Showing papers on "Radiative transfer published in 2016"
01 Jan 2016
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645 citations
California Institute of Technology1, University of Washington2, University of Leeds3, University of Manchester4, Colorado State University5, National Academies6, National Oceanic and Atmospheric Administration7, Pacific Northwest National Laboratory8, University of California, Irvine9, Goddard Space Flight Center10, University of California, San Diego11, Foundation for Research & Technology – Hellas12, University of Michigan13, Hebrew University of Jerusalem14
TL;DR: This work suggests strategies for improving estimates of aerosol−cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.
Abstract: The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth’s clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol−cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol−cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol−cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.
475 citations
TL;DR: The libRadtran as discussed by the authors software package is a widely used software package for radiative transfer calculations, which allows one to compute (polarized) radiances, irradiance, and actinic fluxes in the solar and thermal spectral regions.
Abstract: . libRadtran is a widely used software package for radiative transfer calculations. It allows one to compute (polarized) radiances, irradiance, and actinic fluxes in the solar and thermal spectral regions. libRadtran has been used for various applications, including remote sensing of clouds, aerosols and trace gases in the Earth's atmosphere, climate studies, e.g., for the calculation of radiative forcing due to different atmospheric components, for UV forecasting, the calculation of photolysis frequencies, and for remote sensing of other planets in our solar system. The package has been described in Mayer and Kylling (2005). Since then several new features have been included, for example polarization, Raman scattering, a new molecular gas absorption parameterization, and several new parameterizations of cloud and aerosol optical properties. Furthermore, a graphical user interface is now available, which greatly simplifies the usage of the model, especially for new users. This paper gives an overview of libRadtran version 2.0.1 with a focus on new features. Applications including these new features are provided as examples of use. A complete description of libRadtran and all its input options is given in the user manual included in the libRadtran software package, which is freely available at http://www.libradtran.org .
459 citations
TL;DR: The basic principles of radiative cooling and its performance characteristics for nonradiative contributions, solar radiation, and atmospheric conditions are discussed and the recent advancements over the traditional approaches and their material and structural characteristics are outlined.
Abstract: The recent progress on radiative cooling reveals its potential for applications in highly efficient passive cooling. This approach utilizes the maximized emission of infrared thermal radiation through the atmospheric window for releasing heat and minimized absorption of incoming atmospheric radiation. These simultaneous processes can lead to a device temperature substantially below the ambient temperature. Although the application of radiative cooling for nighttime cooling was demonstrated a few decades ago, significant cooling under direct sunlight has been achieved only recently, indicating its potential as a practical passive cooler during the day. In this article, the basic principles of radiative cooling and its performance characteristics for nonradiative contributions, solar radiation, and atmospheric conditions are discussed. The recent advancements over the traditional approaches and their material and structural characteristics are outlined. The key characteristics of the thermal radiators and solar reflectors of the current state-of-the-art radiative coolers are evaluated and their benchmarks are remarked for the peak cooling ability. The scopes for further improvements on radiative cooling efficiency for optimized device characteristics are also theoretically estimated.
430 citations
TL;DR: In this paper, the exciton dynamics in transition metal dichalcogenide monolayers were investigated using time-resolved photoluminescence experiments performed with optimized time resolution.
Abstract: We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measureτ 0 rad = 1.8 ± 0.2 ps at T = 7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe2 monolayers. Our detailed analysis suggests the following scenario: at low temperature( T ≲ 50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.
403 citations
TL;DR: In this article, the MHD stagnation point flow of a viscoelastic nanofluid towards a stretching surface with nonlinear radiative effects is studied by employing convective condition at the stretching surface.
291 citations
University of St Andrews1, University of Amsterdam2, Centre national de la recherche scientifique3, University of Chile4, Max Planck Society5, University of Vienna6, Hungarian Academy of Sciences7, Autonomous University of Madrid8, University of Toulouse9, University of Cambridge10, INAF11, Leiden University12
TL;DR: In this article, a set of standard assumptions about the shape of the disk, the dust opacities, dust settling, dust size and opacity, gas/dust ratio, etc. were proposed for the modeling of Class II and III protoplanetary disks.
Abstract: We propose a set of standard assumptions for the modelling of Class II and III protoplanetary disks, which includes detailed continuum radiative transfer, thermo-chemical modelling of gas and ice, and line radiative transfer from optical to cm wavelengths. The first paper of this series focuses on the assumptions about the shape of the disk, the dust opacities, dust settling, and polycyclic aromatic hydrocarbons (PAHs). In particular, we propose new standard dust opacities for disk models, we present a simplified treatment of PAHs in radiative equilibrium which is sufficient to reproduce the PAH emission features, and we suggest using a simple yet physically justified treatment of dust settling. We roughly adjust parameters to obtain a model that predicts continuum and line observations that resemble typical multi-wavelength continuum and line observations of Class II T Tauri stars. We systematically study the impact of each model parameter (disk mass, disk extension and shape, dust settling, dust size and opacity, gas/dust ratio, etc.) on all mainstream continuum and line observables, in particular on the SED, mm-slope, continuum visibilities, and emission lines including [OI] 63 μ m, high-J CO lines, (sub-)mm CO isotopologue lines, and CO fundamental ro-vibrational lines. We find that evolved dust properties, i.e. large grains, often needed to fit the SED, have important consequences for disk chemistry and heating/cooling balance, leading to stronger near- to far-IR emission lines in general. Strong dust settling and missing disk flaring have similar effects on continuum observations, but opposite effects on far-IR gas emission lines. PAH molecules can efficiently shield the gas from stellar UV radiation because of their strong absorption and negligible scattering opacities in comparison to evolved dust. The observable millimetre-slope of the SED can become significantly more gentle in the case of cold disk midplanes, which we find regularly in our T Tauri models. We propose to use line observations of robust chemical tracers of the gas, such as O, CO, and H2 , as additional constraints to determine a number of key properties of the disks, such as disk shape and mass, opacities, and the dust/gas ratio, by simultaneously fitting continuum and line observations.
252 citations
TL;DR: In this article, the authors present a consistent optimal estimation retrieval analysis of 10 hot Jupiter exoplanets, each with transmission spectral data spanning the visible to near-infrared wavelength range.
Abstract: We present a consistent optimal estimation retrieval analysis of 10 hot Jupiter exoplanets, each with transmission spectral data spanning the visible to near-infrared wavelength range. Using the NEMESIS radiative transfer and retrieval tool, we calculate a range of possible atmospheric states for WASP-6b, WASP-12b, WASP-17b, WASP-19b, WASP-31b, WASP-39b, HD 189733b, HD 209458b, HAT-P-1b, and HAT-P-12b. We find that the spectra of all 10 planets are consistent with the presence of some atmospheric aerosol; WASP-6b, WASP-12b, WASP-17b, WASP-19b, HD 189733b, and HAT-P-12b are all fit best by Rayleigh scattering aerosols, whereas WASP-31b, WASP-39b and HD 209458b are better represented by a gray cloud model. HAT-P-1b has solutions that fall into both categories. WASP-6b, HAT-P-12b, HD 189733b, and WASP-12b must have aerosol extending to low atmospheric pressures (below 0.1 mbar). In general, planets with equilibrium temperatures between 1300 and 1700 K are best represented by deeper, gray cloud layers, whereas cooler or hotter planets are better fit using high Rayleigh scattering aerosol. We find little evidence for the presence of molecular absorbers other than H2O. Retrieval methods can provide a consistent picture across a range of hot Jupiter atmospheres with existing data, and will be a powerful tool for the interpretation of James Webb Space Telescope observations.
235 citations
TL;DR: In this article, the authors showed that the spin memory formula is a Fourier transform in time of the recently-discovered sub-leading soft graviton theorem, which is a new type of gravitational "spin memory".
Abstract: The conventional gravitational memory effect is a relative displacement in the position of two detectors induced by radiative energy flux. We find a new type of gravitational ‘spin memory’ in which beams on clockwise and counterclockwise orbits acquire a relative delay induced by radiative angular momentum flux. It has recently been shown that the displacement memory formula is a Fourier transform in time of Weinberg’s soft graviton theorem. Here we see that the spin memory formula is a Fourier transform in time of the recently-discovered subleading soft graviton theorem.
229 citations
TL;DR: In this paper, the authors argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas, which can explain its prevalence in galactic halos.
Abstract: The physical origin of high velocity cool gas seen in galactic winds remains unknown. Following Wang (1995), we argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 10^7-10^8K temperature and the shape of the cooling function for T 0.5 cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the beta and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At r_cool, the gas produces X-ray and then UV/optical line emission with a total power bounded by 10^{-2} L_star if the flow is powered by steady-state star formation with luminosity L_star. The wind is thermally unstable at r_cool, potentially leading to a multi-phase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic halos. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing beta, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-beta hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.
217 citations
TL;DR: In this article, a very high rate spherically symmetric accretion flows onto a massive black hole (BH; 10 2. MBH. 10 6 M⊙) embedded in a dense gas cloud with a low abundance of metals, performing one-dimensional hydrodynamical simulations which include multi-frequency radiation transfer and nonequilibrium primordial chemistry.
Abstract: We study very-high rate spherically symmetric accretion flows onto a massive black hole (BH; 10 2 . MBH . 10 6 M⊙) embedded in a dense gas cloud with a low abundance of metals, performing one-dimensional hydrodynamical simulations which include multi-frequency radiation transfer and non-equilibrium primordial chemistry. We find that rapid gas supply from the Bondi radius at a hyper-Eddington rate can occur without being impeded by radiation feedback when (n∞/10 5 cm −3 ) > (MBH/10 4 M⊙) −1 (T∞/10 4 K) 3/2 , where n∞ and T∞ are the density and temperature of ambient gas outside of the Bondi radius. The resulting accretion rate in this regime is steady, and larger than 3000 times the Eddington rate. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is limited below the Eddington rate. For the hyper-Eddington case, the steady solution consists of two parts: a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T ≃ 8000 K. When the emergent luminosity is limited below the Eddington luminosity because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of Tvir & 10 4 K. Once a seed BH forms at the center of the galaxy, it can grow up to a maximum ∼ 10 5 (Tvir/10 4 K) M⊙ via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lyα emitters without X-rays and luminous infrared sources with hot dust emission, respectively.
TL;DR: In this article, scattered light images of the TW Hya disk performed with SPHERE in PDI mode at 063, 079, 124 and 162 micron were presented.
Abstract: We present scattered light images of the TW Hya disk performed with SPHERE in PDI mode at 063, 079, 124 and 162 micron We also present H2/H3-band ADI observations Three distinct radial depressions in the polarized intensity distribution are seen, around 85, 21, and 6~au The overall intensity distribution has a high degree of azimuthal symmetry; the disk is somewhat brighter than average towards the South and darker towards the North-West The ADI observations yielded no signifiant detection of point sources in the disk
Our observations have a linear spatial resolution of 1 to 2au, similar to that of recent ALMA dust continuum observations The sub-micron sized dust grains that dominate the light scattering in the disk surface are strongly coupled to the gas We created a radiative transfer disk model with self-consistent temperature and vertical structure iteration and including grain size-dependent dust settling This method may provide independent constraints on the gas distribution at higher spatial resolution than is feasible with ALMA gas line observations
We find that the gas surface density in the "gaps" is reduced by 50% to 80% relative to an unperturbed model Should embedded planets be responsible for carving the gaps then their masses are at most a few 10 Mearth The observed gaps are wider, with shallower flanks, than expected for planet-disk interaction with such low-mass planets If forming planetary bodies have undergone collapse and are in the "detachted phase" then they may be directly observable with future facilities such as METIS at the E-ELT
TL;DR: 100- to 1,000-fold enhancements in the radiative conductance between parallel-planar surfaces at gap sizes below 100 nm are reported, in agreement with the predictions of near-field theories.
Abstract: Near field-based enhancements in radiative heat conductance that exceed far-field limits by orders of magnitude are demonstrated by manipulating the gap size between plane-parallel dielectric and metallic plates with nanometre precision. Recent experiments1,2,3,4 have demonstrated that radiative heat transfer between objects separated by nanometre-scale gaps considerably exceeds the predictions of far-field radiation theories5. Exploiting this near-field enhancement is of great interest for emerging technologies such as near-field thermophotovoltaics and nano-lithography6,7,8,9,10,11,12,13 because of the expected increases in efficiency, power conversion or resolution in these applications7,11. Past measurements, however, were performed using tip-plate or sphere-plate configurations and failed to realize the orders of magnitude increases in radiative heat currents predicted from near-field radiative heat transfer theory9,14. Here, we report 100- to 1,000-fold enhancements (at room temperature) in the radiative conductance between parallel-planar surfaces at gap sizes below 100 nm, in agreement with the predictions of near-field theories9,14. Our measurements were performed in vacuum gaps between prototypical materials (SiO2–SiO2, Au–Au, SiO2–Au and Au–Si) using two microdevices and a custom-built nanopositioning platform15, which allows precise control over a broad range of gap sizes (from <100 nm to 10 μm). Our experimental set-up will enable systematic studies of a variety of near-field-based thermal phenomena16,17,18, with important implications for thermophotovoltaic applications7,19,20, that have been predicted but have defied experimental verification.
TL;DR: The Radiative Forcing Model Intercomparison Project (RFMIP) as discussed by the authors has been proposed as a foundation for answering the question "How does the Earth system respond to forcing?" through three related activities: (i) accurate characterization of the effective radiative forcing relative to a near-preindustrial baseline and careful diagnosis of the components of this forcing; (ii) assessment of the absolute accuracy of clear-sky radiative transfer parameterizations against reference models on the global scales relevant for climate modeling; and (iii) identification of robust model responses to tightly specified aeros
Abstract: . The phrasing of the first of three questions motivating CMIP6 – “How does the Earth system respond to forcing?” – suggests that forcing is always well-known, yet the radiative forcing to which this question refers has historically been uncertain in coordinated experiments even as understanding of how best to infer radiative forcing has evolved. The Radiative Forcing Model Intercomparison Project (RFMIP) endorsed by CMIP6 seeks to provide a foundation for answering the question through three related activities: (i) accurate characterization of the effective radiative forcing relative to a near-preindustrial baseline and careful diagnosis of the components of this forcing; (ii) assessment of the absolute accuracy of clear-sky radiative transfer parameterizations against reference models on the global scales relevant for climate modeling; and (iii) identification of robust model responses to tightly specified aerosol radiative forcing from 1850 to present. Complete characterization of effective radiative forcing can be accomplished with 180 years (Tier 1) of atmosphere-only simulation using a sea-surface temperature and sea ice concentration climatology derived from the host model's preindustrial control simulation. Assessment of parameterization error requires trivial amounts of computation but the development of small amounts of infrastructure: new, spectrally detailed diagnostic output requested as two snapshots at present-day and preindustrial conditions, and results from the model's radiation code applied to specified atmospheric conditions. The search for robust responses to aerosol changes relies on the CMIP6 specification of anthropogenic aerosol properties; models using this specification can contribute to RFMIP with no additional simulation, while those using a full aerosol model are requested to perform at least one and up to four 165-year coupled ocean–atmosphere simulations at Tier 1.
TL;DR: In this paper, the authors present a Herschel Space Observatory survey of six additional T T Tauri disks in the HD J = 1-0 line at 112 µm and establish upper limits to the line flux for the other four disks, using detailed disk structure and ray-tracing models.
Abstract: The total gas mass of a protoplanetary disk is a fundamental, but poorly determined, quantity. A new technique has been demonstrated to assess directly the bulk molecular gas reservoir of molecular hydrogen using the HD J =1-0 line at 112 µm. In this work we present a Herschel Space Observatory survey of six additional T Tauri disks in the HD line. Line emission is detected at > 3σ significance in two cases: DM Tau and GM Aur. For the other four disks, we establish upper limits to the line flux. Using detailed disk structure and ray-tracing models, we calculate the temperature structure and dust mass from modeling the observed spectral energy distributions, and we include the effect of UV gas heating to determine the amount of gas required to fit the HD line. The ranges of gas masses are 1.0-4.7 × 10^(-2) for DM Tau and 2.5-20.4 × 10^(-2) for GM Aur. These values are larger than those found using CO for GM Aur, while the CO-derived gas mass for DM Tau is consistent with the lower end of our mass range. This suggests a CO chemical depletion from the gas phase of up to a factor of five for DM Tau and up to two orders of magnitude for GM Aur. We discuss how future analysis can narrow the mass ranges further.
TL;DR: In this article, a 3D model of cloud formation in the atmosphere of hot Jupiter exoplanets is presented, which includes the feedback effects of cloud advection and settling, gas phase element deformation and depletion/replenishment, and the radiative effects of clouds opacity.
Abstract: Context. Observations of exoplanet atmospheres have revealed the presence of cloud particles in their atmospheres. 3D modelling of cloud formation in atmospheres of extrasolar planets coupled to the atmospheric dynamics has long been a challenge. Aims. We investigate the thermo-hydrodynamic properties of cloud formation processes in the atmospheres of hot Jupiter exoplanets. Methods. We simulate the dynamic atmosphere of HD 189733b with a 3D model that couples 3D radiative-hydrodynamics with a kinetic, microphysical mineral cloud formation module designed for RHD/GCM exoplanet atmosphere simulations. Our simulation includes the feedback effects of cloud advection and settling, gas phase element advection and depletion/replenishment and the radiative effects of cloud opacity. We model the cloud particles as a mix of mineral materials which change in size and composition as they travel through atmospheric thermo-chemical environments. All local cloud properties such as number density, grain size and material composition are time-dependently calculated. Gas phase element depletion as a result of cloud formation is included in the model. In situ effective medium theory and Mie theory is applied to calculate the wavelength dependent opacity of the cloud component. Results. We present a 3D cloud structure of a chemically complex, gaseous atmosphere of the hot Jupiter HD 189733b. Mean cloud particle sizes are typically sub-micron (0.01−0.5 μ m) at pressures less than 1 bar with hotter equatorial regions containing the smallest grains. Denser cloud structures occur near terminator regions and deeper (~1 bar) atmospheric layers. Silicate materials such as MgSiO3 [s] are found to be abundant at mid-high latitudes, while TiO2 [s] and SiO2 [s] dominate the equatorial regions. Elements involved in the cloud formation can be depleted by several orders of magnitude. Conclusions. The interplay between radiative-hydrodynamics and cloud kinetics leads to an inhomogeneous, wavelength dependent opacity cloud structure with properties differing in longitude, latitude and depth. This suggests that transit spectroscopy would sample a variety of cloud particles properties (sizes, composition, densities).
TL;DR: Fundamental limits to the optical response of absorptive systems are derived, bounding the largest enhancements possible given intrinsic material losses, through basic conservation-of-energy principles.
Abstract: At visible and infrared frequencies, metals show tantalizing promise for strong subwavelength resonances, but material loss typically dampens the response. We derive fundamental limits to the optical response of absorptive systems, bounding the largest enhancements possible given intrinsic material losses. Through basic conservation-of-energy principles, we derive geometry-independent limits to per-volume absorption and scattering rates, and to local-density-of-states enhancements that represent the power radiated or expended by a dipole near a material body. We provide examples of structures that approach our absorption and scattering limits at any frequency; by contrast, we find that common "antenna" structures fall far short of our radiative LDOS bounds, suggesting the possibility for significant further improvement. Underlying the limits is a simple metric, |χ|2/Im χ for a material with susceptibility χ, that enables broad technological evaluation of lossy materials across optical frequencies.
TL;DR: In this article, the authors present a public code, grtrans, for carrying out such calculations in the Kerr metric, including the full treatment of polarised radiative transfer and parallel transport along geodesics.
Abstract: Ray tracing radiative transfer is a powerful method for comparing theoretical models of black hole accretion flows and jets with observations. We present a public code, grtrans, for carrying out such calculations in the Kerr metric, including the full treatment of polarised radiative transfer and parallel transport along geodesics. The code is written in Fortran 90 and efficiently parallelises with OpenMP, and the full code and several components have Python interfaces. We describe several tests which are used for verifiying the code, and we compare the results for polarised thin accretion disc and semi-analytic jet problems with those from the literature as examples of its use. Along the way, we provide accurate fitting functions for polarised synchrotron emission and transfer coefficients from thermal and power law distribution functions, and compare results from numerical integration and quadrature solutions of the polarised radiative transfer equations. We also show that all transfer coefficients can play an important role in predicted images and polarisation maps of the Galactic center black hole, Sgr A*, at submillimetre wavelengths.
TL;DR: In this article, the spectral energy distribution (SED) and optical line strengths of TDEs near peak brightness were modeled with a non-local thermodynamic equilibrium treatment of the excitation and ionization states of hydrogen, helium and oxygen.
Abstract: Observations of luminous flares resulting from the possible tidal disruption of stars by supermassive black holes have raised a number of puzzles. Outstanding questions include the origin of the optical and ultraviolet (UV) flux, the weakness of hydrogen lines in the spectrum, and the occasional simultaneous observation of x-rays. Here we study the emission from tidal disruption events (TDEs) produced as radiation from black hole accretion propagates through an extended, optically thick envelope formed from stellar debris. We analytically describe key physics controlling spectrum formation, and present detailed radiative transfer calculations that model the spectral energy distribution (SED) and optical line strengths of TDEs near peak brightness. The steady-state transfer is coupled to a non local thermodynamic equilibrium treatment of the excitation and ionization states of hydrogen, helium and oxygen (as a representative metal). Our calculations show how an extended envelope can reprocess a fraction of soft x-rays and produce the observed optical fluxes of order 10^43 ergs per second. Variations in the mass or size of the envelope may help explain how the optical flux changes over time with roughly constant color. For high enough accretion luminosities, x-rays can highly ionize the reprocessing region and escape to be observed simultaneously with the optical flux, producing an SED not described by a single blackbody. Due to optical depth effects, hydrogen Balmer line emission is often strongly suppressed relative to helium line emission (with HeII-to-H line ratios of at least 5:1 in some cases) even in the disruption of a solar-composition star. We discuss the implications of our results to understanding the type of stars destroyed in TDEs and the physical processes responsible for producing the observed flares.
TL;DR: This work measures radiative heat transfer for large temperature differences using a custom-fabricated device in which the gap separating two 5 × 5 mm2 intrinsic silicon planar surfaces is modulated from 3,500 to 150 nm and paves the way for the establishment of novel evanescent wave-based systems.
Abstract: Using Rytov’s fluctuational electrodynamics framework, Polder and Van Hove predicted that radiative heat transfer between planar surfaces separated by a vacuum gap smaller than the thermal wavelength exceeds the blackbody limit due to tunnelling of evanescent modes. This finding has led to the conceptualization of systems capitalizing on evanescent modes such as thermophotovoltaic converters and thermal rectifiers. Their development is, however, limited by the lack of devices enabling radiative transfer between macroscale planar surfaces separated by a nanosize vacuum gap. Here we measure radiative heat transfer for large temperature differences (∼120 K) using a custom-fabricated device in which the gap separating two 5 × 5 mm2 intrinsic silicon planar surfaces is modulated from 3,500 to 150 nm. A substantial enhancement over the blackbody limit by a factor of 8.4 is reported for a 150-nm-thick gap. Our device paves the way for the establishment of novel evanescent wave-based systems. Evanescent coupling between surfaces separated by a distance smaller than the thermal wavelength can lead to radiative heat transfer greater than the blackbody limit. Here, the authors demonstrate this between two macroscopic-scale surfaces, paving the way to harnessing the effect in thermal devices.
TL;DR: In this article, the authors present a new version of the MURaM radiative magnetohydrodynamics (MHD) code that allows for simulations spanning from the upper convection zone into the solar corona.
Abstract: We present a new version of the MURaM radiative magnetohydrodynamics (MHD) code that allows for simulations spanning from the upper convection zone into the solar corona. We implement the relevant coronal physics in terms of optically thin radiative loss, field aligned heat conduction, and an equilibrium ionization equation of state. We artificially limit the coronal Alfven and heat conduction speeds to computationally manageable values using an approximation to semi-relativistic MHD with an artificially reduced speed of light (Boris correction). We present example solutions ranging from quiet to active Sun in order to verify the validity of our approach. We quantify the role of numerical diffusivity for the effective coronal heating. We find that the (numerical) magnetic Prandtl number determines the ratio of resistive to viscous heating and that owing to the very large magnetic Prandtl number of the solar corona, heating is expected to happen predominantly through viscous dissipation. We find that reasonable solutions can be obtained with values of the reduced speed of light just marginally larger than the maximum sound speed. Overall this leads to a fully explicit code that can compute the time evolution of the solar corona in response to photospheric driving using numerical time steps not much smaller than 0.1 s. Numerical simulations of the coronal response to flux emergence covering a time span of a few days are well within reach using this approach.
TL;DR: In this paper, the authors investigated the impact of calculating non-equilibrium chemical abundances consistently with the temperature structure for the atmospheres of highly-irradiated, close-in gas giant exoplanets.
Abstract: In this work we investigate the impact of calculating non-equilibrium chemical abundances consistently with the temperature structure for the atmospheres of highly-irradiated, close-in gas giant exoplanets. Chemical kinetics models have been widely used in the literature to investigate the chemical compositions of hot Jupiter atmospheres which are expected to be driven away from chemical equilibrium via processes such as vertical mixing and photochemistry. All of these models have so far used pressure–temperature (P –T ) profiles as fixed model input. This results in a decoupling of the chemistry from the radiative and thermal properties of the atmosphere, despite the fact that in nature they are intricately linked. We use a one-dimensional radiative-convective equilibrium model, ATMO, which includes a sophisticated chemistry scheme to calculate P –T profiles which are fully consistent with non-equilibrium chemical abundances, including vertical mixing and photochemistry. Our primary conclusion is that, in cases of strong chemical disequilibrium, consistent calculations can lead to differences in the P –T profile of up to 100 K compared to the P –T profile derived assuming chemical equilibrium. This temperature change can, in turn, have important consequences for the chemical abundances themselves as well as for the simulated emission spectra. In particular, we find that performing the chemical kinetics calculation consistently can reduce the overall impact of non-equilibrium chemistry on the observable emission spectrum of hot Jupiters. Simulated observations derived from non-consistent models could thus yield the wrong interpretation. We show that this behaviour is due to the non-consistent models violating the energy budget balance of the atmosphere.
TL;DR: In this article, the authors used extensive global two-dimensional hydrodynamic disk gas+dust simulations with embedded planets, coupled with three-dimensional radiative transfer calculations, to model the dust ring and gap structures in the HL Tau protoplanetary disk observed with the Atacama Large Millimeter/Submillimeter Array (ALMA).
Abstract: We use extensive global two-dimensional hydrodynamic disk gas+dust simulations with embedded planets, coupled with three-dimensional radiative transfer calculations, to model the dust ring and gap structures in the HL Tau protoplanetary disk observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We include the self-gravity of disk gas and dust components and make reasonable choices of disk parameters, assuming an already settled dust distribution and no planet migration. We can obtain quite adequate fits to the observed dust emission using three planets with masses of 0.35, 0.17, and 0.26 MJup at 13.1, 33.0, and 68.6 AU, respectively. Implications for the planet formation as well as the limitations of this scenario are discussed.
TL;DR: In this paper, the authors present three-dimensional simulations with nested meshes of the dynamics of the gas around a Jupiter mass planet with the JUPITER and FARGOCA codes.
Abstract: We present three-dimensional simulations with nested meshes of the dynamics of the gas around a Jupiter mass planet with the JUPITER and FARGOCA codes. We implemented a radiative transfer module into the JUPITER code to account for realistic heating and cooling of the gas. We focus on the circumplanetary gas flow, determining its characteristics at very high resolution ($80\%$ of Jupiter's diameter). In our nominal simulation where the temperature evolves freely by the radiative module and reaches 13000 K at the planet, a circumplanetary envelope was formed filling the entire Roche-lobe. Because of our equation of state is simplified and probably overestimates the temperature, we also performed simulations with limited maximal temperatures in the planet region (1000 K, 1500 K, and 2000 K). In these fixed temperature cases circumplanetary disks (CPDs) were formed. This suggests that the capability to form a circumplanetary disk is not simply linked to the mass of the planet and its ability to open a gap. Instead, the gas temperature at the planet's location, which depends on its accretion history, plays also fundamental role. The CPDs in the simulations are hot and cooling very slowly, they have very steep temperature and density profiles, and are strongly sub-Keplerian. Moreover, the CPDs are fed by a strong vertical influx, which shocks on the CPD surfaces creating a hot and luminous shock-front. In contrast, the pressure supported circumplanetary envelope is characterized by internal convection and almost stalled rotation.
TL;DR: In this paper, velocity shifts for the line peaks of various narrow and broad quasar emission lines relative to systemic using a sample of 849 quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project were studied.
Abstract: Quasar emission lines are often shifted from the systemic velocity due to various dynamical and radiative processes in the line-emitting region. The level of these velocity shifts depends both on the line species and on quasar properties. We study velocity shifts for the line peaks of various narrow and broad quasar emission lines relative to systemic using a sample of 849 quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. The coadded (from 32 epochs) spectra of individual quasars have sufficient signal-to-noise ratio (SNR) to measure stellar absorption lines to provide reliable systemic velocity estimates, as well as weak narrow emission lines. The sample also covers a large dynamic range in quasar luminosity (~2 dex), allowing us to explore potential luminosity dependence of the velocity shifts. We derive average line peak velocity shifts as a function of quasar luminosity for different lines, and quantify their intrinsic scatter. We further quantify how well the peak velocity can be measured for various lines as a function of continuum SNR, and demonstrate there is no systematic bias in the line peak measurements when the spectral quality is degraded to as low as SNR~3 per SDSS pixel. Based on the observed line shifts, we provide empirical guidelines on redshift estimation from [OII]3728, [OIII]5008, [NeV]3426, MgII, CIII], HeII1640, broad Hbeta, CIV, and SiIV, which are calibrated to provide unbiased systemic redshifts in the mean, but with increasing intrinsic uncertainties of 46, 56, 119, 205, 233, 242, 400, 415, and 477 km/s, in addition to the measurement uncertainties. These more realistic redshift uncertainties are generally much larger than the formal uncertainties reported by the redshift pipelines for spectroscopic quasar surveys, and demonstrate the infeasibility of measuring quasar redshifts to better than ~200 km/s with only broad lines.
Abstract: We report on the energetics of molecular outflows in 14 local Ultraluminous Infrared Galaxies (ULIRGs) that show unambiguous outflow signatures (P-Cygni profiles or high-velocity absorption wings) in the far-infrared lines of OH measured with the Herschel/PACS spectrometer. Detection of both ground-state (at 119 and 79 um) and one or more radiatively-excited (at 65 and 84 um) lines allows us to model the nuclear gas (<~300 pc) as well as the more extended components using spherically symmetric radiative transfer models. The highest molecular outflow velocities are found in buried sources, in which slower but massive expansion of the nuclear gas is also observed. With the exception of a few outliers, the outflows have momentum fluxes of (2-5)xL_IR/c and mechanical luminosities of (0.1-0.3)% of L_IR. The moderate momentum boosts in these sources (<~3) suggest that the outflows are mostly momentum-driven by the combined effects of AGN and nuclear starbursts, as a result of radiation pressure, winds, and supernovae remnants. In some sources (~20%), however, powerful (10^{10.5-11} Lsun) AGN feedback and (partially) energy-conserving phases are required, with momentum boosts in the range 3-20. These outflows appear to be stochastic strong-AGN feedback events that occur throughout the merging process. In a few sources, the outflow activity in the innermost regions has subsided in the last ~1 Myr. While OH traces the molecular outflows at sub-kpc scales, comparison of the masses traced by OH with those previously inferred from tracers of more extended outflowing gas suggests that most mass is loaded (with loading factors of Mdot/SFR=1-10) from the central galactic cores (a few x 100 pc). Outflow depletion timescales are <10^8 yr, shorter than the gas consumption timescales by factors of 1.1-15, and are anti-correlated with the AGN luminosity.
TL;DR: Theoretical Reims-Tomsk Spectral data ( TheoReTS) as mentioned in this paper is an Internet accessible information system devoted to ab initio based rotationally resolved spectra predictions for some relevant molecular species.
TL;DR: In this article, mass loss from the outer Lagrange point (L2) in binary stellar mergers and their luminous transients were studied by means of radiative hydrodynamical simulations.
Abstract: We study mass loss from the outer Lagrange point (L2) in binary stellar mergers and their luminous transients by means of radiative hydrodynamical simulations. Previously, we showed that for binary mass ratios 0.06 0.15. By contrast, for cold L2 mass-loss (\epsilon 0.8, the equatorial outflow instead remains marginally-bound and falls back to the binary over tens to hundreds of binary orbits, where it experiences additional tidal torqueing and shocking. As the bound gas becomes virialized with the binary, the luminosity of the system increases slowly at approximately constant photosphere radius, causing the temperature to rise. Subsequent evolution depends on the efficiency of radiative cooling. If the bound atmosphere is able to cool efficiently, as quantified by radiative diffusion time being shorter than the advection time (t_diff/t_adv 1 an isotropic wind is formed. Between these two extremes, an inflated envelope transports the heat generated near the binary to the surface by meridional flows. In all cases, the radiated luminosity reaches a fraction ~0.01 to 0.1 of Mdot v_orb^2/2, where Mdot is the mass outflow rate. We discuss the implications of our results for transients in the luminosity gap between classical novae and supernovae, such as V1309 Sco and V838 Mon.
TL;DR: In this article, a three-dimensional Monte-Carlo radiative transfer code (POLARIS) was designed to calculate dust temperature, polarization maps, and spectral energy distributions.
Abstract: Aims. We present POLARIS (POLArized RadIation Simulator), a newly developed three-dimensional Monte-Carlo radiative transfer code. POLARIS was designed to calculate dust temperature, polarization maps, and spectral energy distributions. It is optimized to handle data that results from sophisticated magneto-hydrodynamic simulations. The main purpose of the code is to prepare and analyze multi-wavelength continuum polarization measurements in the context of magnetic field studies in the interstellar medium. An exemplary application is the investigation of the role of magnetic fields in star formation processes.Methods. We combine currently discussed state-of-the-art grain alignment theories with existing dust heating and polarization algorithms. We test the POLARIS code on multiple scales in complex astrophysical systems that are associated with different stages of star formation. POLARIS uses the full spectrum of dust polarization mechanisms to trace the underlying magnetic field morphology.Results. Resulting temperature distributions are consistent with the density and position of radiation sources resulting from magneto-hydrodynamic (MHD) – collapse simulations. The calculated layers of aligned dust grains in the considered cirumstellar disk models are in excellent agreement with theoretical predictions. Finally, we compute unique patterns in synthetic multi-wavelength polarization maps that are dependent on applied dust-model and grain-alignment theory in analytical cloud models.
TL;DR: In this paper, the UK Met Office Global Circulation Model (GCM), the Unified Model (UM), is adapted to exoplanets to study the complexity of hot Jupiter atmospheres revealed by observations of increasing quality.
Abstract: To study the complexity of hot Jupiter atmospheres revealed by observations of increasing quality, we have adapted the UK Met Office Global Circulation Model (GCM), the Unified Model (UM), to these exoplanets. The UM solves the full 3D Navier-Stokes equations with a height-varying gravity, avoiding the simplifications used in most GCMs currently applied to exoplanets. In this work we present the coupling of the UM dynamical core to an accurate radiation scheme based on the two-stream approximation and correlated-k method with state-of-the-art opacities from ExoMol. Our first application of this model is devoted to the extensively studied hot Jupiter HD 209458b. We have derived synthetic emission spectra and phase curves, and compare them to both previous models also based on state-of-the-art radiative transfer, and to observations. We find a reasonable agreement between observations and both our days side emission and hot spot offset, however, our night side emissions is too large. Overall our results are qualitatively similar to those found by Showman et al. (2009, ApJ, 699, 564) with the SPARC/MITgcm, however, we note several quantitative differences: Our simulations show significant variation in the position of the hottest part of the atmosphere with pressure, as expected from simple timescale arguments, and in contrast to the “vertical coherency” found by Showman et al. (2009). We also see significant quantitative differences in calculated synthetic observations. Our comparisons strengthen the need for detailed intercomparisons of dynamical cores, radiation schemes and post-processing tools to understand these differences. This effort is necessary in order to make robust conclusions about these atmospheres based on GCM results.