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We briefly discuss the history of pumping schemes for the common 22-GHz maser transition in o-H2O, and note that radiative mechanisms have been considered in the past, as well as the better known collisional pumping mechanisms associated with shock waves. Substantial IR irradiation is indeed destructive to inversion at 22-GHz under a wide range of physical conditions. We identify a small number of transitions, including 22 GHz, that can be pumped by both collisional and radiative schemes, and, through radiative transfer modelling over a grid of values in kinetic temperature, number density, and dust temperature, identify loci in density-kinetic temperature space where these schemes lead to strong inversions. The radiative pumping scheme generally operates at higher dust temperature, and lower kinetic temperature, than the collisional scheme. We identify a small network of transitions that form a radiative pumping scheme for the 22-GHz maser, involving radiative transitions of wavelength approximately 6  |$\mu$| m. This network is capable of supplying more than 50 per cent of the 22-GHz inversion under typical radiative pumping conditions, and it is consistent with the need for high dust and low kinetic temperatures determined from the modelling. We identify a probable case of radiative pumping in a massive star-forming region from the observation of a positive correlation between the flux densities of 22-GHz H2O masers and 6.7-GHz methanol masers. We discuss possibilities for finding radiatively pumped H2O maser lines in other source types. [ABSTRACT FROM AUTHOR]

The population of the 2 s -level of hydrogen formed in the early Universe is studied within a standard and a non-standard quasi-molecular mechanism of recombination. We demonstrate that, if hydrogen molecular ion |$H_2^+$| temporarily formed during a collision directly dissociates into a hydrogen atom and a proton, both mechanisms of recombination lead to the same probabilities of population. These probabilities differ significantly, however, when attractive quasi-molecular channels are uninvolved and only repulsive channels remain in the treatment. The probabilities of the direct and indirect radiative transitions into the 2 s -level of hydrogen are evaluated. The dominant process in the population of 2 s -level of hydrogen is the direct free–bound radiative transition. [ABSTRACT FROM AUTHOR]

In our recent papers, a non-standard quasi-molecular mechanism was suggested and applied to treat the cosmological recombination. It was assumed that, in the pre-recombination stage of evolution of the Universe, an electron combined with two neighbouring protons and created a hydrogen molecular ion, |$\mathrm{ H}_2^+$| in a highly excited state, which then descended into the lower lying state or dissociated. In this work, we implement a quantitative analysis of this quasi-molecular mechanism of recombination; namely, we elaborate the scheme of calculation for a free–bound radiative transition. We show that the quasi-molecular mechanism played a significant role in the pre-recombination and recombination stages of evolution of the early Universe, and hence must be included into the consideration of the description of a thermal history of the Universe. Together with the earlier developed treatment of bound–bound radiative transitions in |$\mathrm{ H}_2^+$|⁠ , the elaborated scheme of calculation can be used for the design of a rapid and complete cosmological recombination code. [ABSTRACT FROM AUTHOR]

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr ii triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca ii triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr ii and Ca ii lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca ii line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25. [ABSTRACT FROM AUTHOR]

Using known values of rotational and centrifugal distortion constants in conjunction with electric dipole moment of NH2CHO, we have calculated energies of rotational levels in the ground vibrational state, and the probabilities for radiative transitions between the levels. The radiative transition probabilities in conjunction with the scaled values of collisional rate coefficients are used in the Sobolev LVG analysis of NH2CHO. There are some strong lines. For ortho-NH2CHO, we have found one transition 110-111 showing anomalous absorption and five transitions 615-514, 717-616, 716-615, 818-717, 817-716 showing emission feature. For para-NH2CHO, six emission transitions 505-404, 606-505, 707-606, 808-707, 909-808, 100.10-909 are found. Out of these 12 transitions, three transitions, 110-111, 505-404, and 808-707, are already found in the ISM. Other relatively weaker lines are also found in the ISM. In addition to the observed lines, 9 transitions may play important role in the identification of NH2CHO in a cosmic object. [ABSTRACT FROM AUTHOR]

Cyanamide (NH2CN) has pyramidal equilibrium structure with two substates, denoted by 0+ and 0–. For each state of NH2CN, knowing rotational and centrifugal distortion constants in conjunction with electric dipole moment, energies for rotational levels and the probabilities for radiative transitions between the levels are calculated. The radiative transition probabilities in conjunction with scaled values for rate coefficients for collisional transitions between the levels are used in the Large Velocity Gradient analysis for each substate. For each substate, we have found anomalous absorption in three doublets at high temperature and weak MASER action at low temperature. We have also found emission feature in nine transitions in each substate. These transitions, along with the observed ones, may play important role for identification of NH2CN in a cosmic object. [ABSTRACT FROM AUTHOR]

Context. The star-formation rates and stellar densities found in young massive clusters suggest that these stellar systems originate from gas at densities of n(H2) > 106 cm−3. Until today, however, the physical characterization of this ultra high density material remains largely unconstrained in observations. Aims. We investigate the density properties of the star-forming gas in the OMC-1 region located in the vicinity of the Orion Nebula Cluster (ONC). Methods. We mapped the molecular emission at 652 GHz in OMC-1 as part of the APEX-SEPIA660 Early Science. Results. We detect bright and extended N2H+ (J = 7–6) line emission along the entire OMC-1 region. Comparisons with previous ALMA data of the (J = 1–0) transition and radiative transfer models indicate that the line intensities observed in this N2H+ (7–6) line are produced by large mass reservoirs of gas at densities n(H2) > 107 cm−3. Conclusions. The first detection of this N2H+ (7–6) line at parsec-scales demonstrates the extreme density conditions of the star-forming gas in young massive clusters such as the ONC. Our results highlight the unique combination of sensitivity and mapping capabilities of the new SEPIA660 receiver for the study of the ISM properties at high frequencies. [ABSTRACT FROM AUTHOR]

We consider the structure of mildly relativistic shocks in dense media, taking into account the radiation and pair loading, and diffusive radiation energy transfer within the flow. For increasing shock velocity (increasing post-shock temperature), the first important effect is the efficient energy redistribution by radiation within the shock that leads to the appearance of an isothermal jump, whereby the flow reaches the final state through a discontinuous isothermal transition. The isothermal jump, on scales much smaller than the photon diffusion length, consists of a weak shock and a quick relaxation to the isothermal conditions. Highly radiation-dominated shocks do not form isothermal jump. Pair production can mildly increase the overall shock compression ratio to ≈10 (4 for matter-dominated shocks and 7 of the radiation-dominated shocks). [ABSTRACT FROM AUTHOR]

In this study, we investigated the deep (semiinfinite) soil contribution to the brightness temperature at 1.4 GHz calculated through a modified incoherent radiative transfer model. We reproduced the measured brightness temperature collected by a dual L-band radiometer in a bare agricultural field. We found that exclusion of a semi-infinite soil layer in the incoherent model significantly decreased the brightness temperature when the measurement depth in the model was closer to the emitting depth, which is the first few centimeters from the top of soil. The maximum brightness temperature differences between the cases with and without the semi-infinite layer in the incoherent model were computed to be 6.8444 K, 2.8891 K, 0.2477 K and 0.0004 K for the measurement depths of 4 cm, 5 cm, 8 cm and 16 cm, respectively. Based on a comparison with another coherent radiative transfer model, we observed that inclusion of the deep soil layer significantly improved the precision of the incoherent model regardless of the measurement depth. Our results could be one example of improving the accuracy of radiative transfer models, which might be applied to other radiative transfer models and increase the precision of soil moisture retrieval calculations. [ABSTRACT FROM AUTHOR]

Aims. We investigate the increase of the differential emission measure (DEM) towards the chromosphere due to small and cool magnetic loops (height ≲8 Mm, T ≲ 105 K). In a previous paper, we analysed the conditions of existence and stability of these loops through hydrodynamic simulations, focussing on their dependence on the details of the optically thin radiative loss function. Methods. In this paper, we extend those hydrodynamic simulations to verify if this class of loops exists and is stable when using an optically thick radiative loss function. We study two cases: constant background heating and a heating depending on the density. The contribution to the transition region extreme-UV output of these loops is also calculated and presented. Results. We find that stable, quasi-static cool loops can be obtained using an optically thick radiative loss function and a background heating depending on the density. The DEMs of these loops, however, fail to reproduce the observed DEM for temperatures between 4.6 < log T < 4.8. We also show the transient phase of a dynamic loop obtained by considering constant heating rate and find that its average DEM, interpreted as a set of evolving dynamic loops, reproduces the observed DEM very well. [ABSTRACT FROM AUTHOR]

Though H 2 CO, H 2 CS, H 2 CCC, H 2 CCCC, H 2 CCO have been identified in cool interstellar molecular clouds, identification of H 2 CC is still awaited. To analyze its spectrum, collisional rate coefficients are required. We have calculated collisional rate coefficients for rotational transitions between 23 levels of ortho and para H 2 CC for kinetic temperatures 10, 20, 30, 40, and 50 K. The scattering problem is analyzed using the computer code MOLSCAT where the colliding partner is He atom. The interaction between H 2 CC and He has been calculated with GAUSSIAN 2003. For the interaction potential obtained with GAUSSIAN 2003, MOLSCAT is used to derive the parameters q ( L , M , M ′ | E ) as a function of energy E of the colliding partner. After averaging the parameters q ( L , M , M ′ | E ) over a Maxwellian distribution, the parameters Q ( L , M , M ′ | T ) as a function of the kinetic temperature T in the cloud are obtained. Finally, the collisional rate coefficients have been calculated. We have also calculated radiative transition probabilities for the transitions between the rotational levels. Finally, for ortho-H 2 CC, we have solved a set of statistical equilibrium equations coupled with the equations of radiative transfer and found that the H 2 CC may be identified through anomalous absorption of its transition 1 11 – 1 10 at 4.89 GHz. [ABSTRACT FROM AUTHOR]

In order to search for silylidene (H2CSi) in the interstellar medium, Izuha et al. (1996) recorded microwave spectrum of H2CSi in laboratory and made an unsuccessful attempt of its identification in IRC +10216, Ori KL, Sgr B2, through its 717 - 616 transition at 222.055 GHz. For finding out if there are other transitions of H2CSi which may help in its identification in the interstellar medium, we have considered 25 rotational levels of ortho-H2CSi connected by collisional transitions and 35 radiative transitions, and solved radiative transfer problem using the LVG approximation. We have found that the brightness temperatures of 919 - 818, 918 - 817, 101,10 - 919, 1019 - 918, 111,11 - 101,10, 111,10 - 1019 and 121,12 - 111,11 transitions are larger than that of 717 - 616 transition. Thus, these transitions may help in detection of H2CSi in the interstellar medium. [ABSTRACT FROM AUTHOR]

Stars can produce steady-state winds through radiative driving as long as the mechanical luminosity of the wind does not exceed the radiative luminosity at its base. This upper bound on the mass-loss rate is known as the photon-tiring limit. Once above this limit, the radiation field is unable to lift all the material out of the gravitational potential of the star, such that only part of it can escape and reach infinity. The rest stalls and falls back towards the stellar surface, making a steady-state wind impossible. Photon-tiring is not an issue for line-driven winds since they cannot achieve sufficiently high mass-loss rates. It can, however, become important if the star exceeds the Eddington limit and continuum interaction becomes the dominant driving mechanism. This paper investigates the time-dependent behaviour of stellar winds that exceed the photon-tiring limit, using one-dimensional numerical simulations of a porosity-moderated, continuum-driven stellar wind. We find that the regions close to the star show a hierarchical pattern of high-density shells moving back and forth, unable to escape the gravitational potential of the star. At larger distances, the flow eventually becomes uniformly outward, though still quite variable. Typically, these winds have a very high density but a terminal flow speed well below the escape speed at the stellar surface. Since most of the radiative luminosity of the star is used to drive the stellar wind, such stars would appear much dimmer than expected from the super-Eddington energy generation at their core. The visible luminosity typically constitutes less than half of the total energy flow and can become as low as 10 per cent or less for those stars that exceed the photon-tiring limit by a large margin. [ABSTRACT FROM AUTHOR]

Fu–Liou's delta-four-stream (with a two-stream option) radiative transfer model has been implemented in the U.S. Navy's Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) to calculate solar and thermal infrared fluxes in 6 shortwave and 12 longwave bands. The model performance is evaluated at high resolution for clear-sky and overcast conditions against the observations from the Southern Great Plains of the Atmospheric Radiation Measurement Program. In both cases, use of the Fu–Liou model provides significant improvement over the operational implementation of the standard Harshvardhan radiation parameterization in both shortwave and longwave fluxes. A sensitivity study of radiative flux on clouds reveals that the choices of cloud effective radius schemes for ice and liquid water are critical to the flux calculation due to the effects on cloud optical properties. The sensitivity study guides the selection of optimal cloud optical properties for use in the Fu–Liou parameterization as implemented in COAMPS. The new model is then used to produce 3-day forecasts over the continental United States for a winter and a summer month. The verifications of parallel runs using the standard and new parameterizations show that Fu–Liou dramatically reduces the model's systematic warm bias in the upper troposphere in both winter and summer. The resultant cooling modifies the atmospheric stability and moisture transport, resulting in a significant reduction in the upper-tropospheric wet bias. Overall ice and liquid water paths are also reduced. At the surface, Fu–Liou reduces the negative temperature and sea level pressure biases by providing more accurate radiative heating rates to the land surface model. The error reductions increase with forecast length as the impact of improved radiative fluxes accumulates over time. A combination of the two- and fourstream options results in major computational efficiency gains with minimal loss in accuracy. [ABSTRACT FROM AUTHOR]

The purpose of this article is to provide some insight into Ambartsumian’s methods in the theory of radiative transfer, their applications, and further development. Two of these methods are emphasized--the invariance principle and the method of addition of layers, proposed by Ambartsumian in the 1940’s. The difference between these methods and the classical approach for solving radiative transfer problems is discussed. We discuss only a small portion of the subsequent work by others that we believe reveals, in a more intuitive way, the essence and significance of Ambartsumian’s methods and their efficiency for applications. Thus, for example, a separate section is devoted to applications of the Lagrangian formalism to radiative transfer and it is shown that the invariance principle is a special case of a more general variational principle that reflects an invariance with respect to translational transformation of the optical depth. Our discussion of the method of addition of layers points out its generality and the major role it has played in the later creation of such methods as Bellman’s invariant imbedding method and the method for solving radiative transfer problems in inhomogeneous media. The latter method has yielded a number of new analytic results. The concluding section is a brief summary of Ambartsumian’s results in the nonlinear theory of radiative transfer, where he was a pioneer in the study of the class of multilevel problems. This article also sets out to demonstrate the place and role of Ambartsumian’s methods in the theory of radiative transfer, which, to a great extent, set the path along which this theory developed for many years to come. [ABSTRACT FROM AUTHOR]

An a priori study of the turbulence radiation interaction (TRI) is performed on numerical data from Direct Numerical Simulation (DNS) of a turbulent flame. The influence of the various correlations that appear in the radiative emission is investigated and their impact is evaluated in the context of Large Eddy Simulation (LES). In LES, only filtered quantities are computed, where the filter is the grid. The radiative emission is reconstructed first from the exact, then filtered solution variables and the sensitivity to the filter size is evaluated. Three approaches are used to take into account the subgrid scale correlations: the no-TRI, partial TRI and full TRI approaches. Results show that the full TRI is exact compared to the reference emission and that the partial TRI performs worse than the no-TRI for the studied configuration. This indicates that in the studied case, the TRI must be considered in LES in a full formulation. [ABSTRACT FROM AUTHOR]

Formic acid (HCOOH), a simplest carboxylic acid, has great importance as it is a precursor for amino acids (constituents of life). It has two rotameric isomers: trans-HCOOH and cis-HCOOH, each of which lies in a plane due to the delocalization of π -electrons over the heavy atom chain. In each of the isomers, the electric dipole moment is aligned such that there are both a and b type rotational transitions. Further, the energy levels in each type of transitions can be classified into two groups. Thus, there are 8 groups in which the rotational transitions of formic acid may be classified. The trans-HCOOH is detected in Sgr B2, cold dark cloud L134N, Sgr A, comet Hale-Bopp, Orion KL, W51, IRAS 16293-2422 through its a-type transitions. Because of very small value of b-component of electric dipole moment, the b-type transitions of trans-HCOOH may not be detected. To our knowledge, no transitions of cis-HCOOH are yet detected in the interstellar medium, though a and b components of its electric dipole moment are quite large. Using spectroscopic data of trans-HCOOH and cis-HCOOH, we have calculated energies of 100 rotational levels for each of the 8 groups, and the radiative transition probabilities (Einstein A and B coefficients) for radiative transitions between the levels. Since the rate coefficients for collisional transitions between the levels are not available, by using the scaled values for them along with the radiative transition probabilities, we have solved a set of 100 statistical equilibrium equations coupled with the equations of radiative transfer for each group. We have investigated intensities of 16 observed a-type transitions and 12 b-type transitions of trans-HCOOH. We have also found six transitions, 1 10 - 1 11 (1.405 GHz), 2 12 - 3 03 (7.545 GHz), 3 12 - 3 03 (79.744 GHz), 3 21 - 3 12 (222.287 GHz), 1 11 - 2 02 (30.843 GHz) and 4 13 - 4 04 (82.740 GHz) of cis-HCOOH showing anomalous absorption and nine transitions 4 14 - 3 13 (85.042 GHz), 5 15 - 4 14 (106.266 GHz), 3 03 - 2 02 (65.840 GHz), 4 04 - 3 03 (87.694 GHz), 5 05 - 4 04 (109.470 GHz), 5 05 - 4 14 (40.778 GHz), 7 07 - 6 16 (90.910 GHz), 4 04 - 3 13 (16.350 GHz) and 6 06 - 5 15 (65.661 GHz) of cis-HCOOH showing emission feature. These transitions of cis-HCOOH in addition to those of trans-HCOOH may help in the identification of HCOOH in a cosmic object. [ABSTRACT FROM AUTHOR]

Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as Hα 6563 and Ca ii 8542 . However, whether there is a response to the surge appearance and evolution in the Si iv lines or, in fact, in many other transition region lines has not been studied. In this paper, we analyze a simultaneous episode of an Hα surge and a Si iv burst that occurred on 2016 September 03 in active region AR 12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. For the first time, we report emission of Si iv within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si iv patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si iv, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the nonequilibrium ionization of silicon. Through spectral synthesis, we explain several features of the observations. We show that the presence of Si iv emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes. [ABSTRACT FROM AUTHOR]

We examine the effect of different radiative transfer schemes on the properties of three-dimensional (3D) simulations of near-surface stellar convection in the superadiabatic layer, where energy transport transitions from fully convective to fully radiative. We employ two radiative transfer schemes that fundamentally differ in the way they cover the 3D domain. The first solver approximates domain coverage with moments, while the second solver samples the 3D domain with ray integrations. By comparing simulations that differ only in their respective radiative transfer methods, we are able to isolate the effect that radiative efficiency has on the structure of the superadiabatic layer. We find the simulations to be in good general agreement, but they show distinct differences in the thermal structure in the superadiabatic layer and atmosphere. [ABSTRACT FROM AUTHOR]