Simulating 3D Radiation Transport, a modern approach to discretisation and an exploration of probabilistic methods

Light, or electromagnetic radiation in general, is a profound and invaluable resource to investigate our physical world. For centuries, it was the only and it still is the main source of information to study the Universe beyond our planet. With high-resolution spectroscopic imaging, we can identify numerous atoms and molecules, and can trace their physical and chemical environments in unprecedented detail. Furthermore, radiation plays an essential role in several physical and chemical processes, ranging from radiative pressure, heating, and cooling, to chemical photo-ionisation and photo-dissociation reactions. As a result, almost all astrophysical simulations require a radiative transfer model. Unfortunately, accurate radiative transfer is very computationally expensive. Therefore, in this thesis, we aim to improve the performance of radiative transfer solvers, with a particular emphasis on line radiative transfer. First, we review the classical work on accelerated lambda iterations and acceleration of convergence, and we propose a simple but effective improvement to the ubiquitously used Ng-acceleration scheme. Next, we present the radiative transfer library, Magritte: a formal solver with a ray-tracer that can handle structured and unstructured meshes as well as smoothed-particle data. To mitigate the computational cost, it is optimised to efficiently utilise multi-node and multi-core parallelism as well as GPU offloading. Furthermore, we demonstrate a heuristic algorithm that can reduce typical input models for radiative transfer by an order of magnitude, without significant loss of accuracy. This strongly suggests the existence of more efficient representations for radiative transfer models. To investigate this, we present a probabilistic numerical method for radiative transfer that naturally allows for uncertainty quantification, providing us with a mathematical framework to study the trade-off between computational speed and accuracy. Although we cannot yet construct optimal representations for radiative transfer problems, we point out several ways in which this method can lead to more rigorous optimisation

Self-consistent dust and non-LTE line radiative transfer with SKIRT

We introduce Monte Carlo-based non-LTE line radiative transfer calculations in the 3D dust radiative transfer code SKIRT, which was originally set up as a dust radiative transfer code. By doing so, we develop a generic and powerful 3D radiative transfer code that can self-consistently generate spectra with molecular and atomic lines against the underlying continuum. We test the accuracy of the non-LTE line radiative transfer module in the extended SKIRT code using standard benchmarks. We find excellent agreement between the SKIRT results, the published benchmark results, and results obtained using the ray-tracing non-LTE line radiative transfer code MAGRITTE, which validates our implementation. We apply the extended SKIRT code on a 3D hydrodynamic simulation of a dusty AGN torus model and generate multi-wavelength images with CO rotational-line spectra against the underlying dust continuum. We find that the low-J CO emission traces the geometrically thick molecular torus, whereas the higher-J CO lines originate from the gas with high kinetic temperature located in the innermost regions of the torus. Comparing the calculations with and without dust radiative transfer, we find that higher-J CO lines are slightly attenuated by the surrounding cold dust when seen edge-on. This shows that atomic and molecular lines can experience attenuation, an effect that is particularly important for transitions at mid- and near-infrared wavelengths. Therefore, our self-consistent dust and non-LTE line radiative transfer calculations can help interpret the observational data from Herschel, ALMA, and JWST.Comment: 12 pages, 9 figures, Accepted for publication in A&

3D simulations of AGB stellar winds -- II. Ray-tracer implementation and impact of radiation on the outflow morphology

Stars with an initial mass below ~ 8 Msun evolve through the asymptotic giant branch (AGB) phase, during which they develop strong stellar winds. Recent observations have revealed significant morphological complexities in their outflows, most likely caused by a companion. We study the impact of the radiation force on such companion-perturbed AGB outflows. We present the implementation of a ray tracer for radiative transfer in smoothed particle hydrodynamics (SPH) and compared four different descriptions of radiative transfer: the free-wind, the geometrical, the Lucy, and the attenuation approximation. For both low and high mass-loss rates, the velocity profile of the outflow is modified when going from the free-wind to the geometrical approximation, also resulting in a different morphology. In the case of a low mass-loss rate, the effect of the Lucy and attenuation approximation is negligible due to the low densities but morphological differences appear in the high mass-loss rate regime. By comparing the radiative equilibrium temperature and radiation force to full 3D radiative transfer, we show that the Lucy approximation works best. Although, close to the companion, artificial heating occurs and it fails to simulate the shadow cast by the companion. The attenuation approximation produces a lower equilibrium temperature and weaker radiation force, but it produces the shadow cast by the companion. From the predictions of the 3D radiative transfer, we also conclude that a radially directed radiation force is a reasonable assumption. The radiation force thus plays a critical role in dust-driven AGB winds, impacting the velocity profile and morphological structures. For low mass-loss rates, the geometrical approximation suffices, while high mass-loss rates require a more rigorous method, where the Lucy approximation provides the most accurate results although not accounting for all effects

3D Line Radiative Transfer & Synthetic Observations with Magritte

Electromagnetic radiation is a key component in many astrophysical simulations. Not only does it dictate what we can or cannot observe, it can provide radiation pressure, efficient heating and cooling mechanisms, and opens up a range of new chemical pathways due to photo-reactions. Magritte is a software library that can be used as a general-purpose radiative transfer solver, but was particularly designed for line radiative transfer in complex 3D morphologies, such as, for instance, encountered in the stellar winds around evolved stars (see Decin, 2020). It is mainly written in C++ and can either be used as a Python package or as a C++ library. To compute the radiation field, a deterministic ray-tracer and a formal solver are employed, i.e., rays are traced through the model and the radiative transfer equation is solved along those rays (De Ceuster et al., 2019). This is in contrast to most radiative transfer solvers which employ (probabilistic) Monte Carlo techniques (Noebauer & Sim, 2019). By virtue of minimal assumptions about the underlying geometric structure of a model, Magritte can handle structured and unstructured input meshes, as well as smoothed-particle hydrodynamics (SPH) data. Furthermore, tools are provided to optimise different input meshes for radiative transfer (De Ceuster et al., 2020)

The upper Miocene Deurne Member of the Diest Formation revisited : unexpected results from the study of a large temporary outcrop near Antwerp International Airport, Belgium

A 5.50 m thick interval of fossiliferous intensely bioturbated heterogenous glauconiferous sand of the upper Miocene Diest Formation is documented from a very large temporary outcrop just southeast of Antwerp International Airport (northern Belgium), allowing to observe lateral variations over several hundreds of meters and to collect many vertebrate and invertebrate fossils. This paper documents observations on lithology, sedimentary and post-sedimentary structures, and discusses the results of the multi-proxy analyses of the sediment (granulometry, glauconite content, clay mineralogy, Fe content and Fe3+/Fe2+ ratios), the interpretation of the trace fossil assemblage and the sedimentary structures as well as of the large-scale samplings of micro-, meso- and macrofossils. We evidence that the Diest Formation in the Antwerp area consists of two different lithological entities, and that this twofold character can be extrapolated to all previously recorded Deurne Member outcrops. A revised lithostratigraphic scheme for the Diest Formation in the Antwerp area is proposed, with the new Borsbeek member at the base and a redefmed Deurne Member at the top

SPH modelling of AGB wind morphology in hierarchical triple systems \& comparison to observation of R Aql

Asymmetric 3D structures are observed in the outflows of evolved low- and intermediate-mass stars, and are believed to be shaped through the interaction of companions that are hidden within the dense wind. We investigate how triple systems can shape the outflow of AGB stars. We focus on coplanar systems in a hierarchical, stable orbit, consisting of an AGB star with one relatively close companion, and one at large orbital separation. We model a grid of hierarchical triple systems including a wind-launching AGB star, with the smoothed-particle-hydrodynamic Phantom code. We vary the outer companion mass, the AGB wind velocity and the orbital eccentricities to study the impact of these parameters on the wind morphology. Further, we investigate if the outflow of the AGB star R Aql could be shaped by a triple system, by post-processing one of our triple models with a radiative transfer routine, and comparing this to data of the ALMA ATOMIUM programme. The characteristic wind structures resulting from a hierarchical triple system are the following. A large two-edged spiral wake results behind the outer companion star. This structure lies on top of the spiral structure formed by the close binary, which is affected by the orbital motion around the system centre-of-mass. This dense inner spiral pattern interacts with, and strongly impacts, the spiral wake of the outer companion, resulting in a waved-pattern on the outer edge of this spiral wake. From the comparison of our models to the observations of R Aql, we conclude that this circumstellar environment might be shaped by a similar system as the ones modelled in this work, but an elaborate study of the observational data is needed to determine better the orbital parameters and characteristics of the central system

Radiative transfer as a Bayesian linear regression problem

Electromagnetic radiation plays a crucial role in various physical and chemical processes. Hence, almost all astrophysical simulations require some form of radiative transfer model. Despite many innovations in radiative transfer algorithms and their implementation, realistic radiative transfer models remain very computationally expensive, such that one often has to resort to approximate descriptions. The complexity of these models makes it difficult to assess the validity of any approximation and to quantify uncertainties on the model results. This impedes scientific rigour, in particular, when comparing models to observations, or when using their results as input for other models. We present a probabilistic numerical approach to address these issues by treating radiative transfer as a Bayesian linear regression problem. This allows us to model uncertainties on the input and output of the model with the variances of the associated probability distributions. Furthermore, this approach naturally allows us to create reduced-order radiative transfer models with a quantifiable accuracy. These are approximate solutions to exact radiative transfer models, in contrast to the exact solutions to approximate models that are often used. As a first demonstration, we derive a probabilistic version of the method of characteristics, a commonly-used technique to solve radiative transfer problems

MAGRITTE, a modern software library for 3D radiative transfer: I. Non-LTE atomic and molecular line modelling

Radiative transfer is a key component in almost all astrophysical and cosmological simulations. We present MAGRITTE: a modern open-source software library for 3D radiative transfer. It uses a deterministic ray-tracer and formal solver, i.e. it computes the radiation field by tracing rays through the model and solving the radiative transfer equation in its second-order form along a fixed set of rays originating from each point. MAGRITTE can handle structured and unstructured input meshes, as well as smoothed-particle hydrodynamics (SPH) particle data. In this first paper, we describe the numerical implementation, semi-analytic tests and cross-code benchmarks for the non-LTE line radiative transfer module of MAGRITTE. This module uses the radiative transfer solver to self-consistently determine the populations of the quantised energy levels of atoms and molecules using an accelerated Lambda iteration (ALI) scheme. We compare MAGRITTE with the established radiative transfer solvers RATRAN (1D) and LIME (3D) on the van Zadelhoff benchmark and present a first application to a simple Keplerian disc model. Comparing with LIME, we conclude that MAGRITTE produces more accurate and more precise results, especially at high optical depth, and that it is faster.status: publishe

An ALMA view of CS and SiS around oxygen-rich AGB stars

C 2019 The Author(s) We aim to determine the distributions of molecular SiS and CS in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how these distributions differ between stars that lose mass at different rates. In this study, we analyse ALMA observations of SiS and CS emission lines for three oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate of 5 × 10−6 M yr−1, and W Hya and R Dor with low mass-loss rates of ∼1 × 10−7 M yr−1. These molecules are usually more abundant in carbon stars but the high sensitivity of ALMA allows us to detect their faint emission in the low mass-loss rate AGB stars. The high spatial resolution of ALMA also allows us to precisely determine the spatial distribution of these molecules in the circumstellar envelopes. We run radiative transfer models to calculate the molecular abundances and abundance distributions for each star. We find a spread of peak SiS abundances with ∼10−8 for R Dor, ∼10−7 for W Hya, and ∼3 × 10−6 for IK Tau relative to H2. We find lower peak CS abundances of ∼7 × 10−9 for R Dor, ∼7 × 10−8 for W Hya, and ∼4 × 10−7 for IK Tau, with some stratifications in the abundance distributions. For IK Tau, we also calculate abundances for the detected isotopologues: C34S, 29SiS, 30SiS, Si33S, Si34S, 29Si34S, and 30Si34S. Overall, the isotopic ratios we derive for IK Tau suggest a lower metallicity than solar.keywords: stars: AGB and post-AGB, circumstellar matter, stars: evolution, submillimetre: stars, Astrophysics - Solar and Stellar Astrophysics archiveprefix: arXiv primaryclass: astro-ph.SR adsurl: https://ui.adsabs.harvard.edu/#abs/2019MNRAS.484..494D adsnote: Provided by the SAO/NASA Astrophysics Data Systemstatus: publishe

SPH modelling of companion-perturbed AGB outflows including a new morphology classification scheme

info:eu-repo/semantics/publishe