Showing papers on "Radiative transfer published in 2013"

Large contribution of natural aerosols to uncertainty in indirect forcing

Abstract: The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre-industrial-like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates. The impact of aerosol changes on cloud albedo (called the aerosol first indirect forcing) 1 is estimated 2 to exert a global mean radiative forcing of climate over the industrial period between 20.4 W m 22 and 21.8 W m 22 . Other aerosol–cloud interaction effects, involving rapid adjustments, may be of comparable magnitude 3 but their radiative effects are even less well understood on a global scale 3,4 . The uncertainty in the aerosol forcing is much larger than the uncertainty in the well-constrained positive forcing of 1.7 6 0.2 W m 22 that is due to

Ultrabroadband Photonic Structures To Achieve High-Performance Daytime Radiative Cooling

Abstract: If properly designed, terrestrial structures can passively cool themselves through radiative emission of heat to outer space. For the first time, we present a metal-dielectric photonic structure capable of radiative cooling in daytime outdoor conditions. The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m2 at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions.

Leptonic and Hadronic Modeling of Fermi-detected Blazars

Abstract: We describe new implementations of leptonic and hadronic models for the broadband emission from relativistic jets in active galactic nuclei in a temporary steady state. For the leptonic model, a temporary equilibrium between particle injection/acceleration, radiative cooling, and escape from a spherical emission region is evaluated, and the self-consistent radiative output is calculated. For the hadronic model, a temporary equilibrium between particle injection/acceleration, radiative and adiabatic cooling, and escape is evaluated for both primary electrons and protons. A new, semianalytical method to evaluate the radiative output from cascades initiated by internal γγ pair production is presented. We use our codes to fit snapshot spectral energy distributions (SEDs) of a representative set of Fermi-LAT-detected blazars. We find that the leptonic model provides acceptable fits to the SEDs of almost all blazars with parameters close to equipartition between the magnetic field and the relativistic electron population. However, the hard γ-ray spectrum of AO 0235+164, in contrast to the very steep IR-optical-UV continuum, poses a severe problem for the leptonic model. If charge neutrality in leptonic models is provided by cold protons, the kinetic energy carried by the jet should be dominated by protons. We find satisfactory representations of the snapshot SEDs of most blazars in our sample with the hadronic model presented here. However, in the case of two quasars the characteristic break at a few GeV energies cannot be well modeled. All of our hadronic model fits require powers in relativistic protons in the range Lp ~ 1047-1049 erg s–1.

Global monitoring of terrestrial chlorophyll fluorescence from moderate-spectral-resolution near-infrared satellite measurements: methodology, simulations, and application to GOME-2

Abstract: . Globally mapped terrestrial chlorophyll fluorescence retrievals are of high interest because they can provide information on the functional status of vegetation including light-use efficiency and global primary productivity that can be used for global carbon cycle modeling and agricultural applications. Previous satellite retrievals of fluorescence have relied solely upon the filling-in of solar Fraunhofer lines that are not significantly affected by atmospheric absorption. Although these measurements provide near-global coverage on a monthly basis, they suffer from relatively low precision and sparse spatial sampling. Here, we describe a new methodology to retrieve global far-red fluorescence information; we use hyperspectral data with a simplified radiative transfer model to disentangle the spectral signatures of three basic components: atmospheric absorption, surface reflectance, and fluorescence radiance. An empirically based principal component analysis approach is employed, primarily using cloudy data over ocean, to model and solve for the atmospheric absorption. Through detailed simulations, we demonstrate the feasibility of the approach and show that moderate-spectral-resolution measurements with a relatively high signal-to-noise ratio can be used to retrieve far-red fluorescence information with good precision and accuracy. The method is then applied to data from the Global Ozone Monitoring Instrument 2 (GOME-2). The GOME-2 fluorescence retrievals display similar spatial structure as compared with those from a simpler technique applied to the Greenhouse gases Observing SATellite (GOSAT). GOME-2 enables global mapping of far-red fluorescence with higher precision over smaller spatial and temporal scales than is possible with GOSAT. Near-global coverage is provided within a few days. We are able to show clearly for the first time physically plausible variations in fluorescence over the course of a single month at a spatial resolution of 0.5° × 0.5°. We also show some significant differences between fluorescence and coincident normalized difference vegetation indices (NDVI) retrievals.

Effect of a high opacity on the light curves of radioactively powered transients from compact object mergers

Abstract: The coalescence of compact objects is a promising astrophysical source of detectable gravitational wave signals. The ejection of r-process material from such mergers may lead to a radioactively powered electromagnetic counterpart signal which, if discovered, would enhance the science returns. As very little is known about the optical properties of heavy r-process elements, previous light-curve models have adopted opacities similar to those of iron group elements. Here we consider the effect of heavier elements, particularly the lanthanides, which increase the ejecta opacity by several orders of magnitude. We include these higher opacities in time-dependent, multi-wavelength radiative transport calculations to predict the broadband light curves of one-dimensional models over a range of parameters (ejecta masses ~10–3-10–1 M ☉ and velocities ~0.1-0.3 c). We find that the higher opacities lead to much longer duration light curves which can last a week or more. The emission is shifted toward the infrared bands due to strong optical line blanketing, and the colors at later times are representative of a blackbody near the recombination temperature of the lanthanides (T ~ 2500 K). We further consider the case in which a second mass outflow, composed of 56Ni, is ejected from a disk wind, and show that the net result is a distinctive two component spectral energy distribution, with a bright optical peak due to 56Ni and an infrared peak due to r-process ejecta. We briefly consider the prospects for detection and identification of these transients.

Opacities and Spectra of the $r$-process Ejecta from Neutron Star Mergers

Abstract: Material ejected during (or immediately following) the merger of two neutron stars may assemble into heavy elements through the r-process. The subsequent radioactive decay of the nuclei can power transient electromagnetic emission similar to, but significantly dimmer than, an ordinary supernova. Identifying such events is an important goal of future optical surveys, offering new perspectives on the origin of r-process nuclei and the astrophysical sources of gravitational waves. Predictions of the transient light curves and spectra, however, have suffered from the uncertain optical properties of heavy ions. Here we argue that the opacity of an expanding r-process material is dominated by bound-bound transitions from those ions with the most complex valence electron structure, namely the lanthanides. For a few representative ions, we run atomic structure models to calculate the radiative transition rates for tens of millions of lines. The resulting r-process opacities are orders of magnitude larger than that of ordinary (e.g., iron-rich) supernova ejecta. Radiative transport calculations using these new opacities suggest that the light curves should be longer, dimmer, and redder than previously thought. The spectra appear to be pseudo-blackbody, with broad absorption features, and peak in the infrared (~1 μm). We discuss uncertainties in the opacities and attempt to quantify their impact on the spectral predictions. The results have important implications for observational strategies to find and study the radioactively powered electromagnetic counterparts to neutron star mergers.

Radiative transfer simulations of neutron star merger ejecta

Abstract: Mergers of binary neutron stars (NSs) are among the most promising gravitational wave (GW) sources. Next generation GW detectors are expected to detect signals from NS mergers within about 200 Mpc. The detection of electromagnetic wave (EM) counterparts is crucial to understanding the nature of GW sources. Among the possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, predictions so far have not taken into account detailed r-process element abundances in the ejecta. We perform for the first time radiative transfer simulations of the NS merger ejecta including all the r-process elements from Ga to U. We show that the opacity of the NS merger ejecta is about κ = 10 cm2 g–1, which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ~100. As a result, the emission is fainter and lasts longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of the two NSs and a softer equation of state adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, the expected brightness of the emission is i = 22-25 AB mag, z = 21-23 AB mag, and 21-24 AB mag in the NIR JHK bands. Thus, observations with wide-field 4 m- and 8 m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.

Radiative Transfer Simulations for Neutron Star Merger Ejecta

Abstract: The merger of binary neutron stars (NSs) is among the most promising gravitational wave (GW) sources. Next-generation GW detectors are expected to detect signals from the NS merger within 200 Mpc. Detection of electromagnetic wave (EM) counterpart is crucial to understand the nature of GW sources. Among possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, prediction so far does not take into account detailed r-process element abundances in the ejecta. We perform radiative transfer simulations for the NS merger ejecta including all the r-process elements from Ga to U for the first time. We show that the opacity in the NS merger ejecta is about kappa = 10 cm^2 g^{-1}, which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ~ 100. As a result, the emission is fainter and longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of two NSs and a softer equation of states adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, expected brightness of the emission is i = 22 - 25 AB mag, z = 21 - 23 AB mag, and 21 - 24 AB mag in NIR JHK bands. Thus, observations with wide-field 4m- and 8m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.

Leptonic and Hadronic Modeling of Fermi-Detected Blazars

Abstract: We describe new implementations of leptonic and hadronic models for the broadband emission from relativistic jets in AGN in a temporary steady state. For the leptonic model, a temporary equilibrium between particle injection/acceleration, radiative cooling, and escape from a spherical emission region is evaluated, and the self-consistent radiative output is calculated. For the hadronic model, a temporary equilibrium between particle injection/acceleration, radiative and adiabatic cooling, and escape is evaluated for both primary electrons and protons. A new, semi-analytical method to evaluate the radiative output from cascades initiated by internal gamma-gamma pair production is presented. We use our codes to fit snap-shot spectral energy distributions of a representative set of Fermi-LAT detected blazars. We find that the leptonic model provides acceptable fits to the SEDs of almost all blazars with parameters close to equipartition between the magnetic field and the relativistic electron population. However, the hard gamma-ray spectrum of AO 0235+164, in contrast to the very steep IR-optical-UV continuum, poses a severe problem for the leptonic model. If charge neutrality in leptonic models is provided by cold protons, the kinetic energy carried by the jet should be dominated by protons. We find satisfactory representations of the snapshot SEDs of most blazars in our sample with the hadronic model presented here. However, in the case of two quasars the characteristic break at a few GeV energies can not be well modelled. All of our hadronic model fits require powers in relativistic protons in the range L_p ~ 1e47 - 1e49 erg/s.

On the evolution of the H i column density distribution in cosmological simulations

Abstract: We use a set of cosmological simulations combined with radiative transfer calculations to investigate the distribution of neutral hydrogen in the post-reionization Universe. We assess the contributions from the metagalactic ionizing background, collisional ionization and diffuse recombination radiation to the total ionization rate at redshifts z = 0 5. We find that the densities above which hydrogen self-shielding becomes important are consistent with analytic calculations and previous work. However, because of diffuse recombination radiation, whose intensity peaks at the same density, the transition between highly ionized and self-shielded regions is smoother than what is usually assumed. We provide fitting functions to the simulated photoionization rate as a function of density and show that post-processing simulations with the fitted rates yields results that are in excellent agreement with the original radiative transfer calculations. The predicted neutral hydrogen column density distributions agree very well with the observations. In particular, the simulations reproduce the remarkable lack of evolution in the column density distribution of Lyman limit and weak damped Lyα systems below z = 3. The evolution of the low column density end is affected by the increasing importance of collisional ionization with decreasing redshift. On the other hand, the simulations predict the abundance of strong damped Lyα systems to broadly track the cosmic star formation rate density.

The global energy balance from a surface perspective

Abstract: In the framework of the global energy balance, the radiative energy exchanges between Sun, Earth and space are now accurately quantified from new satellite missions. Much less is known about the magnitude of the energy flows within the climate system and at the Earth surface, which cannot be directly measured by satellites. In addition to satellite observations, here we make extensive use of the growing number of surface observations to constrain the global energy balance not only from space, but also from the surface. We combine these observations with the latest modeling efforts performed for the 5th IPCC assessment report to infer best estimates for the global mean surface radiative components. Our analyses favor global mean downward surface solar and thermal radiation values near 185 and 342 Wm−2, respectively, which are most compatible with surface observations. Combined with an estimated surface absorbed solar radiation and thermal emission of 161 and 397 Wm−2, respectively, this leaves 106 Wm−2 of surface net radiation available globally for distribution amongst the non-radiative surface energy balance components. The climate models overestimate the downward solar and underestimate the downward thermal radiation, thereby simulating nevertheless an adequate global mean surface net radiation by error compensation. This also suggests that, globally, the simulated surface sensible and latent heat fluxes, around 20 and 85 Wm−2 on average, state realistic values. The findings of this study are compiled into a new global energy balance diagram, which may be able to reconcile currently disputed inconsistencies between energy and water cycle estimates.

X-ray reflected spectra from accretion disk models. iii. a complete grid of ionized reflection calculations

Abstract: We present a new and complete library of synthetic spectra for modeling the component of emission that is reflected from an illuminated accretion disk. The spectra were computed using an updated version of our code xillver that incorporates new routines and a richer atomic data base. We offer in the form of a table model an extensive grid of reflection models that cover a wide range of parameters. Each individual model is characterized by the photon index Gamma of the illuminating radiation, the ionization parameter zeta at the surface of the disk (i.e., the ratio of the X-ray flux to the gas density), and the iron abundance A(sub Fe) relative to the solar value. The ranges of the parameters covered are: 1.2 <= Gamma <= 3.4, 1 <= zeta <= 104, and 0.5 <= A(sub Fe) <= 10. These ranges capture the physical conditions typically inferred from observations of active galactic nuclei, and also stellar-mass black holes in the hard state. This library is intended for use when the thermal disk flux is faint compared to the incident power-law flux. The models are expected to provide an accurate description of the Fe K emission line, which is the crucial spectral feature used to measure black hole spin. A total of 720 reflection spectra are provided in a single FITS file suitable for the analysis of X-ray observations via the atable model in xspec. Detailed comparisons with previous reflection models illustrate the improvements incorporated in this version of xillver.

Inverse Radiative Heat Transfer

Abstract: Up to this point we have concerned ourselves with radiative heat transfer problems, where the necessary geometry, temperatures, and radiative properties are known, enabling us to calculate the radiative intensity and radiative heat fluxes in such enclosures. Such cases are sometimes called “direct” heat transfer problems. However, there are many important engineering applications where knowledge of one or more input parameters is desired that cause a certain radiative intensity field. For example, it may be desired to control the temperatures of heating elements in a furnace, in order to achieve a specified temperature distribution or radiative heat load on an object being heated. Or the aim may be to deduce difficult to measure parameters (such as radiative properties, temperature fields inside a furnace, etc.) based on measurements of radiative intensity or radiative flux. Such calculations are known as inverse heat transfer analyses.

The Canadian Fourth Generation Atmospheric Global Climate Model (CanAM4). Part I: Representation of Physical Processes

Abstract: The Canadian Centre for Climate Modelling and Analysis (CCCma) has developed the fourth generation of the Canadian Atmospheric Global Climate Model (CanAM4). The new model includes substantially modified physical parameterizations compared to its predecessor. In particular, the treatment of clouds, cloud radiative effects, and precipitation has been modified. Aerosol direct and indirect effects are calculated based on a bulk aerosol scheme. Simulation results for present-day global climate are analyzed, with a focus on cloud radiative effects and precipitation. Good overall agreement is found between climatological mean short- and longwave cloud radiative effects and observations from the Clouds and Earth's Radiant Energy System (CERES) experiment. An analysis of the responses of cloud radiative effects to variations in climate will be presented in a companion paper. [Traduit par la redaction] Le Centre canadien de la modelisation et de l'analyse climatique (CCmaC) a mis au point la quatrieme generation d...

Vertical Photon Transport in Cloud Remote Sensing Problems

Abstract: Photon transport in plane-parallel, vertically inhomogeneous clouds is investigated and applied to cloud remote sensing techniques that use solar reflectance or transmittance measurements for retrieving droplet effective radius. Transport is couched in terms of weighting functions which approximate the relative contribution of individual layers to the overall retrieval. Two vertical weightings are investigated, including one based on the average number of scatterings encountered by reflected and transmitted photons in any given layer. A simpler vertical weighting, based on the maximum penetration of reflected photons, proves useful for solar reflectance measurements. These weighting functions are highly dependent on droplet absorption and solar/viewing geometry. A superposition technique, using adding/doubling radiative transfer procedures, is used to accurately determine both weightings, avoiding time-consuming Monte Carlo methods. Effective radius retrievals from modeled vertically structured liquid water clouds are then made using the standard near-infrared bands and compared with size estimates based on the proposed weighting functions. Agreement between the two methods is generally within several tenths of a micrometer, much better than expected retrieval accuracy. Though the emphasis is on photon transport in clouds, the derived weightings can be applied to any multiple-scattering plane-parallel radiative transfer problem, including arbitrary combinations of cloud, aerosol, and gas layers.

Seismic diagnostics for transport of angular momentum in stars. I. Rotational splittings from the pre-main sequence to the red-giant branch.

Abstract: Context. Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Aims: Our aim is to obtain seismic constraints on the internal transport and surface loss of the angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. Methods: We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of 1.3 Ms models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. Results: We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Conclusions: Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.

An isolated line-shape model to go beyond the Voigt profile in spectroscopic databases and radiative transfer codes

Abstract: We demonstrate that a previously proposed model opens the route for the inclusion of refined non-Voigt profiles in spectroscopic databases and atmospheric radiative transfer codes. Indeed, this model fulfills many essential requirements: (i) it takes both velocity changes and the speed dependences of the pressure-broadening and -shifting coefficients into account. (ii) It leads to accurate descriptions of the line shapes of very different molecular systems. Tests made for pure H2, CO2 and O2 and for H2O diluted in N2 show that residuals are down to ≃ 0.2 % of the peak absorption, (except for the untypical system of H2 where a maximum residual of ±3% is reached), thus fulfilling the precision requirements of the most demanding remote sensing experiments. (iii) It is based on a limited set of parameters for each absorption line that have known dependences on pressure and can thus be stored in databases. (iv) Its calculation requires very reasonable computer costs, only a few times higher than that of a usual Voigt profile. Its inclusion in radiative transfer codes will thus induce bearable CPU time increases. (v) It can be extended in order to take line-mixing effects into account, at least within the so-called first-order approximation.

The Stagger-grid: A grid of 3D stellar atmosphere models - I. Methods and general properties

Abstract: Aims. We present the Stagger-grid, a comprehensive grid of time-dependent, three-dimensional (3D), hydrodynamic model atmospheres for late-type stars with realistic treatment of radiative transfer, covering a wide range in stellar parameters. This grid of 3D models is intended for various applications besides studies of stellar convection and atmospheres per se, including stellar parameter determination, stellar spectroscopy and abundance analysis, asteroseismology, calibration of stellar evolution models, interferometry, and extrasolar planet search. In this introductory paper, we describe the methods we applied for the computation of the grid and discuss the general properties of the 3D models as well as of their temporal and spatial averages (here denoted ⟨3D⟩ models).Methods. All our models were generated with the Stagger-code, using realistic input physics for the equation of state (EOS) and for continuous and line opacities. Our ~ 220 grid models range in effective temperature, T eff , from 4000 to 7000 K in steps of 500 K, in surface gravity, log g , from 1.5 to 5.0 in steps of 0.5 dex, and metallicity, [Fe/H], from − 4.0 to + 0.5 in steps of 0.5 and 1.0 dex.Results. We find a tight scaling relation between the vertical velocity and the surface entropy jump, which itself correlates with the constant entropy value of the adiabatic convection zone. The range in intensity contrast is enhanced at lower metallicity. The granule size correlates closely with the pressure scale height sampled at the depth of maximum velocity. We compare the ⟨3D⟩ models with currently widely applied one-dimensional (1D) atmosphere models, as well as with theoretical 1D hydrostatic models generated with the same EOS and opacity tables as the 3D models, in order to isolate the effects of using self-consistent and hydrodynamic modeling of convection, rather than the classical mixing length theory approach. For the first time, we are able to quantify systematically over a broad range of stellar parameters the uncertainties of 1D models arising from the simplified treatment of physics, in particular convective energy transport. In agreement with previous findings, we find that the differences can be rather significant, especially for metal-poor stars.

RAMSES-RT: Radiation hydrodynamics in the cosmological context

Abstract: We present a new implementation of radiation hydrodynamics (RHD) in the adaptive mesh refinement (AMR) code RAMSES. The multi-group radiative transfer (RT) is performed on the AMR grid with a first-order Godunov method using the M1 closure for the Eddington tensor, and is coupled to the hydrodynamics via non-equilibrium thermochemistry of hydrogen and helium. This moment-based approach has the large advantage that the computational cost is independent of the number of radiative sources - it can even deal with continuous regions of emission such as bound-free emission from gas. As it is built directly into RAMSES, the RT takes natural advantage of the refinement and parallelization strategies already in place. Since we use an explicit advection solver for the radiative transport, the time step is restricted by the speed of light - a severe limitation that can be alleviated using the so--called "reduced speed of light" approximation. We propose a rigorous framework to assess the validity of this approximation in various conditions encountered in cosmology and galaxy formation. We finally perform with our newly developed code a complete suite of RHD tests, comparing our results to other RHD codes. The tests demonstrate that our code performs very well and is ideally suited for exploring the effect of radiation on current scenarios of structure and galaxy formation.

Thermal Hyperbolic Metamaterials

Abstract: We explore the near-field radiative thermal energy transfer properties of hyperbolic metamaterials. The presence of unique electromagnetic states in a broad bandwidth leads to super-planckian thermal energy transfer between metamaterials separated by a nano-gap. We consider practical phonon-polaritonic metamaterials for thermal engineering in the mid-infrared range and show that the effect exists in spite of the losses, absorption and finite unit cell size. For thermophotovoltaic energy conversion applications requiring energy transfer in the near-infrared range we introduce high temperature hyperbolic metamaterials based on plasmonic materials with a high melting point. Our work paves the way for practical high temperature radiative thermal energy transfer applications of hyperbolic metamaterials.

Impact of uncertainties in atmospheric mixing on simulated UTLS composition and related radiative effects

Abstract: [1] The upper troposphere/lower stratosphere (UTLS) region plays an important role in the climate system. Changes in the structure and chemical composition of this region result in particularly large changes in radiative forcings of the atmosphere. Quantifying the processes that control UTLS composition (e.g., stratosphere-troposphere exchange) therefore represents a crucial task. We assess the influence of uncertainties in the atmospheric mixing strength on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multiannual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. Mixing, the irreversible part of transport, is controlled by the local horizontal strain and vertical shear of the atmospheric flow. We find that simulated radiative effects of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m2for water vapor and ozone, respectively. For ozone, the largest impact of mixing uncertainties is observed in the extra-tropical lower stratosphere.

Unveiling the gas-and-dust disk structure in HD 163296 using ALMA observations

Abstract: Aims. The aim of this work is to study the structure of the protoplan etary disk surrounding the Herbig Ae star HD 163296. Methods. We have used high-resolution and high-sensitivity ALMA observations of the CO(3‐2) emission line and the continuum at 850µm, as well as the 3- dimensional radiative transfer code MCFOST to model the data presented in this work. Results. The CO(3‐2) emission unveils for the first time at sub-millim eter frequencies the vertical structure details of a gaseou s disk in Keplerian rotation, showing the back- and the front-side of a flared disk. Continuum emission at 850 µm reveals a compact dust disk with a 240 AU outer radius and a surface brightness profil e that shows a very steep decline at radius larger than 125 AU. The gaseous disk is more than two times larger than the dust disk, with a similar critical radius but with a shallower radial pr ofile. Radiative transfer models of the continuum data confirms the need for a s harp outer edge to the dust disk. The models for the CO(3‐2) channel map require the disk to be slightly more geometrically thick than previous models suggested, and that the temperature at which CO gas becomes depleted (frozen-out) from the outer regions of the disk midplane is T < 20 K, in agreement with previous studies.

Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals

Abstract: CdSe–CdS nanocrystals with a 100% quantum yield and completely quenched Auger processes at low temperatures are demonstrated.

Unveiling the gas and dust disk structure in HD 163296 using ALMA observations

Abstract: Aims: The aim of this work is to study the structure of the protoplanetary disk surrounding the Herbig Ae star HD 163296. Methods: We have used high-resolution and high-sensitivity ALMA observations of the CO(3-2) emission line and the continuum at 850 microns, as well as the 3- dimensional radiative transfer code MCFOST to model the data presented in this work. Results: The CO(3-2) emission unveils for the first time at sub-millimeter frequencies the vertical structure details of a gaseous disk in Keplerian rotation, showing the back- and the front-side of a flared disk. Continuum emission at 850 microns reveals a compact dust disk with a 240 AU outer radius and a surface brightness profile that shows a very steep decline at radius larger than 125 AU. The gaseous disk is more than two times larger than the dust disk, with a similar critical radius but with a shallower radial profile. Radiative transfer models of the continuum data confirms the need for a sharp outer edge to the dust disk. The models for the CO(3-2) channel map require the disk to be slightly more geometrically thick than previous models suggested, and that the temperature at which CO gas becomes depleted (frozen-out) from the outer regions of the disk midplane is T < 20 K, in agreement with previous studies.

Phase-change radiative thermal diode

Abstract: A thermal diode transports heat mainly in one preferential direction rather than in the opposite direction. This behavior is generally due to the non-linear dependence of certain physical properties with respect to the temperature. Here we introduce a radiative thermal diode which rectifies heat transport thanks to the phase transitions of materials. Rectification coefficients greater than 70% and up to 90% are shown, even for small temperature differences. This result could have important applications in the development of future contactless thermal circuits or in the conception of radiative coatings for thermal management.

The Fragmentation of Magnetized, Massive Star-forming Cores with Radiative Feedback

Abstract: We present a set of three-dimensional, radiation-magnetohydrodynamic calculations of the gravitational collapse of massive (300 M ☉), star-forming molecular cloud cores. We show that the combined effects of magnetic fields and radiative feedback strongly suppress core fragmentation, leading to the production of single-star systems rather than small clusters. We find that the two processes are efficient at suppressing fragmentation in different regimes, with the feedback most effective in the dense, central region and the magnetic field most effective in more diffuse, outer regions. Thus, the combination of the two is much more effective at suppressing fragmentation than either one considered in isolation. Our work suggests that typical massive cores, which have mass-to-flux ratios of about 2 relative to critical, likely form a single-star system, but that cores with weaker fields may form a small star cluster. This result helps us understand why the observed relationship between the core mass function and the stellar initial mass function holds even for ~100 M ☉ cores with many thermal Jeans masses of material. We also demonstrate that a ~40 AU Keplerian disk is able to form in our simulations, despite the braking effect caused by the strong magnetic field.

The Formation of IRIS Diagnostics. I. A Quintessential Model Atom of Mg II and General Formation Properties of the Mg II h&k Lines

Abstract: NASA's Interface Region Imaging Spectrograph (IRIS) space mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h and k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations will require forward modeling of Mg II h and k line formation from three-dimensional (3D) radiation-MHD models. This paper is the first in a series where we undertake this forward modeling. We discuss the atomic physics pertinent to h and k line formation, present a quintessential model atom that can be used in radiative transfer computations, and discuss the effect of partial redistribution (PRD) and 3D radiative transfer on the emergent line profiles. We conclude that Mg II h and k can be modeled accurately with a four-level plus continuum Mg II model atom. Ideally radiative transfer computations should be done in 3D including PRD effects. In practice this is currently not possible. A reasonable compromise is to use one-dimensional PRD computations to model the line profile up to and including the central emission peaks, and use 3D transfer assuming complete redistribution to model the central depression.

Semi-implicit scheme for treating radiation under M1 closure in general relativistic conservative fluid dynamics codes

Abstract: A numerical scheme is described for including radiation in multi-dimensional generalrelativistic conservative fluid dynamics codes. In this met hod, a covariant form of the M1 closure scheme is used to close the radiation moments, and the radiative source terms are treated semi-implicitly in order to handle both optically t hin and optically thick regimes. The scheme has been implemented in a conservative general relativistic radiation hydrodynamics codeKORAL. The robustness of the code is demonstrated on a number of test problems, including radiative relativistic shock tubes, static radiation p ressure supported atmosphere, shadows, beams of light in curved spacetime, and radiative Bondi accretion. The advantages of M1 closure relative to other approaches such as Eddington closure and flux-limited di ffusion are discussed, and its limitations are also highlighted.

Three-Dimensional Dust Radiative Transfer ∗

Abstract: Cosmic dust is present in many astrophysical objects, and recent observations across the electromagnetic spectrum show that the dust distribution is often strongly three-dimensional (3D). Dust grains are effective in absorbing and scattering ultraviolet (UV)/optical radiation, and they re-emit the absorbed energy at infrared wavelengths. Understanding the intrinsic properties of these objects, including the dust itself, therefore requires 3D dust radiative transfer (RT) calculations. Unfortunately, the 3D dust RT problem is nonlocal and nonlinear, which makes it one of the hardest challenges in computational astrophysics. Nevertheless, significant progress has been made in the past decade, with an increasing number of codes capable of dealing with the complete 3D dust RT problem. We discuss the complexity of this problem, the two most successful solution techniques [ray-tracing (RayT) and Monte Carlo (MC)], and the state of the art in modeling observational data using 3D dust RT codes. We end with an outlook on the bright future of this field.

Stellar irradiated discs and implications on migration of embedded planets. I. Equilibrium discs

Abstract: Context. The strength and direction of migration of embedded low mass planets depends on the disc’s thermodynamic state. It has been shown that, in discs where the viscous heating is balanced by radiative transport, the migration can be directed outwards, a process which extends the lifetime of growing planetary embryos. Aims. We investigate the influence of opacity and stellar irradiation on the disc thermodynamics. We focus on equilibrium discs, which have no net mass flux. Utilizing the resulting disc structure, we determine the regions of outward migration in the disc. Methods. We performed two-dimensional numerical simulations of equilibrium discs with viscous heating, radiative cooling, and stellar irradiation. We used the explicit/implicit hydrodynamical code NIRVANA that includes a full tensor viscosity and stellar irradiation, as well as a two temperature solver that includes radiation transport in the flux-limited diffusion approximation. The migration of embedded planets was studied by using torque formulae. Results. In the constant opacity case, our code reproduces the analytical results corresponding to a black-body disc: the stellar irradiation dominates in the outer regions – leading to flaring (H /r ∝ r 2/7 ) – while the viscous heating dominates close to the star. In particular, we find that the inner edge of the disc should not be significantly puffed-up by the stellar irradiation. If the opacity depends on the local density and temperature, the structure of the disc is different, and several bumps in the aspect ratio H /r appear, due to transitions between different opacity regimes. The bumps in the disc structure are very important, as they can shield the outer disc from stellar irradiation.Conclusions. Stellar irradiation is an important factor for determining the disc structure and has dramatic consequences for the migration of embedded planets. Compared to discs with only viscous heating and radiative cooling, a stellar irradiated disc features a much smaller region of outward migration for a range of planetary masses. This suggests that the region where the formation of giant planet cores takes place is smaller, which in turn might lead to a shorter growth phase.