Lotfi Ben-Jaffel

EXOPLANET HD 209458b: INFLATED HYDROGEN ATMOSPHERE BUT NO SIGN OF EVAPORATION

Many extrasolar planets orbit closely to their parent star. Their existence raises the fundamental problem of loss and gain in their mass. For exoplanet HD 209458b, reports on an unusually extended hydrogen corona and a hot layer in the lower atmosphere seem to support the scenario of atmospheric inflation by the strong stellar irradiation. However, difficulties in reconciling evaporation models with observations call for a reassessment of the problem. Here we use HST archive data to report a new absorption rate of ~8.9% ± 2.1% by atomic hydrogen during the HD 209458b transit and show that no sign of evaporation could be detected for the exoplanet. We also report evidence of time variability in the HD 209458 Lyα flux, a variability that was not accounted for in previous studies, which corrupted their diagnostics. Mass-loss rates thus far proposed in the literature in the range 5 × (1010–1011) g s-1 must induce a spectral signature in the Lyα line profile of HD 209458 that cannot be found in the present analysis. Either an unknown compensation effect is hiding the expected spectral feature or else the mass-loss rate of neutrals from HD 209458 is modest.

ON THE EXISTENCE OF ENERGETIC ATOMS IN THE UPPER ATMOSPHERE OF EXOPLANET HD209458b

Stellar irradiation and particle forcing strongly affect the immediate environment of extrasolar giant planets orbiting near their parent stars. However, it is not clear how the energy is deposited over the planetary atmosphere, nor how the momentum and energy spaces of the different species that populate the system are modified. Here, we use far-ultraviolet emission spectra from HD209458 in the wavelength range (1180-1710) A to bring new insight to the composition and energetic processes in play in the gas nebula around the transiting planetary companion. In that frame, we consider up-to-date atmospheric models of the giant exoplanet where we implement non-thermal line broadening to simulate the impact on the transit absorption of superthermal atoms (H I, O I, and C II) populating the upper layers of the nebula. Our sensitivity study shows that for all existing models, a significant line broadening is required for O I and probably for C II lines in order to fit the observed transit absorptions. In that frame, we show that O I and C II are preferentially heated compared to the background gas with effective temperatures as large as T O I /TB ~ 10 for O I and T C II /TB ~ 5 for C II. By contrast, the situation is much less clear for H I because several models could fit the Lyα observations including either thermal H I in an atmosphere that has a dayside vertical column [H I] ~ 1.05 × 1021 cm–2, or a less extended thermal atmosphere but with hot H I atoms populating the upper layers of the nebula. If the energetic H I atoms are either of stellar origin or populations lost from the planet and energized in the outer layers of the nebula, our finding is that most models should converge toward one hot population that has an H I vertical column in the range [H I]hot ~ (2-4) × 1013 cm–2 and an effective temperature in the range T H I ~ (1-1.3) × 106 K, but with a bulk velocity that should be rather slow.

Hubble Space Telescope detection of oxygen in the atmosphere of exoplanet HD 189733b

Detecting heavy atoms in the inflated atmospheres of giant exoplanets that orbit close to their parent stars is a key factor for understanding their bulk composition, their evolution, and the processes that drive their expansion and interaction with the impinging stellar wind. Unfortunately, very few detections have been made thus far. Here, we use archive data obtained with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope to report an absorption of ∼6.4%±1.8% by neutral oxygen during the HD 189733b transit. Using published results from a simple hydrodynamic model of HD 189733b, and assuming a mean temperature of ∼(8−12) × 10 3 Kf or the upper atmosphere of the exoplanet, a mean vertical integrated O I density column of ∼8 × 10 15 cm −2 produces only a 3.5% attenuation transit. Much like the case of the hot-Jupiter HD 209458b, super-solar abundances and/or super-thermal broadening of the absorption lines are required to fit the deep transit drop-off observed in most far-ultraviolet lines. We also report evidence of short-time variability in the measured stellar flux, a variability that we analyze using time series derived from the time-tagged exposures, which we then compare to solar flaring activity. In that frame, we find that non-statistical uncertainties in the measured fluxes are not negligible, which calls for caution when reporting transit absorptions. Despite cumulative uncertainties that originate from variability in the stellar and sky background signals and in the instrument response, we also show a possible detection for both a transit and early-ingress absorption in the ion C II 133.5 nm lines. If confirmed, this would be the second exoplanet for which an early ingress absorption is reported. In contrast, such an early ingress signature is not detected for neutral O I. Assuming the HD 189733b magnetosphere to be at the origin of the early absorption, we use the Parker model for the stellar wind and a particle-in-cell code for the magnetosphere to show that its orientation should be deflected ∼10−30 ◦ from the planet-star line, while its nose’s position should be at least ∼16.7 Rp upstream of the exoplanet in order to fit the C II transit light curve. The derived stand-off distance is consistent with a surface magnetic field strength of ∼5.3 Gauss for the exoplanet, and a supersonic stellar wind impinging at ∼250 km s −1 , with a temperature of 1.2 × 10 5 K and a density ∼6.3 × 10 6 cm −3 at the planetary orbit, yet the fit is not unique.

NO DIFFUSE H2 IN THE METAL-DEFICIENT GALAXY I Zw 18

The metal-deficient starburst galaxy I Zw 18 has been observed with the Far Ultraviolet Spectroscopic Explorer (FUSE) in a search for H2 molecules. The spectrum obtained with an aperture covering the full galaxy shows no absorption lines of diffuse H2 at the radial velocity of the galaxy. The upper limit for the diffuse H2 column density is found to be very low, N(H2) 1015 cm-2 (10 σ), unlike our Galaxy where H2 is generally present for even low H I column densities. Although the H I column density here is as high as N(H I) ≈ 2 × 1021 cm-2, we observe 2N(H2)/N(H I) 10-6. We cannot exclude the possibility that some H2 could be in very dense, small, and discrete clumps that cannot be detected with the present observation. However, the remarkable absence of diffuse H2 in this metal-poor galaxy can be explained by the low abundance of dust grains (needed to form this molecule from H atoms), the high ultraviolet flux, and the low density of the H I cloud surrounding the star-forming regions. Thus, having eliminated diffuse H2 as a significant contributor to the total mass, it appears that the gas of the galaxy is dominated by H I and that the high dynamical mass is not composed of cold and diffuse baryonic dark matter.

Transit of Exomoon Plasma Tori: New Diagnosis

In the solar system, moons largely exceed planets in number. The Kepler database has been shown to be sensitive to exomoon detection down to the mass of Mars, but the first search has been unsuccessful. Here, we use a particles-in-cell code to predict the transit of the plasma torus produced by a satellite. Despite the small size of a moon, the spatial extent of its plasma torus can be large enough to produce substantial transit absorptions. The model is used for the interpretation of Hubble Space Telescope early ingress absorptions apparently observed during the WASP-12 b and HD 189733 b UV transits for which no consistent explanation exists. For HD 189733 b an exomoon transiting ~16 Rp ahead of the planet and loading ~1029 C II ions s–1 into space is required to explain the tentative early ingress absorption observed for C II. For WASP-12b, a moon transiting ~6 Rp ahead from the planet and ejecting ~1028 Mg II ions per second is required to explain the NUV early ingress absorption feature. Interestingly, both HD 189733 b and WASP-12b predicted satellites are outside the Hill sphere of their planets, an indication that the moons, if present, were not formed in situ but probably captured later. Finally, our simulations show a strong electromagnetic coupling between the polar regions of planets and the orbital position of the moons, an expected outcome of the unipolar induction DC circuit model. Future observations should test our predictions with a potential opportunity to unambiguously detect the first exomoon plasma torus.

An ultrahot gas-giant exoplanet with a stratosphere

Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum. Conversely, if there is a stratosphere—where temperature increases with altitude—these molecular features will be observed in emission. It has been suggested that stratospheres could form in highly irradiated exoplanets, but the extent to which this occurs is unresolved both theoretically and observationally. A previous claim for the presence of a stratosphere remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.

Effects of interstellar magnetic field B and constant flux of neutral H on the heliosphere

[1] A treatment showing that the local interstellar medium (LISM) bow shock (BS) may exist and the heliosphere is tilted has recently been given by Ben-Jaffel et al. [2000] and used in an attempt to interpret combined Voyager UVS data and Hubble Space Telescope spectra of the Ly α sky background emission as plausible evidence for the existence of BS. However, the authors stressed the need for a more accurate description of the interface region in order to interpret the available data confidently. In this paper we use a more sophisticated Three-Dimensional Time-Dependent Magnetohydrodynamic with Neutral Particles (3DT MHD + N) model of the interaction between the solar wind and the interstellar medium with LISM neutral particles included. We report a parametric study of the effect of the LISM magnetic field strength and inclination angle in the presence of the neutral H atoms on the heliosphere and the heliospheric boundary. Ninety model cases that have been analyzed facilitate in building a diagnostic tool either to interpret archive data on the sky background radiation field or to stimulate new observations related to the interface, observations that may capture some of the fine structure revealed by the new model. Because the inclination angle and strength of the interstellar magnetic field induce unambiguous signatures in the heliospheric structure, our model calculations may serve as a lever to constrain the nature of the LISM from observational data.

Comparison of Heliospheric Models with Observations of the Voyager and IBEX Spacecraft

Results of modeling the heliosphere are compared with observations of the Voyager spacecraft and the IBEX mission simultaneously. The MHD solutions are tested against observational data for different strengths and orientations of the local interstellar magnetic field (LIMF) used in the simulations for asymmetric magnetized solar wind flow. We show that the model reproduces approximately the position of the IBEX ribbon and the termination shock crossing distance for Voyager 2, when the LIMF vector lies in the proximity of the hydrogen deflection plane with the inclination angle to the local interstellar flow equal to 39° ± 9° and its magnitude is 2.4 ± 0.3 μG. In ecliptic coordinates this solution corresponds to the LIMF vector pointing from (longitude, latitude) = (227° ± 7°, 35° ± 7°).

OBSERVATIONS OF THE INTERPLANETARY HYDROGEN DURING SOLAR CYCLES 23 AND 24. WHAT CAN WE DEDUCE ABOUT THE LOCAL INTERSTELLAR MEDIUM

Observations of interstellar helium atoms by the Interstellar Boundary Explorer (IBEX) spacecraft in 2009 reported a local interstellar medium (LISM) velocity vector different from the results of the Ulysses spacecraft between 1991 and 2002. The interplanetary hydrogen (IPH), a population of neutrals that fills the space between planets inside the heliosphere, carries the signatures of the LISM and its interaction with the solar wind. More than 40 yr of space-based studies of the backscattered solar Lyα emission from the IPH provided limited access to the velocity distribution, with the first temporal evolution map of the IPH line-shift during solar cycle 23. This work presents the results of the latest IPH observations made by the Hubble Space Telescopes Space Telescope Imaging Spectrograph during solar cycle 24. These results have been compiled with previous measurements, including data from the Solar Wind Anisotropies instrument on the Solar and Heliospheric Observatory. The whole set has been compared to physically realistic models to test both sets of LISM physical parameters as measured by Ulysses and IBEX, respectively. This comparison shows that the LISM velocity vector has not changed significantly since Ulysses measurements.

HST PanCET Program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b

We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic Comparative Exoplanet Treasury program for WASP-101b, a highly inflated hot Jupiter and one of the community targets proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. From a single HST Wide Field Camera 3 observation, we find that the near-infrared transmission spectrum of WASP-101b contains no significant H2O absorption features and we rule out a clear atmosphere at 13σ. Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at observing strong molecular transmission features. We compare WASP-101b to the well-studied and nearly identical hot Jupiter WASP-31b. These twin planets show similar temperature–pressure profiles and atmospheric features in the near-infrared. We suggest exoplanets in the same parameter space as WASP-101b and WASP-31b will also exhibit cloudy transmission spectral features. For future HST exoplanet studies, our analysis also suggests that a lower count limit needs to be exceeded per pixel on the detector in order to avoid unwanted instrumental systematics.