Showing papers on "Radiative transfer published in 2010"

On the radiative equilibrium of irradiated planetary atmospheres

Abstract: The evolution of stars and planets is mostly controlled by the properties of their atmosphere. This is particularly true in the case of exoplanets close to their stars, for which one has to account both for an (often intense) irradiation flux, and from an intrinsic flux responsible for the progressive loss of the inner planetary heat. The goals of the present work are to help understanding the coupling between radiative transfer and advection in exoplanetary atmospheres and to provide constraints on the temperatures of the deep atmospheres. This is crucial in assessing whether modifying assumed opacity sources and/or heat transport may explain the inflated sizes of a significant number of giant exoplanets found so far. I use a simple analytical approach inspired by Eddington's approximation for stellar atmospheres to derive a relation between temperature and optical depth valid for plane-parallel static grey atmospheres which are both transporting an intrinsic heat flux and receiving an outer radiation flux. The model is parameterized as a function of mean visible and thermal opacities, respectively. The model is shown to reproduce relatively well temperature profiles obtained from more sophisticated radiative transfer calculations of exoplanetary atmospheres. It naturally explains why a temperature inversion (stratosphere) appears when the opacity in the optical becomes significant compared to that in the infrared. I further show that the mean equivalent flux (proportional to T4) is conserved in the presence of horizontal advection on constant optical depth levels. This implies with these hypotheses that the deep atmospheric temperature used as outer boundary for the evolution models should be calculated from models pertaining to the entire planetary atmosphere, not from ones that are relevant to the day side or to the substellar point. In these conditions, present-day models yield deep temperatures that are ~1000K too cold to explain the present size of planet HD 209458b. A tenfold increase in the infrared to visible opacity ratio would be required to slow the planetary cooling and contraction sufficiently to explain its size. However, the mean equivalent flux is not conserved anymore in the presence of opacity variations, or in the case of non-radiative vertical transport of energy: The presence of clouds on the night side or a downward transport of kinetic energy and its dissipation at deep levels would help making the deep atmosphere hotter and may explain the inflated sizes of giant exoplanets.

Physics of Solar Prominences: I-Spectral Diagnostics and Non-LTE Modelling

Abstract: This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (i.e. when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.

Collisional mechanism for gamma‐ray burst emission

Abstract: Nuclear and Coulomb collisions in gamma-ray burst (GRB) jets create a hot e ± plasma. This collisional heating starts when the jet is still opaque, and extends to the transparent region. The e ± plasma radiates its energy. As a result, a large fraction of the jet energy is converted to escaping radiation with a well-defined spectrum. The process is simulated in detail using the known rates of collisions and accurate calculations of radiative transfer in the expanding jet. The result reproduces the spectra of observed GRBs that typically peak near 1 MeV and extend to much higher energies with a photon index β ~ -2.5. This suggests that collisional heating may be the main mechanism for GRB emission.

Seeing the collision of a supernova with its companion star

Abstract: The progenitors of Type Ia and some core collapse supernovae are thought to be stars in binary systems, but little direct observational evidence exists to confirm the hypothesis. We show that the collision of the supernova ejecta with its companion star should produce detectable emission in the hours and days following the explosion. The interaction occurs at distances ~1011-1013 cm and shocks the impacting supernova debris, dissipating kinetic energy and re-heating the gas. Initially, some radiation may escape promptly through the evacuated region of the shadowcone, producing a bright X-ray (0.1-2 keV) burst lasting minutes to hours with luminosity L ~ 1044 ergs s–1. Continuing radiative diffusion from deeper layers of shock-heated ejecta produces a longer lasting optical/UV emission, which exceeds the radioactively powered luminosity of the supernova for the first few days after the explosion. These signatures are prominent for viewing angles looking down upon the shocked region, or about 10% of the time. The properties of the emission provide a straightforward measure of the separation distance between the stars and hence (assuming Roche lobe overflow) the companion's radius. Current optical and UV data sets likely already constrain red giant companions. By systematically acquiring early time data for many supernovae, it may eventually be possible to empirically determine how the parameters of the progenitor system influence the outcome of the explosion.

Theory of excited state decays and optical spectra: application to polyatomic molecules.

Abstract: General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg−Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Qx spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S1 → S0 and S0 → S1 of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by ∼10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promo...

X-ray reflected spectra from accretion disk models. I. Constant density atmospheres

Abstract: We present new models for illuminated accretion disks, their structure, and reprocessed emission. We consider the effects of incident X-rays on the surface of an accretion disk by simultaneously solving the equations of radiative transfer, energy balance, and ionization equilibrium over a large range of column densities. We assume plane-parallel geometry and azimuthal symmetry, such that each calculation corresponds to a ring at a given distance from the central object. Our models include recent and complete atomic data for K-shell processes of the iron and oxygen isonuclear sequences. We examine the effect on the spectrum of fluorescent Kα line emission and absorption in the emitted spectrum. We also explore the dependence of the spectrum on the strength of the incident X-rays and other input parameters, and discuss the importance of Comptonization on the emitted spectrum.

LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths

Abstract: We present a new code for solving the molecular and atomic excitation and radiation transfer problem in a molecular gas and predicting emergent spectra. This code works in arbitrary three dimensional geometry using unstructured Delaunay latices for the transport of photons. Various physical models can be used as input, ranging from analytical descriptions over tabulated models to SPH simulations. To generate the Delaunay grid we sample the input model randomly, but weigh the sample probability with the molecular density and other parameters, and thereby we obtain an average grid point separation that scales with the local opacity. Our code does photon very efficiently so that the slow convergence of opaque models becomes traceable. When convergence between the level populations, the radiation field, and the point separation has been obtained, the grid is ray-traced to produced images that can readily be compared to observations. Because of the high dynamic range in scales that can be resolved using this type of grid, our code is particularly well suited for modeling of ALMA data. Our code can furthermore deal with overlapping lines of multiple molecular and atomic species.

On the radiative equilibrium of irradiated planetary atmospheres

Abstract: The evolution of stars and planets is mostly controlled by the properties of their atmosphere. This is particularly true in the case of exoplanets close to their stars, for which one has to account both for an (often intense) irradiation flux, and from an intrinsic flux responsible for the progressive loss of the inner planetary heat. The goals of the present work are to help understanding the coupling between radiative transfer and advection in exoplanetary atmospheres and to provide constraints on the temperatures of the deep atmospheres. This is crucial in assessing whether modifying assumed opacity sources and/or heat transport may explain the inflated sizes of a significant number of giant exoplanets found so far. I use a simple analytical approach inspired by Eddington's approximation for stellar atmospheres to derive a relation between temperature and optical depth valid for plane-parallel static grey atmospheres which are both transporting an intrinsic heat flux and receiving an outer radiation flux. The model is parameterized as a function of mean visible and thermal opacities, respectively. The model is shown to reproduce relatively well temperature profiles obtained from more sophisticated radiative transfer calculations of exoplanetary atmospheres. It naturally explains why a temperature inversion (stratosphere) appears when the opacity in the optical becomes significant compared to that in the infrared. I further show that the mean equivalent flux (proportional to T^4) is conserved in the presence of horizontal advection on constant optical depth levels. This implies with these hypotheses that the deep atmospheric temperature used as outer boundary for the evolution models should be calculated from models pertaining to the entire planetary atmosphere, not from ones that are relevant to the day side or to the substellar point. In these conditions, present-day models yield deep temperatures that are ~1000K too cold to explain the present size of planet HD 209458b. An tenfold increase in the infrared to visible opacity ratio would be required to slow the planetary cooling and contraction sufficiently to explain its size. However, the mean equivalent flux is not conserved anymore in the presence of opacity variations, or in the case of non-radiative vertical transport of energy: The presence of clouds on the night side or a downward transport of kinetic energy and its dissipation at deep levels would help making the deep atmosphere hotter and may explain the inflated sizes of giant exoplanets.

Precipitation, radiative forcing and global temperature change

Abstract: Radiative forcing is a useful tool for predicting equilibrium global temperature change. However, it is not so useful for predicting global precipitation changes, as changes in precipitation strongly depend on the climate change mechanism and how it perturbs the atmospheric and surface energy budgets. Here a suite of climate model experiments and radiative transfer calculations are used to quantify and assess this dependency across a range of climate change mechanisms. It is shown that the precipitation response can be split into two parts: a fast atmospheric response that strongly correlates with the atmospheric component of radiative forcing, and a slower response to global surface temperature change that is independent of the climate change mechanism, ∼2-3% per unit of global surface temperature change. We highlight the precipitation response to black carbon aerosol forcing as falling within this range despite having an equilibrium response that is of opposite sign to the radiative forcing and global temperature change.

The spatial distribution of mineral dust and its shortwave radiative forcing over North Africa: modeling sensitivities to dust emissions and aerosol size treatments

Abstract: . A fully coupled meteorology-chemistry-aerosol model (WRF-Chem) is applied to simulate mineral dust and its shortwave (SW) radiative forcing over North Africa. Two dust emission schemes (GOCART and DUSTRAN) and two aerosol models (MADE/SORGAM and MOSAIC) are adopted in simulations to investigate the modeling sensitivities to dust emissions and aerosol size treatments. The modeled size distribution and spatial variability of mineral dust and its radiative properties are evaluated using measurements (ground-based, aircraft, and satellites) during the AMMA SOP0 campaign from 6 January to 3 February of 2006 (the SOP0 period) over North Africa. Two dust emission schemes generally simulate similar spatial distributions and temporal evolutions of dust emissions. Simulations using the GOCART scheme with different initial (emitted) dust size distributions require ~40% difference in total emitted dust mass to produce similar SW radiative forcing of dust over the Sahel region. The modal approach of MADE/SORGAM retains 25% more fine dust particles (radius 1.25 μm) than the sectional approach of MOSAIC in simulations using the same size-resolved dust emissions. Consequently, MADE/SORGAM simulates 11% higher AOD, up to 13% lower SW dust heating rate, and 15% larger (more negative) SW dust radiative forcing at the surface than MOSAIC over the Sahel region. In the daytime of the SOP0 period, the model simulations show that the mineral dust heats the lower atmosphere with an average rate of 0.8 ± 0.5 K day−1 over the Niamey vicinity and 0.5 ± 0.2 K day−1 over North Africa and reduces the downwelling SW radiation at the surface by up to 58 W m−2 with an average of 22 W m−2 over North Africa. This highlights the importance of including dust radiative impact in understanding the regional climate of North Africa. When compared to the available measurements, the WRF-Chem simulations can generally capture the measured features of mineral dust and its radiative properties over North Africa, suggesting that the model is suitable for more extensive simulations of dust impact on regional climate over North Africa.

Lyman-alpha Cooling Emission from Galaxy Formation

Abstract: Recent studies have shown that galaxies accrete most of their baryons via the cold mode, from streams with temperatures T~10^4-10^5 K. At these temperatures, the streams should radiate primarily in the Lya line and have therefore been proposed as a model to power the extended, high-redshift objects known as Lya blobs and other high-redshift Lya sources. We introduce a new Lya radiative transfer code, aRT, and apply it to cosmological hydrodynamical simulations. We address physical and numerical issues that are critical to making accurate predictions for the cooling luminosity, but that have been mostly neglected or treated simplistically so far. We highlight the importance of self-shielding and of properly treating sub-resolution models in simulations. Most existing simulations do not self-consistently incorporate these effects, which can lead to order-of-magnitude errors in the predicted cooling luminosity. Using a combination of post-processing ionizing radiative transfer and re-simulation techniques, we develop an approximation to the consistent evolution of the self-shielded gas. We quantify the dependence of the Lya cooling luminosity on halo mass at z=3 for the simplified problem of pure gas accretion. While cooling in massive halos (without additional energy input from star formation and AGN) is in principle sufficient to produce L_alpha~10^43-10^44 erg s^-1 blobs, this requires including energy released in gas of density sufficient to form stars, but which is kept 100% gaseous in our optimistic estimates. Excluding emission from such dense gas yields lower luminosities by up to one to two orders of magnitude at high masses, making it difficult to explain the observed Lya blobs with pure cooling. Resonant scattering produces diffuse Lya halos, even for centrally concentrated emission, and broad double peaked line profiles. [Abridged]

Generation of molecular hot electroluminescence by resonant nanocavity plasmons

Abstract: Control of the radiative properties of functional molecules near metals is a key issue in nano-optics, and is particularly important in the fields of energy transfer and light manipulation at the nanoscale1,2 and the development of plasmonic devices3,4,5. Despite the various vibronic transitions (S1(v′) → S0(v)) available for frequency tuning of fluorescence, the molecular emissions near metals reported to date have been subject to Kasha's rule, with radiative decay from the lowest excited state (S1(0)) (refs 6–10). Here, we show resonant hot electroluminescence arising directly from higher vibronic levels of the singlet excited state (S1(v′ > 0)) for porphyrin molecules confined inside a nanocavity in a scanning tunnelling microscope, by spectrally tuning the frequency of plasmons. We also demonstrate the generation of unexpected upconversion electroluminescence. These observations suggest that the local nanocavity plasmons behave like a strong coherent optical source with tunable energy, and can be used to actively control the radiative channels of molecular emitters by means of intense resonance enhancement of both excitation and emission. Nanocavity plasmons are exploited as a coherent optical source with tunable energy and to actively control the radiative channels of molecules. Intense resonance enhancement of both excitation and emission, in an effect called resonant hot-electroluminescence, is demonstrated for porphyrin molecules confined inside a nanocavity.

GeV emission from gamma‐ray bursts: a radiative fireball?

Abstract: We study the emission observed at energies > 100 MeV of 11 gamma-ray bursts (GRBs) detected by the Fermi-Large Area Telescope (LAT) until 2009 October. The GeV emission has three main properties: (i) its duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor onboard Fermi (this confirms earlier results from the Energetic Gamma-Ray Experiment Telescope); (ii) its spectrum is consistent with F ν ∝ ν ―1 and does not show strong spectral evolution; and (iii) for the brightest bursts the flux detected by the LAT decays as a power law with a typical slope t ―1.5 . We argue that the observed >0.1 GeV flux can be interpreted as afterglow emission shortly following the start of the prompt phase emission as seen at smaller frequencies. The decay slope is what is expected if the fireball emission is produced in the radiative regime, i.e. all dissipated energy is radiated away. We also argue that the detectability in the GeV energy range depends on the bulk Lorentz factor Γ of the bursts, being strongly favoured in the case of large Γ. This implies that the fraction of bursts detected at high energies corresponds to the fraction of bursts having the largest r. The radiative interpretation can help to explain why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase, despite the fact that the collision with the external medium should be more efficient than internal shocks in producing the radiation that we see.

The 6-GHz methanol multibeam maser catalogue – I. Galactic Centre region, longitudes 345° to 6°

Abstract: Context In photodissociation regions (PDRs), the physical conditions and the excitation evolve on short spatial scales as a function of depth within the cloud, providing a unique opportunity to study how the dust and gas populations evolve with the excitation and physical conditions The mapping of the PDRs in NGC 7023 performed during the science demonstration phase of Herschel is part of the “Evolution of interstellar dust” key program The goal of this project is to build a coherent database on interstellar dust emission from diffuse clouds to the sites of star formation Aims We study the far-infrared/submillimeter emission of the PDRs and their fainter surrounding regions We combine the Herschel and Spitzer maps to derive at each position the full emission spectrum of all dust components, which we compare to dust and radiative transfer models in order to learn about the spatial variations in both the excitation conditions and the dust properties Methods We adjust the emission spectra derived from PACS and SPIRE maps using modified black bodies to derive the temperature and the emissivity index β of the dust in thermal equilibrium with the radiation field We present a first modeling of the NGC 7023-E PDR with standard dust properties and abundances Results At the peak positions, a value of β equal to 2 is compatible with the data The detected spectra and the spatial structures are strongly influenced by radiative transfer effects We are able to reproduce the spectra at the peak positions deduced from Herschel maps and emitted by dust particles at thermal equilibrium, and also the evolution of the spatial structures observed from the near infrared to the submillimeter On the other hand, the emission of the stochastically heated smaller particles is overestimated by a factor ~2

Lyα COOLING EMISSION FROM GALAXY FORMATION

Abstract: Recent numerical and analytical studies have shown that galaxies accrete most of their baryons via the cold mode, from streams with temperatures T {approx} 10{sup 4}-10{sup 5} K. At these temperatures, the streams should radiate primarily in the Ly{alpha} line and have therefore been proposed as a model to power the extended, high-redshift objects known as Ly{alpha} blobs, and may also be relevant for powering a range of less luminous Ly{alpha} sources. We introduce a new Ly{alpha} radiative transfer code, {alpha}RT, and calculate the transport of the Ly{alpha} emission from cold accretion in cosmological hydrodynamical simulations. In this paper, we describe our methodology, and address physical and numerical issues that are critical to making accurate predictions for the cooling luminosity, but that have been mostly neglected or treated simplistically so far. In particular, we highlight the importance of self-shielding and of properly treating sub-resolution models in numerical simulations. Most existing simulations do not self-consistently incorporate these effects, which can lead to order-of-magnitude errors in the predicted cooling luminosity. Using a combination of post-processing ionizing radiative transfer and re-simulation techniques, we develop an approximation to the consistent evolution of the self-shielded gas. We quantify the dependence of the Ly{alpha} cooling luminositymore » on halo mass at z = 3 for the simplified problem of pure gas accretion embedded in the cosmic radiation background and without feedback, and present radiative transfer results for a particular system. While pure cooling in massive halos (without additional energy input from star formation and active galactic nuclei) is in principle sufficient to produce L{sub {alpha}} {approx} 10{sup 43}-10{sup 44} erg s{sup -1} blobs, this requires including energy released in gas of density sufficient to form stars, but which is kept 100% gaseous in our optimistic estimates. Excluding emission from such dense gas yields lower luminosities by up to one to two orders of magnitude at high masses, making it difficult to explain the observed Ly{alpha} blobs with pure cooling. Resonant scattering produces diffuse Ly{alpha} halos, even for centrally concentrated emission, and broad double peaked line profiles. In particular, the emergent line widths are in general not representative of the velocity dispersion within galactic halos and cannot be directly used to infer host halo masses.« less

Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling

Abstract: This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (ie when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.

The dusty heart of nearby active galaxies - II. From clumpy torus models to physical properties of dust around AGN

Abstract: With the possibilities of high spatial resolution imaging and spectroscopy as well as infrared (IR) interferometry, the dusty environments (= “dusty torus”) of active galactic nuclei (AGN) are now in reach of observations. Following our Paper I on ground-based mid-IR spectro-photometry, we present an upgrade to our radiative transfer model of three-dimensional clumpy dust tori. The upgrade with respect to earlier work concerns an improved handling of the diffuse radiation field in the torus, which is approximated by a statistical approach. The models are presented as tools to translate classical and interferometric observations into characteristic properties of the dust distribution. We compare model spectral energy distributions (SEDs) for different chemical and grain-size compositions of the dust and find that clouds with standard interstellar matter (ISM) dust and optical depth τV ∼ 50 appear in overall agreement with observed IR SEDs. By studying parameter dependencies, it is shown that type 1 AGN SEDs, in particular the mid-IR spectral index, can be used to constrain the radial dust cloud distribution power law index a, while other parameters are more difficult to assess using SEDs only. Interferometry adds important additional information for modeling when it is interpreted concurrently with the SED. Although type 2 AGN can in principle be used to constrain model parameters as well, obscuration effects make the analysis more ambiguous. We propose a simple, interferometry-based method to distinguish between “compact” and “extended” radial dust distributions without detailed modeling of the data and introduce a way to easily determine individual or sample average model parameters using the observed optical depth in the silicate feature and the mid-IR spectral index.

Evolutionary signatures in the formation of low-mass protostars. II. Toward reconciling models and observations

Abstract: A long-standing problem in low-mass star formation is the luminosity problem, whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L bol, T bol, L bol/L smm) to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions; we include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T bol and L bol/L smm) can be considered reliable indicators of physical stage.

ATMOSPHERIC CIRCULATION OF ECCENTRIC HOT NEPTUNE GJ436b

Abstract: GJ436b is a unique member of the transiting extrasolar planet population being one of the smallest and least irradiated and possessing an eccentric orbit. Because of its size, mass, and density, GJ436b could plausibly have an atmospheric metallicity similar to Neptune (20-60 times solar abundances), which makes it an ideal target to study the effects of atmospheric metallicity on dynamics and radiative transfer in an extrasolar planetary atmosphere. We present three-dimensional atmospheric circulation models that include realistic non-gray radiative transfer for 1, 3, 10, 30, and 50 times solar atmospheric metallicity cases of GJ436b. Low metallicity models (1 and 3 times solar) show little day/night temperature variation and strong high-latitude jets. In contrast, higher metallicity models (30 and 50 times solar) exhibit day/night temperature variations and a strong equatorial jet. Spectra and light curves produced from these simulations show strong orbital phase dependencies in the 50 times solar case and negligible variations with orbital phase in the 1 times solar case. Comparisons between the predicted planet/star flux ratio from these models and current secondary eclipse measurements support a high metallicity atmosphere (30-50 times solar abundances) with disequilibrium carbon chemistry at play for GJ436b. Regardless of the actual atmospheric composition of GJ436b, our models serve to illuminate how metallicity influences the atmospheric circulation for a broad range of warm extrasolar planets.

Gamma-Ray Emission From Crushed Clouds in Supernova Remnants

Abstract: It is shown that the radio and gamma-ray emission observed from newly found GeV-bright supernova remnants (SNRs) can be explained by a model in which a shocked cloud and shock-accelerated cosmic rays (CRs) frozen in it are simultaneously compressed by the supernova blast wave as a result of formation of a radiative cloud shock. Simple reacceleration of pre-existing CRs is generally sufficient to power the observed gamma-ray emission through the decays of π0-mesons produced in hadronic interactions between high-energy protons (nuclei) and gas in the compressed-cloud layer. This model provides a natural account of the observed synchrotron radiation in SNRs W51C, W44, and IC 443 with flat radio spectral index, which can be ascribed to a combination of secondary and reaccelerated electrons and positrons.

Quantum Photocell: Using Quantum Coherence to Reduce Radiative Recombination and Increase Efficiency*

Abstract: The fundamental limit to photovoltaic efficiency is widely thought to be radiative recombination which balances radiative absorption. We here show that it is possible to break detailed balance via quantum coherence, as in the case of lasing without inversion and the photo-Carnot quantum heat engine. This yields, in principle, a quantum limit to photovoltaic operation which can exceed the classical one. The present work is in complete accord with the laws of thermodynamics.

Calculation and experimental test of the cooling factor of tungsten

Abstract: The cooling factor of W is evaluated using state of the art data for line radiation and an ionization balance which has been benchmarked with experiment. For the calculation of line radiation, level-resolved calculations were performed with the Cowan code to obtain the electronic structure and excitation cross sections ( plane-wave Born approximation). The data were processed by a collisional radiative model to obtain electron density dependent emissions. These data were then combined with the radiative power derived from recombination rates and bremsstrahlung to obtain the total cooling factor. The effect of uncertainties in the recombination rates on the cooling factor was studied and was identified to be of secondary importance. The new cooling factor is benchmarked, by comparisons of the line radiation with spectral measurements as well as with a direct measurement of the cooling factor. Additionally, a less detailed calculation using a configuration averaged model was performed. It was used to benchmark the level-resolved calculations and to improve the prediction on radiation power from line radiation for ionization stages which are computationally challenging. The obtained values for the cooling factor validate older predictions from the literature. Its ingredients and the absolute value are consistent with the existing experimental results regarding the value itself, the spectral distribution of emissions and the ionization equilibrium. A table of the cooling factor versus electron temperature is provided. Finally, the cooling factor is used to investigate the operational window of a fusion reactor with W as intrinsic impurity. The minimum value of nT tau(E), for which a thermonuclear burn is possible, is increased by 20% for a W concentration of 3.0 x 10(-5) compared with a plasma without any impurities, except for the He ash which is considered in both cases.

The formation of high-redshift submillimetre galaxies

Abstract: We describe a model for the formation of z ∼ 2 Submillimeter Galaxies (SMGs) which simultaneously accounts for both average and bright SMGs while providing a reasonable match to their mean observed spectral energy distributions (SEDs). By coupling hydrodynamic simulations of galaxy mergers with the high resolution 3D polychromatic radiative transfer code SUNRISE, we find that a mass sequence of merger models which use observ ational constraints as physical input naturally yield objec ts which exhibit black hole, bulge, and H2 gas masses similar to those observed in SMGs. The dominant drivers behind the 850 µm flux are the masses of the merging galaxies and the stellar bir thcloud covering fraction. The most luminous (S850&15 mJy) sources are recovered by ∼10 13 M⊙ 1:1 major mergers with a birthcloud covering fraction close to unity, whereas more average SMGs (S850∼5‐7 mJy) may be formed in lower mass halos (∼5×10 12 M⊙ ). These models demonstrate the need for high spatial resolution hydrodynamic and radiative transfer simulations in matching both the most luminous sources as well as the full SEDs of SMGs. While these models suggest a natural formation mechanism for SMGs, they do not attempt to match cosmological statistics of galaxy populations; future efforts along this line will hel p ascertain the robustness of these models.

Total aerosol effect: radiative forcing or radiative flux perturbation?

Abstract: . Uncertainties in aerosol radiative forcings, especially those associated with clouds, contribute to a large extent to uncertainties in the total anthropogenic forcing. The interaction of aerosols with clouds and radiation introduces feedbacks which can affect the rate of precipitation formation. In former assessments of aerosol radiative forcings, these effects have not been quantified. Also, with global aerosol-climate models simulating interactively aerosols and cloud microphysical properties, a quantification of the aerosol forcings in the traditional way is difficult to define properly. Here we argue that fast feedbacks should be included because they act quickly compared with the time scale of global warming. We show that for different forcing agents (aerosols and greenhouse gases) the radiative forcings as traditionally defined agree rather well with estimates from a method, here referred to as radiative flux perturbations (RFP), that takes these fast feedbacks and interactions into account. Based on our results, we recommend RFP as a valid option to compare different forcing agents, and to compare the effects of particular forcing agents in different models.

Compact metallo-dielectric optical antenna for ultra directional and enhanced radiative emission.

Abstract: We report the design of highly efficient optical antennas employing a judicious synthesis of metallic and dielectric materials. In the proposed scheme, a pair of metallic coupled nanoparticles permits large enhancements in both excitation strength and radiative decay rates, while a high refractive index dielectric microsphere is employed to efficiently collect light without spoiling the emitter quantum efficiency. Our simulations indicate potential fluorescence rate enhancements of 3 orders of magnitude over the entire optical frequency range.

Protostellar collapse: radiative and magnetic feedbacks on small-scale fragmentation

Abstract: Context. Both radiative transfer and magnetic field are understood to have strong impacts on the collapse and the fragmentation of prestellar dense cores, but no consistent calculation exists on these scales. Aims. We perform the first radiation-magneto-hydrodynamics numerical calculations on a prestellar core scale.Methods. We present original AMR calculations including that of a magnetic field (in the ideal MHD limit) and radiative transfer, within the flux-limited diffusion approximation, of the collapse of a 1 dense core. We compare the results with calculations performed with a barotropic EOS. Results. We show that radiative transfer has an important impact on the collapse and the fragmentation, by means of the cooling or heating of the gas, and its importance depends on the magnetic field. A stronger field yields a more significant magnetic braking, increasing the accretion rate and thus the effect of the radiative feedback. Even for a strongly magnetized core, where the dynamics of the collapse is dominated by the magnetic field, radiative transfer is crucial to determine the temperature and optical depth distributions, two potentially accessible observational diagnostics. A barotropic EOS cannot account for realistic fragmentation. The diffusivity of the numerical scheme, however, is found to strongly affect the output of the collapse, leading eventually to spurious fragmentation. Conclusions. Both radiative transfer and magnetic field must be included in numerical calculations of star formation to obtain realistic collapse configurations and observable signatures. Nevertheless, the numerical resolution and the robustness of the solver are of prime importance to obtain reliable results. When using an accurate solver, the fragmentation is found to always remain inhibited by the magnetic field, at least in the ideal MHD limit, even when radiative transfer is included.

RADIATIVE TRANSFER MODELING OF Lyα EMITTERS. I. STATISTICS OF SPECTRA AND LUMINOSITY

Abstract: We combine a cosmological reionization simulation with box size of 100 h {sup -1} Mpc on a side and a Monte Carlo Ly{alpha} radiative transfer code to model Ly{alpha} Emitters (LAEs) at z {approx} 5.7. The model introduces Ly{alpha} radiative transfer as the single factor for transforming the intrinsic Ly{alpha} emission properties into the observed ones. Spatial diffusion of Ly{alpha} photons from radiative transfer results in extended Ly{alpha} emission and only the central part with high surface brightness can be observed. Because of radiative transfer, the appearance of LAEs depends on density and velocity structures in circumgalactic and intergalactic media as well as the viewing angle, which leads to a broad distribution of apparent (observed) Ly{alpha} luminosity for a given intrinsic Ly{alpha} luminosity. Radiative transfer also causes frequency diffusion of Ly{alpha} photons. The resultant Ly{alpha} line is asymmetric with a red tail. The peak of the Ly{alpha} line shifts toward longer wavelength and the shift is anti-correlated with the apparent-to-intrinsic Ly{alpha} luminosity ratio. The simple radiative transfer model provides a new framework for studying LAEs. It is able to explain an array of observed properties of z {approx} 5.7 LAEs in Ouchi et al., producing Ly{alpha} spectra, morphology, and apparentmore » Ly{alpha} luminosity function (LF) similar to those seen in observation. The broad distribution of apparent Ly{alpha} luminosity at fixed UV luminosity provides a natural explanation for the observed UV LF, especially the turnover toward the low luminosity end. The model also reproduces the observed distribution of Ly{alpha} equivalent width (EW) and explains the deficit of UV bright, high EW sources. Because of the broad distribution of the apparent-to-intrinsic Ly{alpha} luminosity ratio, the model predicts effective duty cycles and Ly{alpha} escape fractions for LAEs.« less