Radiative Transfer Research Articles

Consideration of the dielectric response for radiation chemistry simulations.

The spur reaction, a spatially nonhomogeneous chemical reaction following ionization, is crucial in radiolysis or photolysis in liquids, but the spur expansion process has yet to be elucidated. One reason is the need to understand the role of the dielectric response of the solvating molecules surrounding the charged species generated by ionization. The dielectric response corresponds to the time evolution of the permittivity and might affect the chemical reaction-diffusion of the species in a spur expansion process. This study examined the competitive relationship between reaction-diffusion kinetics and the dielectric response by solving the Debye-Smoluchowski equation while considering the dielectric response. The Coulomb force between the charged species gradually decreases with the dielectric response. Our calculation results found a condition where fast recombination occurs before the dielectric response is complete. Although it has been reported that the primary G-values of free electrons depend on the static dielectric constant under low-linear-energy transfer radiation-induced ionization, we propose that considering the dielectric response can provide a deeper insight into fast recombination reactions under high-linear-energy transfer radiation- or photo-induced ionization. Our simulation method enables the understanding of fast radiation-induced phenomena in liquids.

A conclusive non-detection of magnetic field in the Am star o Peg with high-precision near-infrared spectroscopy

The A-type metallic-line (Am) stars are typically considered to be non-magnetic or to possess very weak sub-G magnetic fields. This view has been repeatedly challenged in the literature; most commonly for the bright hot Am star Several studies claim to have detected 1--2 kG field of unknown topology in this object, possibly indicating a new process of magnetic-field generation in intermediate-mass stars. In this study, we revisit the evidence of a strong magnetic field in using new high-resolution spectropolarimetric observations and advanced spectral fitting techniques. We estimated the mean magnetic field strength in from the high-precision CRyogenic InfraRed Echelle Spectrograph $) measurement of near-infrared (NIR) sulphur lines. We modelled this observation with a polarised radiative transfer code, including treatment of the departures from local thermodynamic equilibrium. In addition, we used the least-squares deconvolution multi-line technique to derive longitudinal field measurements from archival optical spectropolarimetric observations of this star. Our analysis of the NIR S i lines reveals no evidence of Zeeman broadening, ruling out magnetic field with a strength exceeding 260 G. This null result is compatible with the relative intensification of Fe ii lines in the optical spectrum, taking into account blending and uncertain atomic parameters of the relevant diagnostic transitions. Longitudinal field measurements on three different nights also yield null results with a precision of 2 G. This study refutes the claims of kG-strength dipolar or tangled magnetic field in This star therefore appears to be non-magnetic, with surface magnetic field characteristics no different from those of other Am stars.

Prominence and coronal rain formation by steady versus stochastic heating and how we can relate it to observations

Prominences and coronal rain are two forms of coronal condensations for which we still lack satisfactory details on the formation pathways and conditions under which the two come to exist. Even more so, it is unclear why prominences and filaments appear in so many different shapes and sizes, with a vertical rather than a horizontal structure or vice-versa. It is also not clear why coronal rain is present in some cases and not in others. Our aim is to understand the formation process of prominences and coronal rain in more detail by exploring what influence two specific heating prescriptions can have on the resulting formation and evolution, using simulations. We try to determine why we see prominences with such a variety in their properties, particularly by looking at the large-scale topology and dynamics. We attempted to recreate some of these aspects by simulating different types of localised heating. Besides the differences we see on a large scale, we also attempted to determine what the smaller-scale phenomena are, such as reconnection, the influence of resistivity (or lack thereof), the influence of flows and oscillations. We compared prominences that formed via a steady versus stochastic type of heating. We performed 2.5D simulations using the open-source MPI-AMRVAC code. To further extend the work and allow for future direct comparison with observations, we used Lightweaver to form spectra of the filament view of our steady case prominence. With that, we analysed a reconnection event that shares certain characteristics with nanojets. We show how different forms of localised heating that induce thermal instability result in prominences with different properties. The steady form of heating results in prominence with a clear vertical structure stretching across the magnetic field lines. On the other hand, stochastic heating produces many threads that predominantly have a horizontal motion along the field lines. Furthermore, the specific type of heating also influences the small-scale dynamics. In the steady heating case, the prominence is relatively static; however, there is evidence of reconnection happening almost the entire time the prominence is present. In the case of stochastic heating, the threads are highly dynamic, with them also exhibiting a form of transverse oscillation (strongly resembling the decayless type) similar to the vertically polarised oscillations previously found in observations. The fact that the threads in the stochastic heating case are constantly moving along the field lines suppresses any conditions for reconnection. It, therefore, appears that, to first order, the choice of heating prescription defines whether the prominence-internal dynamics are oriented vertically or horizontally. We closely inspected a sample reconnection event and computed the synthetic optically thick radiation using the open-source Lightweaver radiative transfer framework. We find the associated dynamics to imprint clear signatures, both in Doppler and emission, on the resulting spectra that should be testable with state-of-the-art instrumentation such as DKIST.

Monte Carlo simulation of polarized lidar returns for atmospheric clouds sensing

Abstract The paper deals with Monte Carlo simulation of polarized radiative transfer and lidar returns in sensing of atmospheric clouds. The Stokes parameters were used to describe the transfer of polarized radiation, and computation of the lidar returns was based on flux-at-point estimators. We consider modifications of the numerical algorithm with scattering according to the phase function for unpolarized light and with scattering depending on polarization. Numerical experiments were performed for lidar sensing of waterdrop clouds.

The many colors of the TNG100 simulation

Abstract We apply the 3D dust radiative transfer code SKIRT to the low-redshift (z ≤ 0.1) galaxy population in the TNG100 cosmological simulation, the fiducial run of the IllustrisTNG project. We compute global fluxes and spectral energy distributions (SEDs) from the far-ultraviolet to the sub-millimeter for ≈60 000 galaxies. Our post-processing methodology follows the study of Trčka et al. (2022) of the higher-resolution TNG50 simulation. We verify that TNG100 reproduces observational luminosity functions at low redshifts to excellent precision, unlike TNG50. Additionally, we test the realism of our TNG100 plus SKIRT fluxes by comparing various flux and color relations to data from the GAMA survey. TNG100 broadly reproduces the observed distributions, but we predict ultraviolet colors that are too blue by ≈0.4 mag, possibly related to the extinction in the star-forming regions subgrid model not being selective enough. Furthermore, we find that the simulated galaxies exhibit mid-infrared fluxes elevated by up to ≈0.5 mag that we attribute to overly effective stochastic heating of the diffuse dust. All synthetic broadband fluxes and SEDs are made publicly available in three orientations and four apertures, and can readily be used to study TNG100 galaxies in a mock observational fashion.

Particle scale modeling of heat transfer between mesoscale clusters and surfaces in fluidized beds

The distribution of heat absorption and the mechanisms of conductive, convective, and radiative heat transfers between cold particle clusters (Tp = 300 K) and high-temperature surfaces (Tw = 1173 K) in fluidized beds were investigated through numerical simulations using the MFiX-DEM approach. The influence of cluster characteristics (εc and Dc), restitution coefficients (ep and ew), and particle properties (dp, ρp, kp, and cp) on heat absorption and various heat transfers was examined. The simulations indicate that convection predominantly governed the heat transfer behavior during the process of mesoscale clusters impacting the surfaces. A significant increase in heat transfer can be attained by reducing the cluster volume fraction, cluster size, particle diameter, and particle density. Besides, decreasing the restitution coefficients allowed for a relatively small enhancement of heat absorption. The maximum difference in ep was 2.11 kW/kg, while the maximum difference in ew was 0.99 kW/kg. The specific heat capacity and thermal conductivity of the particle had little influence on the distribution of heat absorption and heat transfer with a maximum relative error of up to 5.8 %. Finally, the mathematical correlation during surface-to-cluster heat transfer was established. This work introduces a novel application of the MFiX-DEM approach to analyze surface-to-cluster heat transfer at the particle scale, providing valuable insights for optimizing fluidized bed systems.

Canopy light transmittance-based crop growth monitoring and leaf area index estimation in a garlic field

ABSTRACT A smart agricultural system is necessary for monitoring crop growth and stress conditions using near-ground-based remote sensing techniques. Crop growth can be estimated using several standard crop growth parameters. However, obtaining timed sequential data for observing leaf area index (LAI) is challenging, and normalized difference vegetation index (NDVI) estimation in crop fields requires the installation of sensors on frames structure that are taller than the crop. Canopy light transmittance (CLT) indicates the degree of decrease in the amount of light passing through some material. It was conventionally used to understand canopy radiative transfer. This study examined the viability of CLT as a novel crop parameter for monitoring crop growth conditions. The CLT, LAI, and NDVI of a garlic field were recorded for five years. The correlation between daily CLT and LAI was higher than that between NDVI and LAI. Thus, CLT has the potential to sequentially estimate crop LAI values, particularly for capturing the temporal patterns of LAI. In addition, like NDVI, CLT showed sensitivity in representing the canopy structure and the amount of biomass because CLT is conceptually related to the sky gap fraction. Thus, CLT has the potential to serve as a novel growth parameter for continuous crop growth monitoring.

Multiple solution of MHD Casson fluid flow in porous channel with chemical reactions and thermal radiation effects

This study focused on evaluating the synergistic effects of magnetohydrodynamics, Casson fluid properties, thermal radiation, chemical reactions, and heat transfer mechanisms within a porous channel. Through the application of similarity transformation, nonlinear governing equations are converted into nonlinear ordinary differential equations. These modified equations were then numerically solved in MATLAB using the Runge–Kutta method alongside the shooting technique to ensure precise results. The investigation yielded various numerical solutions for the dimensionless velocity, temperature, concentration, skin friction coefficient, heat transfer coefficient, and Sherwood number, all of which are presented in both tabular and graphical formats. Three distinct solutions emerged from the analysis, differentiated by variations in the Reynolds number, Casson number, expansion or contraction ratio, and magnetic parameters. These solutions demonstrate differences in the parameters under investigation. Solutions within the ranges of γ[5,∞] and R[31.07,∞] are influenced by the specific values of the magnetic number M[0,2.0]. For each M value, there exists a unique set of solutions for γ and R that satisfy the established parameters. As the magnetic number varies, so does the range of feasible solutions, highlighting the sensitivity of the system to magnetic influence. The values of γ and R are adjusted according to the chosen M value.

A Unified Gas-Kinetic Particle Method for Frequency-Dependent Radiative Transfer Equations with Isotropic Scattering Process on Unstructured Mesh

A Unified Gas-Kinetic Particle Method for Frequency-Dependent Radiative Transfer Equations with Isotropic Scattering Process on Unstructured Mesh

ASTRAEUS

Context. Observations with the James Webb Space Telescope (JWST) have revealed an abundance of bright z > 10 galaxy candidates, challenging the predictions of most theoretical models at high redshifts. Aims. Since massive stars dominate the observable ultraviolet (UV) emission, we explore whether a stellar initial mass function (IMF) that becomes increasingly top-heavy towards higher redshifts and lower gas-phase metallicities results in a higher abundance of bright objects in the early universe and how it influences the evolution of galaxy properties compared to a constant Salpeter IMF. Methods. We parameterised the IMF based on the findings from hydrodynamical simulations that track the formation of stars in differently metal-enriched gas clouds in the presence of the cosmic microwave background (CMB) at different redshifts. We incorporated this evolving IMF into the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionisation in N-body dArk mattEr simUlationS) framework, which couples galaxy evolution and reionisation in the first billion years. Our implementation accounts for the IMF dependence of supernova (SN) feedback, metal enrichment, and ionising and UV radiation emission. We conducted two simulations: one with a Salpeter IMF and the other with the evolving IMF. In both, we adjusted the free model parameters to reproduce key observables. Results. Compared to a constant Salpeter IMF, we find that (i) the higher abundance of massive stars in the evolving IMF results in more light per unit stellar mass, resulting in a slower build-up of the stellar mass and lower stellar-to-halo mass ratio; (ii) due to the self-similar growth of the underlying dark matter (DM) halos, the evolving IMF’s star formation main sequence scarcely deviates from that of the Salpeter IMF; (iii) the evolving IMF’s stellar mass to gas-phase metallicity relation shifts to higher metallicities, while its halo mass to gas-phase metallicity relation remains unchanged; (iv) the evolving IMF’s median dust-to-metal mass ratio is lower due to its stronger SN feedback; and (v) the evolving IMF requires lower values of the escape fraction of ionising photons and exhibits a flatter median relation and smaller scatter between the ionising photons emerging from galaxies and the halo mass. However, the ionising emissivities of the galaxies mainly driving reionisation (Mh ∼ 1010 M⊙) are comparable to those of a Salpeter IMF, resulting in minimal changes to the topology of the ionised regions. Conclusions. These results suggest that a top-heavier IMF alone is unlikely to explain the higher abundance of bright z > 10 sources, since the lower mass-to-light ratio driven by the greater abundance of massive stars is counteracted by stronger stellar feedback.

Feasibility of detecting shadows in disks induced by infall

Observations performed with high-resolution imaging techniques have revealed the existence of shadows in circumstellar disks that can be explained by the misalignment of an inner disk with respect to an outer one. The cause of misalignment, however, is still a matter of debate. In this study, we investigate the feasibility of observing shadows induced by one prominent scenario that may lead to misalignment, which involves the late infall of material onto a protostellar system. In particular, we used previously performed hydrodynamical simulations of such events and we generated flux maps in the visible, near-infrared, submillimeter, and millimeter wavelength ranges using Monte Carlo radiative transfer. Based on those results, we derived synthetic observations of these systems performed with the instruments SPHERE/VLT and ALMA, which we used as a basis for our subsequent analysis. We find that near-infrared observations with SPHERE are particularly well suited for detecting shadows via direct imaging alongside other features such as gaps, arcs, and streamers. On the contrary, performing a shadow detection based on reconstructed ALMA observations is very challenging due to the high sensitivity that is required for this task. Thus, in cases that allow for a detection, sophisticated analyses may be needed, for instance by the utilization of carefully constructed azimuthal profiles, aiding the search for potentially shallow shadows. Lastly, we conclude that a late infall-induced disk misalignment offers a plausible explanation for the emergence of shadows observed in various systems.

Stochastic Averaging of Radiative Transfer Coefficients for Relativistic Electrons

Synchrotron emissivities, absorptivities, and Faraday rotation and conversion coefficients are needed in modeling a variety of astrophysical sources, including Event Horizon Telescope (EHT) sources. We develop a method for estimating transfer coefficients that exploits their linear dependence on the electron distribution function, decomposing the distribution function into a sum of parts each of whose emissivity can be calculated easily. We refer to this procedure as stochastic averaging and apply it in two contexts. First, we use it to estimate the emissivity of an isotropic κ distribution function with a high-energy cutoff. The resulting coefficients can be evaluated efficiently enough to be used directly in ray-tracing calculations, and we provide an example calculation. Second, we use stochastic averaging to assess the effect of subgrid turbulence on the volume-averaged emissivity and along the way provide a prescription for a turbulent emissivity. We find that for parameters appropriate to EHT sources turbulence reduces the emissivity slightly. In the infrared, turbulence can dramatically increase the emissivity.

Magnetic activity of red giants: Correlation between the amplitude of solar-like oscillations and chromospheric indicators

Previous studies have found that red giants (RGs) in close binary systems undergoing spin-orbit resonance exhibit an enhanced level of magnetic activity with respect to single RGs rotating at the same rate, from measurements of photometric variability, Sph′, and the chromospheric emission S-index, SCa II. Here, we consider a sample of 4465 RGs observed by the NASA Kepler mission, for which previous studies have measured Sph′ and SCa II, in order to measure additional activity indicators that probe different heights in the chromosphere: the near-ultraviolet (NUV) excess from NASA GALEX photometric data, and chromospheric indices based on the depth of Hα, Mg I, and infared Ca II absorption lines from LAMOST spectroscopic data. Firstly, as for Ca II H&K, we observe that RGs belonging to close binaries in a state of spin-orbit resonance display larger chromospheric emission than the cohort of RGs, as is illustrated by an NUV excess and shallower Hα and infrared Ca II lines. We report no excess of Mg I emission. This result reinforces previous claims that tidal locking leads to enhanced magnetic fields, and allows us to provide criteria to classify active RGs – single or binary – based on their rotation periods and magnetic activity indices. Secondly, we strikingly observe that the depths of the Mg I and Hα lines are anticorrelated and correlated, respectively, with the amplitude of solar-like oscillations for a given surface gravity, log g, regardless of the presence of photometric rotational modulation. Such a correlation opens up future possibilities of estimating the value of magnetic fields at the surface of RG stars, whether quiet or active, by combining spectroscopic and asteroseismic measurements with three-dimensional atmospheric models that include radiative transfer.

Establishing a mass-loss rate relation for red supergiants in the Large Magellanic Cloud

Context. The high mass-loss rates of red supergiants (RSGs) drastically affect their evolution and final fate, but their mass-loss mechanism remains poorly understood. Various empirical prescriptions scaled with luminosity have been derived in the literature, yielding results with a dispersion of two to three orders of magnitude. Aims. We determine an accurate mass-loss rate relation with luminosity and other parameters using a large, clean sample of RSGs. In this way, we shed light into the underlying physical mechanism and explain the discrepancy between previous works. Methods. We assembled a sample of 2219 RSG candidates in the Large Magellanic Cloud, with ultraviolet to mid-infrared photometry in up to 49 filters. We determined the luminosity of each RSG by integrating the spectral energy distribution and the mass-loss rate using the radiative transfer code DUSTY. Results. Our derived RSG mass-loss rates range from approximately 10−9 M⊙ yr−1 to 10−5 M⊙ yr−1, mainly depending on the luminosity. The average mass-loss rate is 9.3 × 10−7 M⊙ yr−1 for log(L/L⊙) > 4, corresponding to a dust-production rate of ∼3.6 × 10−9 M⊙ yr−1. We established a mass-loss rate relation as a function of luminosity and effective temperature. Furthermore, we found a turning point in the relation of mass-loss rate versus luminosity at approximately log(L/L⊙) = 4.4, indicating enhanced rates beyond this limit. We show that this enhancement correlates with photometric variability. We compared our results with prescriptions from the literature, finding an agreement with works assuming steady-state winds. Additionally, we examined the effect of different assumptions on our models and found that radiatively driven winds result in mass-loss rates higher by two to three orders of magnitude, which is unrealistically high for RSGs. For grain sizes < 0.1 μm, the predicted mass-loss rates are higher by a factor of 25−30 than larger grain sizes. Finally, we found that 21% of our sample constitute current binary candidates. This has a minor effect on our mass-loss relation.

The need for spatially resolved observations of polycyclic aromatic hydrocarbons in protoplanetary discs

Context. The signatures of polycyclic aromatic hydrocarbons (PAHs) have been observed in protoplanetary discs, and their emission features obtained from spectral energy distributions (SED) have been used in the literature to characterise their size and determine their abundance. Aims. Two simple disc models (uniform PAH distribution against a PAH gap in the inner disc) are compared to investigate the difference of their SED and obtainable information. Methods. We used the radiative transfer code RADMC-3D to model the SED of two protoplanetary discs orbiting a typical Herbig star, one of which features a depletion of PAHs in the inner disc. We further created artificial images of the discs at face-on view to extract radial profiles of the PAH emission in the infrared. Results. We find that the extracted PAH features from an SED provide limited information about the PAHs in protoplanetary disc environments, except for the ionisation state. The distribution of PAHs in a protoplanetary disc influences the total observed PAH luminosity in a non-linear fashion and alters the relative strength between the 3.3 µm and 11.3 µm features. Furthermore, we produced radial profiles at the 3 µm, 6 µm and, 11 µm PAH emission features and find that they follow a double power-law profile where the slope reflects the radiative environment (single photon regime vs. multi-photon regime) in which the PAHs lie. Conclusions. Using spatially resolved techniques such as IFU or imaging in the era of the James Webb Space Telescope, we find that multi-wavelength radial emission profiles will not only provide information on the spatial distribution of the PAHs, but may also provide information on their size and underlying UV environment, which is crucial for photo-evaporative disc wind models.

Satellite-Based Estimation of the Role of Cloud-Radiative Interaction in Accelerating Tropical Cyclone Development

Abstract Recent modeling studies have suggested a potentially important role of cloud-radiative interactions in accelerating tropical cyclone (TC) development, but there has been only limited investigation of this in observations. Here, we investigate this by performing radiative transfer calculations based on cloud property retrievals from the CloudSat Tropical Cyclone (CSTC) dataset. We examine the radius–height structure of radiative heating anomalies, compute the resulting radiatively driven circulations, and use the moist static energy variance budget to compute radiative feedbacks. We find that inner-core midlevel ice water content and anomalous specific humidity increase with TC intensification rate, resulting in enhanced inner-core deep-layer longwave warming anomalies and shortwave cooling anomalies in rapidly intensifying TCs. This leads to a stronger radiatively driven deep in-up-and-out overturning circulation and inner-core radiative feedback in rapidly intensifying TCs. The longwave-driven circulation provides radially inward momentum fluxes and upward moisture fluxes, which benefit TC development, while the shortwave-driven circulation suppresses TC development. The longwave anomalies, which dominate the inner-core positive radiative feedback, are mainly generated from cloud-radiative interactions, with ice particles dominating the deep-layer circulation and liquid droplets and water vapor contributing to the shallow circulation. Moreover, the variability in ice water content, as opposed to the variability in liquid water content and the effective radii of ice particles and liquid droplets, dominates the uncertainty in TC-radiative interaction. These results provide observational evidence for the importance of cloud-radiative interactions in TC development and suggest that the amount and spatial structure of ice water content are critical for determining the strength of this interaction. Significance Statement The limited investigation of tropical cyclone (TC)-radiative interaction in observations impedes our understanding of TC development. This study aims to quantitatively show the spatial variation in radiation in TCs and their effect on TC development by using a set of satellite-based observations. We relate TC-radiative interaction to TC intensification and emphasize the inner-core features. Moreover, we quantitatively demonstrate the relative contribution from clouds, liquid droplets, ice particles, and water vapor to TC-radiative interaction as well as the source of the variation in radiative properties. These results provide an additional observational foundation for the importance of cloud-radiative interactions in TC development and support a quantitative validation for numerical modeling.

Complex organic molecules uncover deeply embedded precursors of hot cores

Context. During the process of star formation, the dense gas undergoes significant chemical evolution leading to the emergence of a rich variety of molecules associated with hot cores and hot corinos. However, the physical conditions and the chemical processes involved in this evolution are poorly constrained. In particular, the early phases, corresponding to a stage prior to the emergence of any strong ionising emission from the protostar, are still poorly studied. Aims. In this work, we provide a full inventory of the emission from complex organic molecules (COMs) to investigate the physical structure and chemical composition of six high-mass protostellar envelopes. We aim to investigate the conditions for the emergence of COMs in hot cores. Methods. We performed an unbiased spectral survey towards six infrared-quiet massive clumps between 159 GHz and 374 GHz with the APEX 12 m telescope, covering the entire atmospheric windows at 2 mm, 1.2 mm, and 0.8 mm. To identify the spectral lines, we used rotational diagrams and radiative transfer modelling assuming local thermodynamic equilibrium. Results. We detect up to 11 COMs plus three isotopologues, of which at least five COMs (CH3OH, CH3CN, CH3OCHO, CH3OCH3, and CH3CHO) are detected towards all sources. Towards all the objects, most of the COM emission is found to be cold, with respect to the typical temperatures at which COMs are found, with a temperature of 30 K and extended with a size of ~0.3 pc. Although the bulk of the gas for our sample of young massive clumps has a cold temperature, we also detect emission from COMs originating from the immediate vicinity of the protostar. This warm component of the envelope is best traced by methanol and methyl cyanide, in particular methyl cyanide traces a compact (~1″) and the hottest (T ~200 K) component of the envelope. Only three out of the six sources exhibit a robustly detected hot gas component (T > 100 K) traced by several COMs. We find a gradual emergence of the warm component in terms of size and temperature, together with an increasing molecular complexity, allowing us to establish an evolutionary sequence for our sample based on COMs. While they can already be well characterised by an emerging molecular richness, gas temperatures of COMs in the hot gas and molecular abundances suggest that COMs may become abundant in the gas phase at temperatures below the thermal desorption temperature. Conclusions. Our findings confirm that the sources of our sample of infrared-quiet massive clumps are in an early evolutionary stage during which the bulk of the gas is cold. The presence of COMs is found to be characteristic of these early evolutionary stages accompanying high-mass star and cluster formation. While the extent of the compact heated regions resembles that of hot cores, the molecular abundances, except for complex cyanides, resemble those of hot corinos and are lower than the peak COM abundances of hot cores. We suggest that the emergence of hot cores is preceded by a phase in which mostly O-bearing COMs appear first with similar abundances to hot corinos albeit with larger source sizes.

Molecular line polarisation from the circumstellar envelopes of asymptotic giant branch stars

Context. Polarisation observations of masers in the circumstellar envelopes (CSEs) around asymptotic giant branch (AGB) stars have revealed strong magnetic fields. However, masers probe only specific lines of sight through the CSE. Non-masing molecular line polarisation observations can more directly reveal the large-scale magnetic field morphology and hence probe the effect of the magnetic field on AGB mass loss and the shaping of the AGB wind. Aims. Observations and models of CSE molecular line polarisation can now be used to describe the magnetic field morphology and estimate its strength throughout the entire CSE. Methods. We used observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) of molecular line polarisation in the envelope of two AGB stars: CW Leo and R Leo. We modelled the observations using the multi-dimensional polarised radiative transfer tool PORTAL. Results. We found linearly polarised emission, with maximum fractional polarisation on the order of a few percent, in several molecular lines towards both stars. Towards R Leo, we also found a high level of linear polarisation (up to ∼35%) for one of the SiO v = 1 maser transitions. We can explain the observed differences in polarisation structure between the different molecular lines by alignment of the molecules through a combination of the Goldreich-Kylafis effect and radiative alignment effects. We specifically show that the polarisation of CO traces the morphology of the magnetic field. Competition between the alignment mechanisms allowed us to describe the behaviour of the magnetic field strength as a function of the radius throughout the circumstellar envelope of CW Leo. The magnetic field strength derived using this method is inconsistent with the magnetic field strength derived using a structure-function analysis of the CO polarisation and the strength previously derived using CN Zeeman observations. In contrast with CW Leo, the magnetic field in the outer envelope of R Leo appears to be advected outwards by the stellar wind. Conclusions. The ALMA observations and our polarised radiative transfer models show the power of using multiple molecular species to trace the magnetic field behaviour throughout the circumstellar envelope. While the observations appear to confirm the existence of a large-scale magnetic field, further observations and modelling are needed to understand the apparent inconsistency of the magnetic field strength derived with different methods in the envelope of CW Leo.

Monte Carlo modelling of cyclotron and radioisotope center (CYRIC) at Tohoku University: a radiation protection study

Protection against ionizing radiations is important in laboratories with radioactive materials and high energy cyclotron beams. The Cyclotron and Radioisotope Center (CYRIC) located in Tohoku University in Miyagi prefecture, Japan and is a well-known nuclear science laboratory with cyclotron beams and substantial number of high activity radioactive materials. Considering this, it is important to perform complete radiation transport computations to ensure the safety of non-occupational and occupational workers. In the present work, we have developed a complete 3-dimensional model of the main cyclotron building and radiation labs using Monte Carlo method. We have found that the dispersed photons and neutrons inside and in the surrounding of the CYRIC building pose no significant risk to occupational and non-occupational workers. The present work and the developed models would be useful in the field of radiation protection.

Full correction of the self-absorption of laser-induced plasma beryllium emissions via sample preparation

Self-absorption effect is always encountered in laser-induced breakdown spectroscopy (LIBS) and immediately distorts the calibration curves especially for the elemental determination of complex materials. With the aim to provide an expeditious approach for LIBS measurements susceptible to self-absorption effect, an exploratory study for sample preparation by powder mixing is implemented, and the self-absorption of laser-induced plasma beryllium emissions is investigated as a function of the dilution factor. For comparison, boric acid and wax powder are separately used as typical binding agent to mix with beryl powder to produce two sets of pressed pellets with sequential gradient of beryllium content variation. For the resonant lines most prone to self-absorption of Be II emission doublet at 313.042 nm and 313.107 nm, the beryllium spectral line shapes can be directly regulated and improved in the case of both sets of pellets as the dilution factor increases. In addition, the self-absorption effect for the strongly emitting beryllium spectral lines is further assessed by calculating the self-absorption coefficients based on the radiation transport equation. With regard to the both types of binder as diluent, when the dilution factor for beryl increases to 10 with a weight percent of 0.386% for beryllium in the pellet, the SA values may exponentially increase in general to around 0.8 and eventually asymptotically approaches to 1 without exception. Thus the self-absorption effect of laser-induced beryl plasma emissions can be readily overcome with sample preparation by powder mixing. By contrast with laser pulse irradiance in the present work, the dilution factor for powder mixing plays a dominant role in eliminating self-absorption effect. This research proposes a potentially effective approach to reduce self-absorption in laser-induced plasma emissions.