Showing papers on "Radiative transfer published in 1999"

The NextGen Model Atmosphere Grid for 3000 ≤ Teff ≤ 10,000 K

Abstract: We present our NextGen Model Atmosphere grid for low-mass stars for effective temperatures larger than 3000 K. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants. The models were calculated from 3000 to 10,000 K (in steps of 200 K) for 3.5{le}logthinspg{le}5.5 (in steps of 0.5) and metallicities of {minus}4.0{le}[M/H]{le}0.0. We discuss the results of the model calculations and compare our results to the Kurucz grid. Some comparisons to standard stars like Vega and the Sun are presented and compared with detailed NLTE calculations. {copyright} {ital {copyright} 1999.} {ital The American Astronomical Society}

Improved general circulation models of the Martian atmosphere from the surface to above 80 km

Abstract: We describe a set of two “new generation” general circulation models of the Martian atmosphere derived from the models we originally developed in the early 1990s. The two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations. The vertical resolution near the surface has been refined, and the vertical domain has been extended to above 80 km. These changes are accompanied by the inclusion of state-of-the-art parameterizations to better simulate the dynamical and physical processes near the surface (boundary layer scheme, subgrid-scale topography parameterization, etc.) and at high altitude (gravity wave drag). In addition, radiative transfer calculations and the representation of polar processes have been significantly improved. We present some examples of zonal-mean fields from simulations using the model at several seasons. One relatively novel aspect, previously introduced by Wilson [1997], is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi-global, angular-momentum conserving Hadley cell which has no terrestrial equivalent. Within such a cell the Coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km. This winter polar warming had been observed but not properly modeled until recently. In fact, thermal inversions are generally predicted above one, and often both, poles around 60–70 km. However, the Mars middle atmosphere above 40 km is found to be very model-sensitive and thus difficult to simulate accurately in the absence of observations.

Estimation of the remote-sensing reflectance from above-surface measurements.

Abstract: The remote-sensing reflectance Rrs is not directly measurable, and various methodologies have been employed in its estimation. I review the radiative transfer foundations of several commonly used methods for estimating Rrs, and errors associated with estimating Rrs by removal of surface-reflected sky radiance are evaluated using the Hydrolight radiative transfer numerical model. The dependence of the sea surface reflectance factor ρ, which is not an inherent optical property of the surface, on sky conditions, wind speed, solar zenith angle, and viewing geometry is examined. If ρ is not estimated accurately, significant errors can occur in the estimated Rrs for near-zenith Sun positions and for high wind speeds, both of which can give considerable Sun glitter effects. The numerical simulations suggest that a viewing direction of 40 deg from the nadir and 135 deg from the Sun is a reasonable compromise among conflicting requirements. For this viewing direction, a value of ρ ≈ 0.028 is acceptable only for wind speeds less than 5 m s-1. For higher wind speeds, curves are presented for the determination of ρ as a function of solar zenith angle and wind speed. If the sky is overcast, a value of ρ ≈ 0.028 is used at all wind speeds.

Incorporation of mineralogical composition into models of the radiative properties of mineral aerosol from UV to IR wavelengths

Abstract: We describe a technique to model the radiative properties of mineral aerosols which accounts for their composition. We compile a data set of refractive indices of major minerals and employ it, along with data on mineralogical composition of dust from various locations, to calculate spectral optical and radiative properties of mineral aerosol mixtures. Such radiative properties are needed for climate modeling and remote sensing applications. We consider external mixtures of individual minerals, as well as mixtures of aggregates. We demonstrate that an external mixture of individual minerals must contain unrealistically high amounts of hematite to have a single scattering albedo lower than 0.9 at 500 nm wavelength. In contrast, aggregation of hematite with quartz or clays can strongly enhance absorption by dust at solar wavelengths. We also simulate the daily mean net (solar + infrared) forcing by dust of varying compositions. We found that, for a given composition and under similar atmospheric conditions, a mixture of aggregates can cause the positive radiative forcing while a mixture of individual minerals gives the negative forcing.

Population Diagram Analysis of Molecular Line Emission

Abstract: We develop the use of the population diagram method to analyze molecular emission in order to derive physical properties of interstellar clouds. We focus particular attention on how the optical depth affects the derived total column density and the temperature. We derive an optical depth correction factor that can be evaluated based on observations and that incorporates the effect of saturation on derived upper level populations. We present analytic results for linear molecules in local thermodynamic equilibrium (LTE). We investigate numerically how subthermal excitation influences the population diagram technique, studying how the determination of kinetic temperature is affected when the local density is insufficient to achieve LTE. We present results for HC3N and CH3OH, representative of linear and nonlinear molecules, respectively. In some cases, alternative interpretations to the standard optically thin and thermalized picture yield significantly different results for column density and kinetic temperature, and we discuss this behavior. The population diagram method can be a very powerful tool for determining physical conditions in dense clouds if proper recognition is given to effects of saturation and subthermal excitation. We argue that the population diagram technique is, in fact, superior to fitting intensities of different transitions directly, and we indicate how it can be effectively employed.

The expulsion of stellar envelopes in core-collapse supernovae

Abstract: We examine the relation between presupernova stellar structure and the distribution of ejecta in core-collapse supernovae of Types Ib, Ic, and II, under the approximations of adiabatic, spherically symmetric flow. We develop a simple yet accurate analytical formula for the velocity of the initial forward shock that traverses the stellar envelope. For material that does not later experience a strong reverse shock, the entropy deposited by this forward shock persists into the final, freely expanding state. We demonstrate that the final density distribution can be approximated with simple models for the final pressure distribution, in a way that matches the results of simulations. Our results indicate that the distribution of density and radiation pressure in a star's ejecta depends on whether the outer envelope is radiative or convective, and if convective, on the composition structure of the star. Our models are most accurate for the high-velocity ejecta cast away from the periphery of a star. For stellar structures that limit to a common form in this region, the resulting ejecta limit to a common distribution at high velocities because the blast wave forgets its history as it approaches the stellar surface. We present formulae for the final density distribution of this material as a function of mass, for both radiative and efficiently convective envelopes. These formulae limit to the well-known planar, self-similar solutions for mass shells approaching the stellar surface. However, the assumption of adiabatic flow breaks down for shells of low optical depth, so this planar limit need not be attained. The event of shock emergence, which limits adiabatic flow, also produces a soft X-ray burst of radiation. Formulae are given for the observable properties of this burst and their dependence on the parameters of the explosion. Motivated by the relativistic expansion recently inferred by Kulkarni et al. for the synchrotron shell around SN 1998bw, we estimate the criterion for relativistic mass ejection and the rest mass of relativistic ejecta. We base our models for the entire ejecta distribution on the high-velocity solution, on our shock-velocity formula, and on realistic radiation pressure distributions. We also present simpler, but less flexible, analytical approximations for ejecta distributions. We survey the ejecta of the polytropic hydrogen envelopes of red supergiants. Our models will be useful for studies of the light curves and circumstellar or interstellar interactions of core-collapse supernovae, and of the birth of pulsar nebulae in their ejecta.

Multiple scattering of classical waves: microscopy, mesoscopy, and diffusion

Abstract: important corrections are presented. These corrections are calculated with the radiative transfer or Schwarzschild-Milne equation, which describes intensity transport at the ‘‘mesoscopic’’ level and is derived from the ‘‘microscopic’’ wave equation. A precise treatment of the diffuse intensity is derived which automatically includes the effects of boundary layers. Effects such as the enhanced backscatter cone and imaging of objects in opaque media are also discussed within this framework. This approach is extended to mesoscopic correlations between multiple scattered intensities that arise when scattering is strong. These correlations arise from the underlying wave character. The derivation of correlation functions and intensity distribution functions is given and experimental data are discussed. Although the focus is on light scattering, the theory is also applicable to microwaves, sound waves, and noninteracting electrons. [S0034-6861(99)00601-7]

Evolution of Nonradiative Supernova Remnants

Abstract: We conduct an analytic and numerical study of the dynamics of supernova remnant (SNR) evolution from the ejecta-dominated stage through the Sedov-Taylor (ST) stage, the stages that precede the onset of dynamically significant radiative losses and/or pressure confinement by the ambient medium. We assume spherical symmetry and focus on the evolution of ejecta described by a power-law density distribution expanding into a uniform ambient medium. We emphasize that all nonradiative remnants of a given power-law structure evolve according to a single unified solution, and we discuss this general property in detail. Use of dimensionless quantities constructed from the characteristic dimensional parameters of the problem—the ejecta energy, ejecta mass, and ambient density—makes the unified nature of the solution manifest. It is also possible to obtain a unified solution for the ST and radiative stages of evolution, and we place our work in the context of scaling laws for solutions for SNR evolution in those stages. We present numerical simulations of the flow and approximate analytic solutions for the motions of both the reverse shock and blast-wave shock. These solutions follow the shocks through the nonradiative stages of remnant evolution across periods of self-similar flow linked by non-self-similar behavior. We elucidate the dependence of the ejecta-dominated evolution on the ejecta power-law index n by developing a general trajectory for all n and explaining its relation to the solutions of Chevalier and Nadyozhin for n>5 and Hamilton & Sarazin for n=0. We demonstrate excellent agreement between our analytic solutions and numerical simulations. These solutions should be valuable in describing remnants such as SN 1006, Tycho, Kepler, Cassiopeia A, and other relatively young SNRs that are between the early ejecta-dominated stage and the late Sedov-Taylor stage. In appendices, we extend our results to power-law ambient media, and we describe an early period of the evolution in which the SNR is radiative and evolves according to a nonunified solution.

The Role of Solar Radiation in Atmospheric Chemistry

Abstract: Solar radiation at visible and ultraviolet wavelengths drives the chemistry of the atmosphere, by photo-dissociating relatively stable molecules into highly reactive radical fragments. Knowledge of photo-dissociation rate coefficients (J values) is crucial to understanding the behavior of global stratospheric and tropospheric ozone, the atmospheric lifetimes of gases such as carbon monoxide, methane, and non-methane hydrocarbons, and the formation of oxidants at urban and regional scales. J values depend on molecular parameters (absorption cross sections and photo-dissociation quantum yields) that are specific to the photo-reaction of interest, and on the availability of solar radiation at any specific location in the atmosphere. Advances in computer modeling of atmospheric radiative transfer now allow rapid calculation of J values for use in photo-chemistry models, and routinely include the effects of molecular absorbers and scatterers, clouds, aerosols, and surface reflections, for any location and time of the year. However, actual atmospheric conditions needed as input to the calculation are often not available. Direct measurements of J values, while in principle preferable, are technically difficult and limited in their temporal/spatial coverage, but generally support the theoretical calculations at least under optimal conditions (e. g., cloud free skies).

Radiative Transfer in the Atmosphere and Ocean

Abstract: Preface 1. Basic properties of radiation, atmospheres and oceans 2. Basic state variables 3. Interaction of radiation with matter 4. Formulation of radiative transfer problems 5. Approximate solutions of prototype problems 6. Accurate numerical solutions of prototype problems 7. Emission-dominated radiative processes 8. Radiative transfer in spectrally-complex media 9. Solar radiation driving photochemistry and photobiology 10. The role of radiation in climate Appendix 1. A primer on radiative transfer: absorption and scattering opacity Appendix 2. Stokes parameters, Poincare sphere, and the Mueller matrix Appendix 3. Nomenclature: glossary of symbols Appendix 4. Principle of reciprocity for the bidirectional reflectance Appendix 5. Isolation of the azimuth-dependence Appendix 6. The streaming term in spherical geometry Appendix 7. Reflectance and transmittance of the invariant intensity (I n2) Appendix 8. Scaling transformation for anisotropic scattering Appendix 9. Reciprocity, duality and effects of surface reflection Appendix 10. Removal of overflow problems in the intensity formulas.

The NEXTGEN Model Atmosphere Grid. II. Spherically Symmetric Model Atmospheres for Giant Stars with Effective Temperatures between 3000 and 6800 K

Abstract: We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to that of the NextGen dwarf atmosphere models; however, spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We revisit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of non-LTE calculations for a few selected models.

MODTRAN4 radiative transfer modeling for atmospheric correction

Abstract: MODTRAN4, the latest publicly released version of MODTRAN, provides many new and important options for modeling atmospheric radiation transport. A correlated-k algorithm improves multiple scattering, eliminates Curtis-Godson averaging, and introduces Beer's Law dependencies into the band model. An optimized 15 cm-1 band model provides over a 10-fold increase in speed over the standard MODTRAN 1 cm-1 band model with comparable accuracy when higher spectral resolution results are unnecessary. The MODTRAN ground surface has been upgraded to include the effects of Bidirectional Reflectance Distribution Functions (BRDFs) and Adjacency. The BRDFs are entered using standard parameterizations and are coupled into line-of-sight surface radiance calculations.

Hydrodynamical non-radiative accretion flows in two dimensions

Abstract: Two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows that cannot cool through emission of radiation are presented. The calculations begin from an equilibrium configuration consisting of a thick torus with constant specific angular momentum. Accretion is induced by the addition of a small anomalous azimuthal shear stress which is characterized by a function ν. We study the flows generated as the amplitude and form of ν are varied. A spherical polar grid which spans more than two orders of magnitude in radius is used to resolve the flow over a wide range of spatial scales. We find that convection in the inner regions produces significant outward mass motions that carry away both the energy liberated by and a large fraction of the mass participating in the accretion flow. Although the instantaneous structure of the flow is complex and dominated by convective eddies, long-time averages of the dynamical variables show remarkable correspondence to certain steady-state solutions. The two-dimensional structure of the time-averaged flow is marginally stable to the Hoiland criterion, indicating that convection is efficient. Near the equatorial plane, the radial profiles of the time-averaged variables are power laws with an index that depends on the radial scaling of the shear stress. A stress in which ν∝r1/2 recovers the widely studied self-similar solution corresponding to an ‘α-disc’. We find that, regardless of the adiabatic index of the gas, or the form or magnitude of the shear stress, the mass inflow rate is a strongly increasing function of radius, and is everywhere nearly exactly balanced by mass outflow. The net mass accretion rate through the disc is only a fraction of the rate at which mass is supplied to the inflow at large radii, and is given by the local, viscous accretion rate associated with the flow properties near the central object.

Bidirectional Reflectance of Flat, Optically Thick Particulate Layers: An Efficient Radiative Transfer Solution and Applications to Snow and Soil Surfaces

Abstract: We describe a simple and highly efficient and accurate radiative transfer technique for computing bidirectional reflectance of a macroscopically flat scattering layer composed of nonabsorbing or weakly absorbing, arbitrarily shaped, randomly oriented and randomly distributed particles. The layer is assumed to be homogeneous and optically semi-infinite, and the bidirectional reflection function (BRF) is found by a simple iterative solution of the Ambartsumian's nonlinear integral equation. As an exact Solution of the radiative transfer equation, the reflection function thus obtained fully obeys the fundamental physical laws of energy conservation and reciprocity. Since this technique bypasses the computation of the internal radiation field, it is by far the fastest numerical approach available and can be used as an ideal input for Monte Carlo procedures calculating BRFs of scattering layers with macroscopically rough surfaces. Although the effects of packing density and coherent backscattering are currently neglected, they can also be incorporated. The FORTRAN implementation of the technique is available on the World Wide Web at http://ww,,v.giss.nasa.gov/-crmim/brf.html and can be applied to a wide range of remote sensing, engineering, and biophysical problems. We also examine the potential effect of ice crystal shape on the bidirectional reflectance of flat snow surfaces and the applicability of the Henyey-Greenstein phase function and the 6-Eddington approximation in calculations for soil surfaces.

Resonance Energy Transfer

Abstract: 1. Resonance energy transfer in proteins introduction some basic considerations a short history of FRET determinations the components of the Foorster equation quantum yield determining spectral overlap steady state or time--resolved measurements? resonance energy transfer using intrinsic amino acids homotransfer between intrinsic probes heterotransfer the range of distances determined by resonance energy transfer precise location of resonance energy transfer probes properties of probes labeling specific residues in proteins resonance energy transfer experiments using lanthanide ions measurements in radially symmetrical systems comparison with crystallographic distances using resonance energy transfer to constrain molecular models resonance energy transfer with single fluorophores: new wave experiments intramolecular energy transfer in proteins bound to membranes green fluorescent protein resonance energy transfer and biosensors: a new and promising technique shortcomings the future of FRET summary dedication acknowledgements references. 2. Unified theory and radiative and raditionless energy transfer introduction background the basis of the unified theory spectral features refraction and dissipation dynamics of energy transfer between a pair of molecules in a dielectric medium conclusion appendix A: Heitler--MA method for analysis of the transition operator Appendix B: modified approach to the transition operator references. 3. Dynamics of radiative transport introduction overview of atomic and molecular radiative transport the Holstein--Biberman equation multiple scattering representation stochastic approach combined radiative and nonradiative transport conclusion appendix A: probablitity of emission of a photon between t + dt for an nth generation molecule appendix B: depolarization factor for radiative transferaccording to classical electrodynamics references. 4. Orientational aspects in pair energy transfer introduction Kappa--squared and probability, Kappa--squared and anisotropy notes on the effects of order and motion acknowledgements references. 5. Polarization in molecular complexes with incoherent energy transfer introduction interaction of light with single molecules or chromophores bichromophore molecular complexes trichromophore complexes multichromophore complexes with C3 symmetry conclusion appendix A appendix B appendix C appendix D references. 6. Theory of coupling in multichromophoric systems introduction reactant and product states: LMO model the origin of coupling matrix elements paradigmatic results coulombic coupling superexchange interpretation of steady state spectra calculation of couplings acknowledgements references. 7. Exciton annihilation in molecular aggregates introduction theory applications discussion acknowledgements references. 8. Energy transfer and localization: applications to photosynthetic systems introduction optical properties of dimers and aggregates energy and localization in antenna complexes and reaction centers acknowledgements references. 9.Excitation energy transfer in photosynthesis introduction the structure of light--harvesting complexes the mechanism of energy transfer and trapping in photosynthesis dynamics of excitation energy transfer conclusions acknowledgements references. 10. The Fenna--Matthews--Olson protein: a strongly coupled photosynthetic antenna introduction steady state spectroscopy FMO exciton simulations FMO primary processes epilog and future prospects acknowledgements references. 11. Use of a Monte carlo method in the problem of energy migration in molecular complexes introduction an illustration of Monte Carlo calculations in the problem of fluorescence decay energy transfer in CME: major algorithm applications of monte Carlo simulations conclusion acknowledgements references. Index

The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K

Abstract: We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to the NextGen dwarf atmosphere models, however spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We re-visit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of NLTE calculations for a few selected models.

Supernova Remnants in Molecular Clouds

Abstract: Massive (8 M☉) stars may end their lives in the molecular clouds in which they were born. O-type stars probably have sufficient photoionizing radiation and wind power to clear a region more than 15 pc in radius of molecular material. Early B stars (B1-B3 on the main sequence, or 8-12 M☉ stars) are not capable of this and may interact directly with molecular gas when they explode. Molecular clouds are known to be clumpy, with dense molecular clumps occupying only a few percent of the volume. A supernova remnant then evolves primarily in the interclump medium, which has a density nH=5-25 H atoms cm-3. The remnant becomes radiative at a radius of ~6 pc, forming a shell that is magnetically supported. The structure of the shell can be described by a self-similar solution. When this shell interacts with the dense clumps, the molecular shock fronts are driven by a considerable overpressure compared to the pressure in the rest of the remnant. The expected range of clump sizes leads to a complex velocity distribution, with the possibility of molecular gas accelerated to a high velocity. Observations of the remnants W44 and IC 443 can be understood in this model. W44 has a shell expanding at ~150 km s-1 into a medium with density 4-5 cm-3. The shock emission expected in such a model is consistent with the observed Hα surface brightness and the [O I] 63 μm line luminosity. The clump interaction is seen in OH maser emission, which shows a magnetic field strength that is consistent with that expected in the model. IC 443 appears to be expanding at a lower velocity, 100 km s-1, into an interclump medium with a higher density, ~15 cm-3. The interaction of the radiative shell with molecular clumps can produce the molecular emission that is observed from IC 443. Both remnants are shell sources of radio synchrotron emission, which can be attributed to relativistic electrons in the cool radiative shell. If ambient cosmic-ray electrons are further accelerated by the shock front and by the postshock compression, the radio fluxes and the flat spectral indices of W44 and IC 443 can be explained. The energetic electrons are in a high-density shell and their bremsstrahlung emission can approximately produce the γ-ray fluxes observed by EGRET. Molecular clouds have a significant uniform magnetic field component so that heat conduction is likely to be important in the hot interior and can explain the isothermal X-ray emission observed from the remnants.

Near-Field Effects in Spatial Coherence of Thermal Sources

Abstract: We present an exact calculation of the cross-spectral density tensor of the near field thermally emitted into free space by an opaque planar surface. The approach, based on fluctuational electrodynamics and the fluctuation-dissipation theorem, yields novel near-field correlation properties. We show that the spatial coherence length of the field close to the surface at a given wavelength $\ensuremath{\lambda}$ may be much smaller than the well-known $\ensuremath{\lambda}/2$ of blackbody radiation. We also show that a long-range correlation may exist, when resonant surface waves, such as surface-plasmon or surface-phonon polaritons, are excited. These results should have important consequences in the study of coherence in thermal emission and in the modeling of nanometer scale radiative transfer.

Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5) × 106 K

Abstract: The three-dimensional structure of solar active region NOAA 7986 observed on 1996 August 30 with the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) is analyzed. We develop a new method of dynamic stereoscopy to reconstruct the three-dimensional geometry of dynamically changing loops, which allows us to determine the orientation of the mean loop plane with respect to the line of sight, a prerequisite to correct properly for projection effects in three-dimensional loop models. With this method and the filter-ratio technique applied to EIT 171 and 195 A images we determine the three-dimensional coordinates [x(s), y(s), z(s)], the loop width w(s), the electron density ne(s), and the electron temperature Te(s) as a function of the loop length s for 30 loop segments. Fitting the loop densities with an exponential density model ne(h) we find that the mean of inferred scale height temperatures, Tλe=1.22 ± 0.23 MK, matches closely that of EIT filter-ratio temperatures, TEITe=1.21 ± 0.06 MK. We conclude that these cool and rather large-scale loops (with heights of h≈30-225 Mm) are in hydrostatic equilibrium. Most of the loops show no significant thickness variation w(s), but we measure for most of them a positive temperature gradient (dT/ds>0) across the first scale height above the footpoint. Based on these temperature gradients we find that the conductive loss rate is about 2 orders of magnitude smaller than the radiative loss rate, which is in strong contrast to hot active region loops seen in soft X-rays. We infer a mean radiative loss time of τrad≈40 minutes at the loop base. Because thermal conduction is negligible in these cool EUV loops, they are not in steady state, and radiative loss has entirely to be balanced by the heating function. A statistical heating model with recurrent heating events distributed along the entire loop can explain the observed temperature gradients if the mean recurrence time is 10 minutes. We computed also a potential field model (from SOHO/MDI magnetograms) and found a reasonable match with the traced EIT loops. With the magnetic field model we determined also the height dependence of the magnetic field B(h), the plasma parameter β(h), and the Alfven velocity vA(h). No correlation was found between the heating rate requirement EH0 and the magnetic field Bfoot at the loop footpoints.

Albedo and Reflection Spectra of Extrasolar Giant Planets

Abstract: We generate theoretical albedo and reflection spectra for a full range of extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects. Our albedo modeling utilizes the latest atomic and molecular cross sections, a Mie theory treatment of extinction by condensates, a variety of particle size distributions, and an extension of the Feautrier radiative transfer method which allows for a general treatment of the scattering phase function. We find that due to qualitative similarities in the compositions and spectra of objects within each of five broad effective temperature ranges, it is natural to establish five representative EGP albedo classes: a ``Jovian'' class (T$_{\rm eff} \lesssim 150$ K; Class I) with tropospheric ammonia clouds, a ``water cloud'' class (T$_{\rm eff} \sim 250$ K; Class II) primarily affected by condensed H$_2$O, a ``clear'' class (T$_{\rm eff} \gtrsim 350$ K; Class III) which lacks clouds, and two high-temperature classes: Class IV (900 K $\lesssim$ T$_{\rm{eff}}$ $\lesssim$ 1500 K) for which alkali metal absorption predominates, and Class V (T$_{\rm{eff}}$ $\gtrsim$ 1500 K and/or low surface gravity ($\lesssim$ 10$^3$ cm s$^{-2}$)) for which a high silicate layer shields a significant fraction of the incident radiation from alkali metal and molecular absorption. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III, IV, and V objects. We derive Bond albedos and effective temperatures for the full set of known EGPs and explore the possible effects of non-equilibrium condensed products of photolysis above or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixing ratios.

Constraints on the Evolution of Massive Stars through Spectral Analysis. I. The WC5 Star HD 165763

Abstract: Using a significantly revised non-LTE radiative transfer code that allows for the effects of line blanketing by He, C, O, Si, and Fe, we have performed a detailed analysis of the Galactic Wolf-Rayet (W-R) star HD 165763 (WR 111, WC5) Standard W-R models consistently overestimate the strength of the electron scattering wings, especially on strong lines, so we have considered models where the wind is both homogeneous and clumped The deduced stellar parameters for HD 165763 are as follows: The stellar parameters deduced for HD 165763 are significantly different from earlier analyses The deduced luminosity is a factor of 2 larger, and a smaller core radius is found The smaller radius is in better agreement with expectations from stellar evolution calculations Both of these changes can be attributed to the effects of line blanketing The deduced C/He abundance is similar to earlier calculations, and the O/He abundance had not previously been determined The observed iron spectrum, principally due to Fe V and Fe VI, is well reproduced using a solar iron mass fraction, although a variation of at least a factor of 2 about this value cannot be precluded In particular, the models naturally produce the Fe V emission feature centered on 1470 A and the complex Fe emission/absorption spectrum at shorter wavelengths Also, Fe strongly modifies the line strengths and profile shapes shortward of 1800 A and must be taken into account if we are to successfully model this region The reddening toward HD 165763 does not follow the mean Galactic extinction law We determine the reddening law toward HD 165763 by comparing our model continuum levels to observations In order to simultaneously match the UV, optical, and particularly the infrared fluxes, we used the parameterized reddening law of Cardelli, Clayton, and Mathis with EB-V = 03 and R = 45, where R = AV/EB-V Based on both observational and theoretical suggestions we have considered models in which the wind is still accelerating at large radii In particular, we discuss models in which the velocity law can be characterized by β = 1 for r < 10R* but that undergo a substantial velocity increase (~600 km s-1) beyond 10R* These models appear to give slightly better fits to the line profiles, but the improvements are small, and it is difficult to gauge whether observational data require such a velocity law We cannot yet determine whether the winds of W-R stars are driven by radiation pressure, because we neglect many higher levels of iron in our model ions, and we do not include important elements such as cobalt and nickel However, the wind problem in W-R stars is less severe than previously assumed if clumping occurs For our clumped model, the single scattering limit is only exceeded by a factor of 10 compared to 3 times this value for a homogeneous wind Clumping appears to be the key to explaining the apparent high mass-loss rates determined for W-R stars and is extremely important in understanding how or even whether W-R winds are driven by radiation pressure A reduction in W-R mass-loss rates has important implications for stellar evolution calculations

Aerosol properties and radiative effects in the United States East Coast haze plume: An overview of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX)

Abstract: Aerosol effects on atmospheric radiation are a leading source of uncertainty in predicting climate change. The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) was designed to reduce this uncertainty by measuring and analyzing aerosol properties and effects on the United States eastern seaboard, where one of the world's major plumes of urban/industrial haze moves from the continent over the Atlantic Ocean. The TARFOX intensive field campaign was conducted July 10–31, 1996. It included coordinated measurements from four satellites (GOES-8, NOAA-14, ERS-2, Landsat), four aircraft (ER-2, C-130, C-131A, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean, behind frontal passages, to moderately polluted, with aerosol optical depths exceeding 0.5 at midvisible wavelengths. Gradients of aerosol optical thickness were sampled to aid in separating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals. Early results from TARFOX include demonstration of the unexpected importance of carbonaceous compounds and water condensed on aerosol in the United States East Coast haze plume, chemical apportionment of the aerosol optical depth, measurements of aerosol-induced changes in upwelling and downwelling shortwave radiative fluxes, and generally good agreement between measured flux changes and those calculated from measured aerosol properties. This overview presents the TARFOX objectives, rationale, overall experimental approach, and key initial findings as a guide to the more complete results reported in this special section and elsewhere.

The k-distribution method and correlated-k approximation for a shortwave radiative transfer model.

Abstract: Absorption cross sections are tabulated for water vapor, including continuum absorption, ozone, oxygen and carbon dioxide in the solar spectral region by adopting the k-distribution method. These tables are generated based on line-by-line code results for ranges of total pressure, temperature and water vapor concentration typical of values throughout the troposphere. These tables are incorporated into a shortwave radiative transfer code, which has 32 wavelength intervals across the solar spectrum, by using the correlated-k approximation in order to evaluate the accuracy in the broad band direct normal irradiance computation. A comparison of the direct normal irradiance with MODTRAN3 demonstrates that these tables can be used for shortwave broad band irradiance computations; the difference in the transmissivity is within 0.01 throughout most of the solar spectral region.

Optical transitions of rare earth ions for amplifiers: how the local structure works in glass

Abstract: Several properties of 4f optical transitions of rare earth ions in glasses utilized as optical amplifiers for telecommunications are discussed. For the Er 3+ : 1.55 μm transition, the role of Judd–Ofelt Ω 6 parameters is presented. With compositional change in structure of aluminosilicate glasses, the changes of the radiative cross section, quantum efficiency, and flatness, is observed, which are especially important for the amplifiers in the wavelength-division-multiplexing (WDM) network system. Moreover, the energy level structures and resultant spectral properties of Pr 3+ , Nd 3+ and Dy 3+ ions, all of which are 1.3 μm-active ions, are compared. The hypersensitivity of Dy 3+ transitions appears especially in chalcogenide glasses, where the non-radiative loss due to multiphonon emissions is minimized. Compositional variations of the branching ratio and radiative cross sections, and their dependence on the Ω 2 parameters, which are affected by the asymmetry of the rare earth ion sites, are discussed.

A model for the natural and anthropogenic aerosols over the tropical Indian Ocean derived from Indian Ocean Experiment data

Abstract: The physical, chemical and radiative properties of aerosols are investigated over the tropical Indian Ocean during the first field phase (FFP) of the international Indian Ocean Experiment. The FFP was conducted during February 20 to March 31, 1998. The results shown here are from the Kaashidhoo Climate Observatory (KCO), a new surface observatory established on the tiny island of Kaashidhoo (4.965°N, 73.466°E) in the Republic of Maldives. From simultaneous measurements of aerosol physical, chemical, and radiative properties and the vertical structure from lidar, we have developed an aerosol model which, in conjunction with a Monte Carlo radiative transfer model, successfully explains (within a few percent) the observed solar radiative fluxes at the surface and at the top of the atmosphere. This agreement demonstrates the fundamental importance of measuring aerosol physical and chemical properties for modeling radiative fluxes. KCO, during the northeast monsoon period considered here, is downwind of the Indian subcontinent and undergoes variations in the aerosol visible optical depth τν from ∼0.1 to 0.4, with a monthly mean of ∼0.2. Lidar data suggest that the aerosol is confined largely to the first 3 kms. Sulfate and ammonium contribute ∼29% to τν; sea-salt and nitrate contributes ∼17%; mineral dust contributes ∼15%; and the inferred soot, organics, and fly ash contribute 11%, 20%, and 8% respectively. We estimate that anthropogenic sources may contribute as much as 65% to the observed τν. We consider both an externally and an internally mixed aerosol model with very little difference between the two in the computed radiative forcing. The observed scattering coefficients are in the upper range of those reported for other oceanic regions, the single-scattering albedos are as low as 0.9, and the Angstrom wavelength exponents of ∼1.2 indicate the abundance of submicron aerosols. In summary, the data and the model confirm the large impact of anthropogenic sources. The surface global fluxes (for overhead Sun) decrease by as much as 50 to 80 W m−2 owing to the presence of the aerosols, and the top of the atmosphere fluxes increase by as much as 15 W m−2, thus indicating that anthropogenic aerosols are having a large impact on the tropical Indian Ocean.

The cooling of astrophysical media by H2

Abstract: The results of recent quantum mechanical calculations of cross-sections for rotational transitions within the vibrational ground state of HD are used to evaluate the rate of radiative energy loss from gas containing HD, in addition to H, He and H2. The cooling function for HD (i.e. the rate of cooling per HD molecule) is evaluated in steady state on a grid of values of the relevant parameters of the gas, namely the gas density and temperature, the atomic to molecular hydrogen abundance ratio and the ortho:para-H2 density ratio. The corresponding cooling function for H2, previously computed by Le Bourlot et al., is slightly revised to take account of transitions induced by collisions with ground-state ortho-H2 (J=1). The cooling functions and the data required for their calculation are available from http://ccp7.dur.ac.uk/. We then make a study of the rate of cooling of the primordial gas through collisions with H2 and HD molecules. In this case, radiative transitions induced by the cosmic background radiation field and, in the case of H2, collisional transitions induced by H+ ions should additionally be included.