Tähtienvälisten pilvien säteilynkuljetusmallinnus

Almost all information on astrophysical objects is obtained through observation of electromagnetic radiation. The observed radiation has been altered in interactions with matter, and understanding the transport of radiation is a key prerequisite for understanding the physical conditions in the observed objects. The transport of radiation is described by the radiative transfer equation. Owing to its complex nature, solving the radiative transfer equation is difficult, and it is usually necessary to resort to numerical calculations. In this thesis, the focus is on the modelling of radiation transport in interstellar clouds. The dense gas and dust in interstellar clouds scatter, absorb, and emit radiation, and understanding the radiative transfer effects is crucial in the interpretation of observations. Four of the five articles that are contained in this thesis concern various applications of radiative transfer modelling. Two articles focus on the modelling of spectral line radiation. We study the use of OH Zeeman splitting observations in the determination of magnetic field strengths in molecular clouds. The role of magnetic fields in the process of star formation is still largely an open question with two competing models: the turbulence dominated scenario where magnetic fields are weak, and the ambipolar diffusion driven model with stronger magnetic fields. By combining magneto-hydrodynamical calculations with radiative transfer simulations, we show that the turbulence dominated scenario is consistent with the observed magnetic field strengths. Two articles concern the dust radiative transfer. We study the dust density distribution and grain properties in the dust envelope surrounding the carbon star IRC +10216. By modelling the surface brightness distribution of the scattered light in the dust envelope, we can infer the mass-loss history of the star and improve models of newly formed dust grains. In another article we use magneto-hydrodynamical calculations and radiative transfer simulations to study the reliability of cloud core mass estimates. Observations of dust thermal emission at the far-infrared and sub-millimetre wavelengths are commonly used to determine the masses of molecular cloud cores. By constructing synthetic observations of a model cloud and comparing the estimated masses to the true masses that are obtained directly from the cloud model, we can determine the robustness of mass estimates. Instead of focusing on the applications of radiative transfer modelling, one article describes new numerical methods for efficient radiative transfer simulations. We describe new algorithms for radiative transfer on hierarchical grids. The new algorithms, in particular the use of sub-iterations, are faster by a factor of several compared to the old methods.Lähes kaikki tieto tähtitieteellisistä kohteista saadaan havaitsemalla sähkömagneettista säteilyä. Havaittava säteily on muuttunut vuorovaikutuksissa aineen kanssa, joten havaintojen tulkinta edellyttää säteilyn siirtymisen ymmärtämistä. Säteilyn siirtymistä kuvataan säteilynkuljetysyhtälöllä. Yhtälön monimutkaisuuden vuoksi ratkaisussa joudutaan yleensä käyttämään numeerisia menetelmiä. Tässä väitöskirjassa keskitytään mallintamaan säteilynkuljetusta tähtienvälisissä pilvissä. Niiden tiheä kaasu ja pöly sirottavat, absorboivat ja emittoivat säteilyä, joten säteilynkuljetusmallinnus on avainasemassa pilvien tutkimisessa. Neljä väitöskirjan viidestä artikkelista käsittelee säteilynkuljetusmallinnuksen sovelluksia. Kaksi artikkelia käsittelee spektriviivasäteilyä. Niin sanotun Zeemanin ilmiön ansiosta hydroksyyliradikaalin (OH) spektriviivahavainnoista voidaan johtaa magneettikentän voimakkuus tähtienvälisissä pilvissä. Magneettikenttien rooli tähtien synnyssä on yhä suurelta osin avoin kysymys, joten magneettikenttien mittaus on hyvin tärkeää. Yhdistämällä magnetohydrodynaamiset simulaatiot ja säteilynkuljetusmallinnuksen osoitamme, että malli jonka mukaan pilvien magneettikentät ovat heikkoja ja melko merkityksettömiä tähtien syntyprosessissa sopii kirjallisuudessa esitettyihin spektriviivahavaintoihin. Kaksi artikkelia käsittelee tähtienvälistä pölyä. Tutkimme valonsirontaa hiilitähteä IRC +10216 ympäröivässä pölykuoressa. Mallintamalla kuoren kirkkausjakaumaa voimme selvittää pölykuoren rakennetta sekä pölyhiukkasten ominaisuuksia. Toisessa artikkelissa magnetohydrodynaamisia malleja säteilynkuljetuslaskuja käytetään pilviytimien massa-arvioiden luotettavuuden tutkimiseen. Viides artikkeli keskittyy sovellusten sijasta tehokkaiden säteilynkuljetusalgoritmien kehitykseen. Esittelemme uusia menetelmiä, joilla säteilynkuljetusongelma voidaan ratkaista murto-osassa vanhojen menetelmien vaatimasta ajasta

Composite biasing in Monte Carlo radiative transfer

Biasing or importance sampling is a powerful technique in Monte Carlo radiative transfer, and can be applied in different forms to increase the accuracy and efficiency of simulations. One of the drawbacks of the use of biasing is the potential introduction of large weight factors. We discuss a general strategy, composite biasing, to suppress the appearance of large weight factors. We use this composite biasing approach for two different problems faced by current state-of-the-art Monte Carlo radiative transfer codes: the generation of photon packages from multiple components, and the penetration of radiation through high optical depth barriers. In both cases, the implementation of the relevant algorithms is trivial and does not interfere with any other optimisation techniques. Through simple test models, we demonstrate the general applicability, accuracy and efficiency of the composite biasing approach. In particular, for the penetration of high optical depths, the gain in efficiency is spectacular for the specific problems that we consider: in simulations with composite path length stretching, high accuracy results are obtained even for simulations with modest numbers of photon packages, while simulations without biasing cannot reach convergence, even with a huge number of photon packages.Comment: 12 pages, accepted for publication in A&

Perturbation monte carlo in quantitative photoacoustic tomography

Publisher Copyright: © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.In this work, use of perturbation Monte Carlo is extended to solving inverse problem of quantitative photoacoustic tomography. The approach is demonstrated feasible for estimating optical absorption and scattering distribution.Peer reviewe

Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

The gas kinetic temperature in the centres of starless, high-density cores is predicted to fall as low as 5-6 K. The aim of this study was to determine the kinetic temperature distribution in the low-mass prestellar core Oph D where previous observations suggest a very low central temperature. The densest part of the Oph D core was mapped in the NH3(1,1) and (2,2) inversion lines using the Very Large Array (VLA). The physical quantities were derived from the observed spectra by fitting the hyperfine structure of the lines, and subsequently the temperature distribution of Oph D was calculated using the standard rotational temperature techniques. A physical model of the cores was constructed, and the simulated spectra after radiative transfer calculations with a 3D Monte Carlo code were compared with the observed spectra. Temperature, density, and ammonia abundance of the core model were tuned until a satisfactory match with the observation was obtained. The high resolution of the interferometric data reveals that the southern part of Oph D comprises of two small cores. The gas kinetic temperatures, as derived from ammonia towards the centres of the southern and northern core are 7.4 and 8.9 K, respectively. The observed masses of the cores are only 0.2 M_Sun. Their potential collapse could lead to formation of brown dwarfs or low-mass stars.Comment: Accepted for publication in A&A; 10 pages, 9 figure

The Super-Alfv\'enic Model of Molecular Clouds: Predictions for Zeeman Splitting Measurements

We present synthetic OH Zeeman splitting measurements of a super-Alfvenic model of molecular clouds. We select dense cores from synthetic 13CO maps computed from the largest simulation to date of supersonic and super-Alfvenic turbulence. The synthetic Zeeman splitting measurements in the cores yield a relation between the magnetic field strength, B, and the column density, N, in good agreement with the observations. The large scatter in B at a fixed value of N is partly due to intrinsic variations in the magnetic field strength from core to core. We also compute the relative mass-to-flux ratio between the center of the cores and their envelopes, ${\cal R}_{\mu}$, and show that super-Alfvenic turbulence produces a significant scatter also in ${\cal R}_{\mu}$, including negative values (field reversal between core center and envelope). We find ${\cal R}_{\mu}<1$ for 70% of the cores, and ${\cal R}_{\mu}10$ \muG, 81% have ${\cal R}_{\mu}<1$. These predictions of the super-Alfvenic model are in stark contrast to the ambipolar drift model of core formation, where only ${\cal R}_{\mu}>1$ is allowed.Comment: 4 pages, 6 figures, submitted to ApJL. Minor corrections and additions, updated reference

Gas and dust in the star-forming region ρ Oph A ∗, ∗∗, ∗∗∗: The dust opacity exponent β and the gas-to-dust mass ratio g2d

© ESO, 2015. Aims. We aim at determining the spatial distribution of the gas and dust in star-forming regions and address their relative abundances in quantitative terms. We also examine the dust opacity exponent β for spatial and/or temporal variations. Methods. Using mapping observations of the very dense ρ Oph A core, we examined standard 1D and non-standard 3D methods to analyse data of far-infrared and submillimetre (submm) continuum radiation. The resulting dust surface density distribution can be compared to that of the gas. The latter was derived from the analysis of accompanying molecular line emission, observed with Herschel from space and with APEX from the ground. As a gas tracer we used N<inf>2</inf>H⁺, which is believed to be much less sensitive to freeze-out than CO and its isotopologues. Radiative transfer modelling of the N<inf>2</inf>H⁺ (J = 3-2) and (J = 6-5) lines with their hyperfine structure explicitly taken into account provides solutions for the spatial distribution of the column density N(H<inf>2</inf>), hence the surface density distribution of the gas. Results. The gas-to-dust mass ratio is varying across the map, with very low values in the central regions around the core SM 1. The global average, = 88, is not far from the canonical value of 100, however. In ρ Oph A, the exponent β of the power-law description for the dust opacity exhibits a clear dependence on time, with high values of 2 for the envelope-dominated emission in starless Class -1 sources to low values close to 0 for the disk-dominated emission in Class III objects. β assumes intermediate values for evolutionary classes in between. Conclusions. Since β is primarily controlled by grain size, grain growth mostly occurs in circumstellar disks. The spatial segregation of gas and dust, seen in projection toward the core centre, probably implies that, like C¹⁸O, also N<inf>2</inf>H⁺ is frozen onto the grains