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-α line of neutral hydrogen (H i Lyα). The unexpected size of this comet-like exosphere prevented observing the full transit of its tail. In this Letter, we present new Lyα observations of GJ 436 obtained with the Space Telescope Imaging Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The stability of the Lyα 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 Lyα 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 Lyα and Si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star.
The transit spectrum of the exoplanet HD 189733b has recently been obtained between 0.55 and 1.05 μm. 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 ($\propto$$\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 ~10-2 and ~10-1 μm. 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.
We present transmission spectra of the hot Jupiter HD 189733b taken with the Space Telescope Imaging Spectrograph (STIS) aboard Hubble Space Telescope (HST). The spectra cover the wavelength range ...5808-6380 Å with a resolving power of R= 5000. We detect absorption from the Na i doublet within the exoplanet's atmosphere at the 9σ confidence level within a 5 Å band (absorption depth 0.09 ± 0.01 per cent) 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 Na i abundance hiding the line wings beneath an H2 Rayleigh signature. These observations are consistent with previous broad-band spectroscopy from Advanced Camera for Surveys (ACS) and STIS, where a featureless spectrum was seen. We also investigate the effects of starspots on the Na i line profile, finding that their impact is minimal and within errors in the sodium feature.
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 Na i doublet line wings corresponding to altitudes below ∼500 km, and a 2800 ± 400 K region derived from the Na i doublet line cores corresponding to altitudes from ∼500 to 4000 km. The zero altitude is defined by the white-light radius of R
P/R
★= 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.
Atmospheric escape has been detected from the exoplanet HD 209458b through transit observations of the hydrogen Lyman-α line. Here we present spectrally resolved Lyman-α transit observations of the ...exoplanet HD 189733b at two different epochs. These HST/STIS observations show for the first time that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4 ± 3.6% between velocities of −230 to −140 km s-1. Contrary to HD 209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 109 g s-1, a stellar wind with a velocity of 190 km s-1 and a temperature of ~105 K. An X-ray flare from the active star seen with Swift/XRT 8 h before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet’s atmosphere and stellar variations.
ABSTRACT
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 Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) instrument, enabling differential spectrophotometric transit light curves 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 Å. 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 Å regions. Wider long slits or multi‐object grism spectroscopy with wide masks will likely prove effective in minimizing the observed slit‐loss trends. During both transits, we detect significant absorption in the planetary atmosphere of XO‐2b using a 50‐Å bandpass centred on the Na i doublet, with absorption depths of Δ(Rpl/R★)2 = 0.049 ± 0.017 per cent using the R500R grism and 0.047 ± 0.011 per cent using the R500B grism (combined 5.2σ 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 Å region surrounding the doublet. Combined with narrow‐band photometric measurements, XO‐2b is the first hot Jupiter with evidence for both sodium and potassium present in the planet's atmosphere.
The naked-eye star 55 Cancri hosts a planetary system with five known planets, including a hot super-Earth (55 Cnc e) extremely close to its star and a farther out giant planet (55 Cnc b), found in ...milder irradiation conditions with respect to other known hot Jupiters. This system raises important questions on the evolution of atmospheres for close-in exoplanets, and the dependence with planetary mass and irradiation. These questions can be addressed by Lyman-α transit observations of the extended hydrogen planetary atmospheres, complemented by contemporaneous measurements of the stellar X-ray flux. In fact, planet “e” has been detected in transit, suggesting the system is seen nearly edge-on. Yet, planet “b” has not been observed in transit so far. Here, we report on Hubble Space Telescope STIS Lyα and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous observations cover two transits of 55 Cnc e and two inferior conjunctions of 55 Cnc b. They reveal the star as a bright Lyα target and a variable X-ray source. While no significant signal is detected during the transits of 55 Cnc e, we detect a surprising Lyα absorption of 7.5 ± 1.8% (4.2σ) at inferior conjunctions of 55 Cnc b. The absorption is only detected over the range of Doppler velocities where the stellar radiation repels hydrogen atoms towards the observer. We calculate a false-alarm probability of 4.4%, which takes the a-priori unknown transit parameters into account. This result suggests the possibility that 55 Cnc b has an extended upper H i atmosphere, which undergoes partial transits when the planet grazes the stellar disc. If confirmed, it would show that planets cooler than hot Jupiters can also have extended atmospheres.
The planet HD 209458 b is one of the most well studied hot-Jupiter exoplanets. The upper atmosphere of this planet has been observed through ultraviolet/optical transit observations with H i ...observation of the exosphere revealing atmospheric escape. At lower altitudes just below the thermosphere, detailed observations of the Na i absorption line has revealed an atmospheric thermal inversion. This thermal structure is rising toward high temperatures at high altitudes, as predicted by models of the thermosphere, and could reach ~ 10 000 K at the exobase level. Here, we report new near ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations of atmospheric absorptions during the planetary transit of HD 209458 b. We report absorption in atomic magnesium (Mg i), while no signal has been detected in the lines of singly ionized magnesium (Mg ii). We measure the Mg i atmospheric absorption to be 6.2 ± 2.9% in the velocity range from − 62 to − 19 km s-1. The detection of atomic magnesium in the planetary upper atmosphere at a distance of several planetary radii gives a first view into the transition region between the thermosphere and the exobase, where atmospheric escape takes place. We estimate the electronic densities needed to compensate for the photo-ionization by dielectronic recombination of Mg+ to be in the range of 108−109 cm-3. Our finding is in excellent agreement with model predictions at altitudes of several planetary radii. We observe Mg i atoms escaping the planet, with a maximum radial velocity (in the stellar rest frame) of −60 km s-1. Because magnesium is much heavier than hydrogen, the escape of this species confirms previous studies that the planet’s atmosphere is undergoing hydrodynamic escape. We compare our observations to a numerical model that takes the stellar radiation pressure on the Mg i atoms into account. We find that the Mg i atoms must be present at up to ~7.5 planetari radii altitude and estimate an Mg i escape rate of ~3 × 107 g s-1. Compared to previous evaluations of the escape rate of H i atoms, this evaluation is compatible with a magnesium abundance roughly solar. A hint of absorption, detected at low level of significance, during the post-transit observations, could be interpreted as a Mg i cometary-like tail. If true, the estimate of the absorption by Mg i would be increased to a higher value of about 8.8 ± 2.1%.
We report Hubble Space Telescope optical to near-infrared transmission spectroscopy of the hot-Jupiter WASP-6b, measured with the Space Telescope Imaging Spectrograph and Spitzer's InfraRed Array ...Camera. The resulting spectrum covers the range 0.29–4.5 μm. We find evidence for modest stellar activity of WASP-6 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 Δ (R
p / R
*) = 0.0071 from 0.33 to 4.5 μm. The spectrum suggests an effective extinction cross-section with a power law of index consistent with Rayleigh scattering, with temperatures of 973 ± 144 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 Mg2SiO4, MgSiO3, KCl and Na2S dust condensates. WASP-6b is the second planet after HD 189733b which has equilibrium temperatures near ∼1200 K and shows prominent atmospheric scattering in the optical.
In 1981, β Pictoris showed strong and rapid photometric variations possibly due to a transiting giant planet. Later, a planetary mass companion to the star, β 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 β 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 β 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 ± 0.08 years along with an eccentricity of around 0.12 and (ii) an orbit with a period of 36.38 ± 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.
We present Hubble Space Telescope optical and near-IR transmission spectra of the transiting hot-Jupiter WASP-31b. The spectrum covers 0.3-1.7 mu m at a resolution R similar to 70, which we combine ...with Spitzer photometry to cover the full-optical to IR. The spectrum is dominated by a cloud deck with a flat transmission spectrum which is apparent at wavelengths > 0.52 mu m. The cloud deck is present at high altitudes and low pressures, as it covers the majority of the expected optical Na line and near-IR H2O features. While Na I absorption is not clearly identified, the resulting spectrum does show a very strong potassium feature detected at the 4.2- sigma confidence level. Broadened alkali wings are not detected, indicating pressures below similar to 10 mbar. The lack of Na and strong K is the first indication of a sub-solar Na/K abundance ratio in a planetary atmosphere (lnNa/K = -3.3 plus or minus 2.8), which could potentially be explained by Na condensation on the planet's night side, or primordial abundance variations. A strong Rayleigh scattering signature is detected at short wavelengths, with a 4- sigma significant slope. Two distinct aerosol size populations can explain the spectra, with a smaller sub-micron size grain population reaching high altitudes producing a blue Rayleigh scattering signature on top of a larger, lower lying population responsible for the flat cloud deck at longer wavelengths. We estimate that the atmospheric circulation is sufficiently strong to mix micron size particles upwards to the required 1-10 mbar pressures, necessary to explain the cloud deck. These results further confirm the importance of clouds in hot Jupiters, which can potentially dominate the overall spectra and may alter the abundances of key gaseous species.
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