Atmosphere Of HD 209458b Research Articles

TiO and VO broad band absorption features in the optical spectrum of the atmosphere of the hot-Jupiter HD 209458b

Context. The presence of titanium oxide (TiO) and vanadium oxide (VO) gas phase species is searched for in the atmosphere of the hot Jupiter HD 209458b. Aims. We compared a model for the planet’s transmitted spectrum to multi-wavelength eclipse-depth measurements (from 3 000 to 10 000 A), obtained by Sing et al. (2008a) using archived HST-STIS time series spectra. We make use of these observations to search for spectral signatures from extra absorbers in the planet a tmosphere between 6 000 and 8 000 A. Methods. Along with sodium depletion and Rayleigh scattering recently published for this exoplanet atmosphere, an extra absorb er of uncertain origin, redward of the sodium lines, resides in the atmosphere of the planet. Furthermore, this planet has a stratosphere experiencing a thermal inversion caused by the capture of optical stellar flux by absorbers that resides at altitude. Rec ent models have predicted that the presence of TiO and VO in the atmosphere of HD 209458b may be responsible for this temperature inversion. Although no specific TiO and VO spectral band head signatures have been identified unambiguously in the observed spectrum , we suggest here that the opacities of those molecules are possi ble candidates to explain the remaining continuous broad band absorption observed between 6 200 and 8 000 A. To match reasonably well the data, the abundances of TiO and VO molecules are evaluated from ten to one thousand times below solar. This upper limit result is in agreement with expected variations with altitude due to depletion effects such as condensation. Results.

Rayleigh scattering by H2 in the extrasolar planet HD 209458b

Transiting planets, such as HD 209458b, 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 HD 209458b obtained by Sing et al. (2008a, ApJ, submitted) which provides a measurement of the absorption depth in the 3000-6200 A wavelength range. We show that the rise in absorption depth at short wavelengths can be interpreted as Rayleigh scattering within the atmosphere of HD 209458b. 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.

Ground-based detections of sodium in HD 209458b's atmosphere in two data sets

AbstractWe present two separate ground-based detections of sodium in the transmission spectrum of HD 209458b. First we reanalyzed an archival data set from the HDS spectrograph on Subaru, which shows sodium at a >5σ level. Secondly, our preliminary results of a UVES/VLT data set indicate sodium absorption at a similar level, although the data cover the eclipse only partially. Both results are fully consistent with the HST results of Charbonneau et al. (2002). The Na D absorption feature seems to be resolved in the narrowest passband.

Atmospheric composition and structure of HD209458b

AbstractTransiting planets like HD209458b offer a unique opportunity to scrutinize their atmospheric composition and structure. Transit spectroscopy probes the transition region between the day and night sides, called the limb. We present a re-analysis of existing HST/STIS transmission spectra of HD209458b's atmosphere. From these observations we identify H2 Rayleigh scattering, derive the absolute Sodium abundance and quantify its depletion in the upper atmosphere, extract a stratospheric T-P profile and find a temperature inversion and explain broad band absorptions with the presence of TiO and VO molecules.

Exoplanet HD 209458b (Osiris): Evaporation Strengthened

Following reanalysis of Hubble Space Telescope observations of primary transits of the extrasolar planet HD 209458b at Lyα, Ben-Jaffel 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 and coworkers, 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. Vidal-Madjar derives a 15% ± 4% absorption depth during transit over the core of the stellar Lyα line (from –130 to +100 km s−1), and this result agrees with the 8.9% ± 2.1% absorption depth reported by Ben-Jaffel from a slightly expanded data set but over a larger wavelength range (±200 km s−1). These measurements agree also with the 5% ± 2% absorption reported by Vidal-Madjar over the whole Lyα line from independent, lower resolution data. We show that stellar Lyα variability is unlikely to significantly affect those detections. The H I atoms must necessarily have velocities above the escape velocities and/or be outside the Roche lobe, given the lobe shape and orientation. Absorption by H I in HD 209458b's atmosphere has thus been detected with different data sets, and now with independent analyses. All these results strengthen the concept of evaporating hot Jupiters, as well as the modeling of this phenomenon.

Identification of Absorption Features in an Extrasolar Planet Atmosphere

Water absorption is identified in the atmosphere of HD 209458b by comparing models for the planet's transmitted spectrum to recent, multiwavelength, eclipse-depth measurements (from 0.3 to 1 μm) published by Knutson et al. A cloud-free model that includes solar abundances, rainout of condensates, and photoionization of sodium and potassium is in good agreement with the entire set of eclipse-depth measurements from the ultraviolet to near-infrared. Constraints are placed on condensate removal by gravitational settling, the bulk metallicity, and the redistribution of absorbed stellar flux. Comparisons are also made to the Charbonneau et al. sodium measurements.

Physical and chemical aeronomy of HD 209458b

We report on the physical and chemical aeronomy of the hot Jupiter HD 209458b, a prominent case in the growing sample of known extrasolar planets. Our work is motivated by the recent detections of hydrogen, carbon and oxygen atoms obscuring about one tenth of the disk of the host star at the detection wavelengths and which have been interpreted as evidence for an escaping atmosphere. We model the escape and composition of the irradiated atmosphere by solving the equations of mass, momentum and energy conservation. At an orbital distance a ∼ 0.05 AU , intense Extreme Ultraviolet stellar irradiation may lead to the massive escape of its atmosphere. It is shown that for a planet of the characteristics of HD 209458b at small enough orbital distances, tidal forces may enhance the escape rate over the 1 / a 2 law inferred from simple energetic arguments, shortening the lifetime of the planet to a few Gigayears. This conclusion is contingent upon the premise of supersonic escape, on which we have based our calculations. It is expected that the atmosphere of HD 209458b contains hydrogen, helium and trace amounts of heavier elements such as carbon, oxygen and nitrogen. Indeed, the observations indicate that some of the heavier species reach as far above the surface of the planet as the lighter hydrogen atoms. We evaluate the abundances of the likely species forming from these elements and from the deuterium isotope throughout the upper atmosphere. Beyond a few planetary radii, all elements are strongly ionized, the atoms of carbon, helium and nitrogen being the first to do so. Our model, in the scenario of solar abundance for heavy constituents appears to be consistent with the observation depths of the three detected atoms. We have implemented a mass-consistent treatment of molecular and ambipolar diffusion suitable for multi-temperature multi-component gases that can be readily implemented in the modelling of planetary atmospheres.

Atmosphere, Interior, and Evolution of the Metal‐rich Transiting Planet HD 149026b

We investigate the atmosphere and interior of the new transiting planet HD 149026b, which appears to be very rich in heavy elements. We first compute model atmospheres at metallicities ranging from solar to 10 times solar and show how for cases with high metallicity or inefficient redistribution of energy from the dayside, the planet may develop a hot stratosphere due to absorption of stellar flux by TiO and VO. The spectra predicted by these models are very different than cooler atmosphere models without stratospheres. The spectral effects are potentially detectable with the Spitzer Space Telescope. In addition, the models with hot stratospheres lead to a large limb brightening, rather than darkening. We compare the atmosphere of HD 149026b to other well-known transiting planets, including the recently discovered HD 189733b, which we show has a planet-to-star flux ratio twice that of HD 209458 and TrES-1. The methane abundance in the atmosphere of HD 189733b is a sensitive indicator of atmospheric temperature and metallicity and can be constrained with Spitzer IRAC observations. We then turn to interior studies of HD 149026b and use a grid of self-consistent model atmospheres and high-pressure equations of state for all components to compute thermal evolution models of the planet. We estimate that the mass of heavy elements within the planet is in the range of 60-93 M⊕. Finally, we discuss trends in the radii of transiting planets with metallicity in light of this new member of the class.

A New Search for Carbon Monoxide Absorption in the Transmission Spectrum of the Extrasolar Planet HD 209458b

We have revisited the search for carbon monoxide absorption features in transmission during the transit of the extrasolar planet HD 209458b. In 2002 August-September we acquired a total of 1077 high-resolution spectra (λ/δλ ~ 25,000) in the K-band (2 μm) wavelength region using NIRSPEC on the Keck II telescope during three transits. These data are more numerous and of better quality than the data analyzed in an initial search by Brown et al. Our analysis achieves a sensitivity sufficient to test the degree of CO absorption in the first-overtone bands during transit on the basis of plausible models of the planetary atmosphere. We analyze our observations by comparison with theoretical tangent geometry absorption spectra, computed by adding height-invariant ad hoc temperature perturbations to the model atmosphere of Sudarsky et al. and by treating cloud height as an adjustable parameter. We do not detect CO absorption. The strong 2-0 R-branch lines between 4320 and 4330 cm-1 have depths during transit less than 1.6 parts in 104 in units of the stellar continuum (3 σ limit) at a spectral resolving power of 25,000. Our analysis indicates a weakening similar to that found in the case of sodium, suggesting that a general masking mechanism is at work in the planetary atmosphere. Under the interpretation that this masking is provided by high clouds, our analysis defines the maximum cloud-top pressure (i.e., minimum height) as a function of the model atmospheric temperature. For the relatively hot model used by Charbonneau et al. to interpret their sodium detection, our CO limit requires cloud tops at or above 3.3 mbar, and these clouds must be opaque at a wavelength of 2 μm. High clouds comprised of submicron-sized particles are already present in some models but may not provide sufficient opacity to account for our CO result. Cooler model atmospheres, having smaller atmospheric scale heights and lower CO mixing ratios, may alleviate this problem to some extent. However, even models 500 K cooler than the Sudarsky et al. model require clouds above the 100 mbar level to be consistent with our observations. Our null result therefore requires clouds to exist at an observable level in the atmosphere of HD 209458b, unless this planet is dramatically colder than current belief.

Source of Atomic Hydrogen in the Atmosphere of HD 209458b

Atomic hydrogen loss at the top of HD 209458b's atmosphere has been recently suggested (Vidal-Madjar et al.). We have developed a one-dimensional model to study the chemistry in the upper atmosphere of this extrasolar hot The three most abundant elements (other than He) as well as four parent molecules are included in this model, viz., H, C, O, H_2, CO, H_2O, and CH_4. The higher temperatures (~1000 K) and higher stellar irradiance (~6 × 10^5 W m^-2) strongly enhance and modify the chemical reaction rates in this atmosphere. The main result is that the production of atomic hydrogen in the atmosphere is mainly driven by H_2O photolysis, and the reaction of OH with H_2, and is insensitive to the exact abundances of CO, H_2O, and CH_4. For comparison, the bulk H concentration for hot Jupiters is 3 orders of magnitude higher than that of Jupiter.

Non-LTE Effects of N[CLC]a[/CLC] [CSC]i[/CSC] in the Atmosphere of HD 209458[CLC]b[/CLC

The recent announcement that sodium absorption has been observed in the atmosphere of HD 209458b, the only extrasolar giant planet (EGP) observed to transit its parent star, is the first direct detection of an EGP atmosphere. We explore the possibility that neutral sodium is not in local thermodynamic equilibrium (LTE) in the outer atmosphere of irradiated EGPs and that the sodium concentration may be underestimated by models that make the LTE assumption. Our results indicate that it may not be necessary to invoke excessive photoionization, low metallicity, or even high-altitude clouds to explain the observations.