Polarization of resonance lines formed in extenned spherical atmospheres

The problem of polarization of the resonance lines formed in extended spherical atmospheres is studied in detail. In this paper, the atmosphere is assumed to be at rest. The basic problem of resonance line polarization in spherical atmospheres as compared to the conventionally used plane-parallel atmospheres, is studied in Nagendra (1988). Our main interest in this paper is to understand the behaviour of polarized radiation fields in extended model spherical atmospheres so that some constraints can be placed on the model parameters in the modelling work conected with observations of polarization across resonance lines. A comparison of polarized lines formed under three kinds of line-scattering mechanisms is also made. They are CS=coherent scatteirng, CRD=complete redistribution, and PRD=partial frequency redistribution which, in the increasing order of generality, provide a good approximation in the two-level atom approach, to the resonance line polarization. The dependence of polarization on the opacity laws, extendedness and on optical depth is studied in detail. The distribution of line intensity and polarization across the visible disk of an extended model stellar atmosphere is studied, in view of the possible disk-resolved observations in future, of the extended atmospheres of the stars.     

Modelling the Jovian planets by a simple analytical algorithm

This paper models the structures of the Jovian atmospheres and cores by a simple analytical algorithm, which does not require the use of a computer. The method known as the MDV (Motz-Doorish Variable) method was first reported in the Irish Astronomical Journal (1989). It is a method to solve the Stellar Interior Equations (SIE) which dictate the structure of a star and/or the Jovian planets (Bodenheimer, 1986). The results are compared mainly, to the work of Stevenson (1982b) and found to be very agreeable. We include tabulated results of both the atmospheres and cores of Jupiter, Saturn, Uranus, and Neptune.     

Chemical modeling of exoplanet atmospheres

The past twenty years have revealed the diversity of planets that exist in the Universe. It turned out that most of exoplanets are different from the planets of our Solar System and thus, everything about them needs to be explored. Thanks to current observational technologies, we are able to determine some information about the atmospheric composition the thermal structure and the dynamics of these exoplanets, but many questions remain still unanswered. To improve our knowledge about exoplanetary systems, more accurate observations are needed and that is why the Exoplanet Characterisation Observatory (EChO) is an essential space mission. Thanks to its large spectral coverage and high spectral resolution, EChO will provide exoplanetary spectra with an unprecedented accuracy, allowing to improve our understanding of exoplanets. In this work, we review what has been done to date concerning the chemical modeling of exoplanet atmospheres and what are the main characteristics of warm exoplanet atmospheres, which are one of the main targets of EChO. Finally we will present the ongoing developments that are necessary for the chemical modeling of exoplanet atmospheres.     

Group-theoretical description of radiative transfer in one-dimensional media

Group theory is used to describe a procedure for adding inhomogeneous absorbing and scattering atmospheres in a one-dimensional approximation. The inhomogeneity originates in the variation of the scattering coefficient with depth. Group representations are derived for the composition of media in three different cases: inhomogeneous atmospheres in which the scattering coefficient varies continuously with depth, composite or multicomponent atmospheres, and the special case of homogeneous atmospheres. We extend an earlier proposal to solve problems in radiative transfer theory by first finding global characteristics of a medium (reflection and transmission coefficients) and then determining the internal radiation field for an entire family of media without solving any new equations. Semi-infinite atmospheres are examined separately. For some special depth dependences of the scattering coefficients it is possible to obtain simple analytic solutions expressed in terms of elementary functions. An algorithm for numerical solution of radiative transfer problems in inhomogeneous atmospheres is described.     

The radio occultation method for the study of planetary atmospheres

Planets, the Moon and the Sun have a number of ‘atmospheres’ which may be measured by the radio occultation method, using radio links to spacecraft which are being occulted by the body as seen from Earth. Molecular atmospheres, ionospheres, magnetospheres, particulate atmospheres and several general-relativistic atmospheres can all affect radio signal charecteristics. Measured Doppler frequencies, signal amplitudes and wave polarizations contain information on these atmospheres. From such occultation measurements it is possible, for example, to derive profiles which are related to the changes with height of temperature, pressure, density, free electron concentration and the fractional volume occupied by particulate matter. Important clues for identifying molecular or particulate constituents can also be obtained. Even though one type of measurement may be sensitive to several different atmospheric characteristics, these characteristics can often be separated due to differences in their dependence on radio wavelength or on height above the surface.     

A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin.

We present our current understanding of the composition, vertical mixing, cloud structure and the origin of the atmospheres of Jupiter and Saturn. Available observations point to a much more vigorous vertical mixing in Saturn's middle-upper atmosphere than in Jupiter's. The nearly cloud-free nature of the Galileo probe entry site, a 5-micron hotspot, is consistent with the depletion of condensable volatiles to great depths, which is attributed to local meteorology. Somewhat similar depletion of water may be present in the 5-micron bright regions of Saturn also. The supersolar abundances of heavy elements, particularly C and S in Jupiter's atmosphere and C in Saturn's, as well as the progressive increase of C from Jupiter to Saturn and beyond, tend to support the icy planetesimal model of the formation of the giant planets and their atmospheres. However, much work remains to be done, especially in the area of laboratory studies, including identification of possible new microwave absorbers, and modelling, in order to resolve the controversy surrounding the large discrepancy between Jupiter's global ammonia abundance, hence the nitrogen elemental ratio, derived from the earth-based microwave observations and that inferred from the analysis of the Galileo probe-orbiter radio attenuation data for the hotspot. We look forward to the observations from Cassini-Huygens spacecraft which are expected to result not only in a rich harvest of information for Saturn, but a better understanding of the formation of the giant planets and their atmospheres when these data are combined with those that exist for Jupiter.     

A numerical method for determining the temperature structure of planetary atmospheres

A numerical method for calculating the time-average, vertical temperature structure of planetary atmospheres is presented. It is assumed that the atmospheres are in radiative-convective equilibrium, which is a good first approximation to many situations. Numerical tests of the rate of convergence and accuracy of the answer are presented. The method can readily handle molecular sources of opacity. Accurate results can be obtained with a minimum of computer time, because the number of iterations needed (～ 4) is small and the number of pressure levels at which the net flux needs to be evaluated (～ 10) is small. As an application of this procedure, we have calculated some model atmospheres of Jupiter.     

Elemental abundance analyses with DAO spectrograms — XVIII. The double-lined spectroscopic binary 46 Draconis

Equipartition magnetic fields can dramatically affect the polarization of radiation emerging from accretion disc atmospheres in active galactic nuclei. We extend our previous work on this subject by exploring the interaction between Faraday rotation and absorption opacity in local, plane-parallel atmospheres with parameters appropriate for accretion discs. Faraday rotation in pure scattering atmospheres acts to depolarize the radiation field by rotating the polarization planes of photons after last scattering. Absorption opacity in an unmagnetized atmosphere can increase or decrease the polarization compared to the pure scattering case, depending on the thermal source function gradient. Combining both Faraday rotation and absorption opacity, we find the following results. If absorption opacity is much larger than scattering opacity throughout the atmosphere, then Faraday rotation generally has only a small effect on the emerging polarization because of the small electron column density along a photon mean free path. However, if the absorption opacity is not too large and it acts alone to increase the polarization, then the effects of Faraday rotation can be enhanced over those in a pure scattering atmosphere. Finally, while Faraday rotation often depolarizes the radiation field, it can in some cases increase the polarization when the thermal source function does not rise too steeply with optical depth. We confirm the correctness of the analytic calculation by Silant'ev of the high magnetic field limit of the pure scattering atmosphere, which we incorrectly disputed in our previous paper.     

The total and relative contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres in the UV and VUV regions

The main aim of this work is to estimate the total contribution of the processes of     molecular ion photodissociation and     collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of     and other relevant radiative absorption processes included in standard models.
The method for the calculations of the molecular ion     photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in     collisions is based on the quasi-static approximation.
Absorption coefficients are calculated in the region  50 nm ≤λ≤ 850 nm  and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.
We examined the spectral ranges in which the total     and     absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of  λ≲ 70 nm  the process of     atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is  1:10⁵  .     

Polarization of radiation from stellar atmospheres. The grey case

An accurate numerical method is presented for the solution of the transfer equations in a plane-parallel atmosphere in which scattering occurs according to Rayleigh's law. Some results are given for the polarization and limb darkening of both integrated and monochromatic radiation emerging from grey atmospheres with various ratios of scattering to absorption. The method is equally applicable to non-grey atmospheres.     

The Origin and Evolution of the Atmospheres of Venus, Earth and Mars

The chemical compositions of the primordial atmospheres of Venus, Earth and Mars have long been a topic of debate between the experts. Some believe that the original atmospheres were a product of outgassed volatiles from the newly accreted terrestrial planets and that these atmospheres consisted primarily of carbon dioxide, nitrogen, water vapor and residual hydrogen and helium (e.g., Lewis and Prinn, <it>Planets and their Atmospheres,</it> Academic Press, Orlando, FL, 1984, pp. 62–63, 81–84, 228–231, 383). Still others think the earliest atmospheres were composed of the gas components of the solar nebula from which the solar system formed (i.e., hydrogen, helium, methane, ammonia and water). I consider the latter to be the correct scenario. Presented herein is a proposed mechanism by which the original atmospheres of Venus, Earth and Mars were transformed to atmospheres rich in carbon dioxide and nitrogen. An explanation is proposed for why water is so common on the surface of Earth and so scarce on the surfaces of Venus and Mars. Also presented are the effects the “great impact” (single cataclysmic event that was responsible for producing the Earth–Moon system) had upon the early atmosphere of Earth. The origin, structure and composition of the impacting object are determined through deductive analyses.     

Modelling the VUV emission spectrum of N2: preliminary results on the effects of rotational interactions on line intensities

A theoretical model of the excited singlet ungerade electronic states of the nitrogen molecule is presented. This work is an extension of a previous study (D. Stahel, M. Leoni and K. Dressler, J. Chem. Phys. 79, 2541-2558, 1983) with the addition of rotational interactions between states of different symmetry. These rotational interactions together with the homogeneous couplings frequently lead to extreme mixing of the states. This model is being used to analyse the high resolution vacuum ultraviolet emission spectrum which is currently being recorded photoelectrically. The ultimate goal of this work is the reliable interpretation of the low resolution emission spectra observed from planetary atmospheres, notably that of Titan, and the transitions assigned as being important in the Voyager 1 spectra of Titan are discussed in detail.     

Atmospheric variations and meteorite production on Mars

Through a combination of aerobraking (drag deceleration) and ablation, meteoroids which enter planetary atmospheres may be slowed sufficiently to soft-land as meteorites. Results of an earlier study suggest that the current 6 mbar atmosphere of Mars is sufficient to aerobrake significant numbers of small (<10 kg) asteroidal-type meteoroids into survivable, low-velocity (<500 m s⁻¹) impacts with the planet's surface. Since rates of meteorite production depend upon the density of Mars's atmosphere, they must also change as the martian climate changes. However, to date, martian meteorite production has received relatively little attention in the literature Here we expand upon our previous work to study martian meteorite production rates and how they depend upon variations of the martian atmosphere, and to estimate the ranges of mass, velocity and entry-angle that produce meteorites. We find that even the current atmosphere of Mars is sufficient to soft-land significant fractions of incident stony and iron objects, and that these fractions increase dramatically for denser martian atmospheres. Therefore, like impact cratering, meteorite populations may preserve evidence of past martian climates.     

Modeling of Stark–Zeeman Lines in Magnetized Hydrogen Plasmas

The action of electric and magnetic fields on atomic species results in a perturbation of the energy level structure, which alters the shape of spectral lines. In this work, we present the Zeeman–Stark line shape simulation method and perform new calculations of hydrogen Lyman and Balmer lines, in the framework of magnetic fusion research. The role of the Zeeman effect, fine structure and the plasma’s non-homogeneity along the line-of-sight are investigated. Under specific conditions, our results are applicable to DA white dwarf atmospheres.     

Reflected light from 3D exoplanetary atmospheres and simulation of HD 209458b

We present radiation transfer models that demonstrate that reflected light levels from 3D exoplanetary atmospheres can be more than 50 per cent lower than those predicted by models of homogeneous or smooth atmospheres. Compared to smooth models, 3D atmospheres enable starlight to penetrate to larger depths resulting in a decreased probability for the photons to scatter back out of the atmosphere before being absorbed. The increased depth of penetration of starlight in a 3D medium is a well-known result from theoretical studies of molecular clouds and planetary atmospheres. For the first time we study the reflectivity of 3D atmospheres as a possible explanation for the apparent low geometric albedos inferred for extrasolar planetary atmospheres. Our models indicate that 3D atmospheric structure may be an important contributing factor to the non-detections of scattered light from exoplanetary atmospheres. We investigate the self-shadowing radiation transfer effects of patchy cloud cover in 3D scattered light simulations of the atmosphere of HD 209458b. We find that, for a generic planet, geometric albedos can be as high as 0.45 in some limited situations, but that in general the geometric albedo is much lower. We conclude with some explanations on why extrasolar planets are likely dark at optical wavelengths.     

On nonstationary radiative transfer in a spectral line in stellar atmospheres

We consider nonstationary radiative transfer in a line in stellar atmospheres simulated as a stationary semi-infinite plane-parallel medium. We assume complete frequency redistribution in the elementary act of scattering. We assume that the time a photon spends in the medium is determined only by the mean time spent in the absorbed state. We obtain an explicit expression for the resolvent of the nonstationary integral equation of transfer, which is a bilinear expansion with respect to the eigenfunctions found in [12] for the corresponding stationary transfer equation.Translated fromAstrofizika, Vol. 37, No. 3, 1994.I am grateful to the American Astronomical Society for financial support of this work.     

About Superrotation in Venus

In this work we study in a general view slow rotating planets as Venus or Titan which present superrotating winds in their atmospheres. We are interested in understanding what mechanisms are candidates to be sources of net angular momentum to generate this kind of dynamics. In particular, in the case of Venus, in its atmosphere around an altitude of 100 km relative to the surface, there exists winds that perform a full rotation around the planet in four terrestrial days, whereas the venusian day is equivalent to 243 terrestrial ones. This phenomenon called superrotation is known since many decades. However, its origin and behaviour is not completely understood. In this article we analise and ponderate the importance of different effects to generate this dynamics.     

Sodium in the atmospheres of thick-disk red giants

The atmospheric parameters and elemental abundances for ten thick-disk red giants have been determined from high-resolution spectra by the method of model stellar atmospheres. The results of a comparative analysis of the [Na/Fe] abundances in the atmospheres of the investigated stars and thin-disk red giants are presented. Sodium in the atmospheres of thick-disk red giants is shown to have no overabundances typical of thin-disk red giants.     

Oxygen airglow emission on Venus and Mars as seen by VIRTIS/VEX and OMEGA/MEX imaging spectrometers

Imaging spectrometers are highly effective instruments for investigation of planetary atmospheres. They present the advantage of coupling the compositional information to the spatial distribution, allowing simultaneous study of chemistry and dynamics in the atmospheres of Venus and Mars. In this work, we summarize recent results about the O₂(a¹Δ_g) night and day glows, respectively obtained by VIRTIS/Venus Express and OMEGA/Mars Express, the imaging spectrometers currently in orbit around Venus and Mars. The case of the O₂(a¹Δ_g - X³Σ_g⁻) IR emission at 1.27 μm on the night side of Venus and the day side of Mars is analyzed, pointing out dynamical aspects of these planets, like the detection of gravity waves in their atmospheres. The monitoring of seasonal and daily airglow variations provides hints about the photochemistry on these planets.     

Radiative Transfer in Inhomogeneous Atmospheres. I

This series of papers is devoted to multiple scattering of light in plane parallel, inhomogeneous atmospheres. The approach proposed here is based on Ambartsumyan's method of adding layers. The main purpose is to show that one can avoid difficulties with solving various boundary value problems in the theory of radiative transfer, including some standard problems, by reducing them to initial value problems. In this paper the simplest one dimensional problem of diffuse reflection and transmission of radiation in inhomogeneous atmospheres with finite optical thicknesses is considered as an example. This approach essentially involves first determining the reflection and transmission coefficients of the atmosphere, which, as is known, are a solution of the Cauchy problem for a system of nonlinear differential equations. In particular, it is shown that this system can be replaced with a system of linear equations by introducing auxiliary functions P and S. After the reflectivity and transmissivity of the atmosphere are determined, the radiation field in it is found directly without solving any new equations. We note that this approach can be used to obtain the required intensities simultaneously for a family of atmospheres with different optical thicknesses. Two special cases of the functional dependence of the scattering coefficient on the optical thickness, for which the solutions of the corresponding equations can be expressed in terms of elementary functions, are examined in detail. Some numerical calculations are presented and interpreted physically to illustrate specific features of radiative transport in inhomogeneous atmospheres.