Ly? profile. We find log N(D ) = 15.47 ? 0.06, log N(O ) = 16.73 ? 0.10, and log N(H ) = 20.14 (all errors 2 ?). This implies D/H = (2.14 ? 0.82) ? 10-5, D/O = ? 10-2, and O/H = (3.89 ? 1.67) ? 10-4. Taken with the FUSE results reported in companion papers and previous measurements of the local interstellar medium (LISM), this suggests the possibility of spatial variability in D/H for sight lines exceeding ~100 pc. This result may constrain models which characterize the mixing timescales and length scales of material in the LISM.