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Efficient Monte Carlo continuum radiative transfer with SKIRT. Maarten Baes. 2 nd East-Asia Numerical Astrophysics Meeting, Daejeon, Korea 3 November 2006. Brussels. Why continuum radiative transfer…. the ISM is ex t remely dus t y.

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Efficient monte carlo continuum radiative transfer with skirt
Efficient Monte Carlo continuum radiative transfer with SKIRT

Maarten Baes

2nd East-Asia Numerical Astrophysics Meeting, Daejeon, Korea3 November 2006



Why continuum radiative transfer…

  • the ISM is extremely dusty

  • dust strongly affects the radiation field at all wavelengths - X-ray: scattering - UV and optical: extinction - IR and submm: emission

Detailed continuum radiative transfer simulations are necessary to investigate the effect of dust on observable properties of all dusy systems…


Radiative transfer equation

  • we take into account the effects of - extinction

  • condition of thermal equilibrium:


Radiative transfer equation

  • we take into account the effects of - extinction - multiple anisotropic scattering

  • condition of thermal equilibrium:


Radiative transfer equation

  • we take into account the effects of - extinction - multiple anisotropic scattering - thermal dust re-emission, assuming thermal equilibrium

  • condition of thermal equilibrium:


Radiative transfer equation

  • we take into account the effects of - extinction - multiple anisotropic scattering - thermal dust re-emission, assuming thermal equilibrium - multiple dust grain populations

  • condition of thermal equilibrium:


Monte carlo radiative transfer

  • the radiation field is reconstructed by classifying the photons by position, propagation direction, wavelength…

Monte Carlo radiative transfer

  • probabilisitic technique >< deterministic technique

  • RT simulations in which a large number of photons are followed individually through the dusty medium

  • the trajectory of each photon is determined by (pseudo) random numbers


Monte carlo radiative transfer1
Monte Carlo radiative transfer

  • ADVANTAGES

  • conceptually simple, natural treatment of emission, absorption and scattering

  • all geometries possible (3D simulations)

  • rather economic in memory → large grids are possible

  • very flexible: multiple anisotropic scattering, polarization, kinematics, dust clumping…

  • DISADVANTAGES

  • Poisson noise

    • error analysis is difficult

    • accuracy goes as N-1/2→ efficiency !?


Skirt

Steinacker et al. 2003

SKIRT

  • Stellar Kinematics Including Radiative Transfer

  • allows all geometries for sources and sinks: dust cells

  • several dust cell geometries: spherical, cylindrical, cuboidal,…


Skirt1
SKIRT

  • strongly optimized through the use of deterministic elements

    • forced (first) scattering Witt 1977

    • peeling-off technique Yusef-Zadeh et al. 1984

    • continuous absorption Lucy 1999

    • partlypolychromatic photon packages Baes 2006, MNRAS, submitted

  • computing power: dedicated cluster with 16 x 2 Gb memory

  • two major modes:

    • LTE → modelling the dust temperature distribution and the SED of dusty systems

    • KINE → modelling the observed kinematics of dusty galaxies


Skirt in lte mode
SKIRT in LTE mode

  • LTE radiative transfer:

    • radiative equilibrium: energy absorbed = energy emitted

    • the absorbed energy determines the dust temperature

  • frequency distribution adjustment techniqueBjorkman & Wood 2001 Baes et al. 2005, NewA, 10, 523

    • no iteration is necessary

    • immediate re-emission: guaranteed flux conservation

    • works with all optical depths

  • polychromatic photon packages: very efficient


1d benchmark tests
1D benchmark tests

  • Ivezić et al. (1997) benchmark tests

    • star + spherical envelope

    • V-band optical depths 1-1000


Polychromatic photon packages
Polychromatic photon packages

(re-)emissioneach photon package initially contains photons of all wavelengths

exitif it leaves the galaxy: contribution to the SED at all wavelengths

scatteringloss of polychromatism

minimal computational overheadsignificant gain in efficiency

Baes 2006, MNRAS, submitted


2d benchmark tests
2D benchmark tests

  • Pascucci et al. (2004) benchmark tests

    • star + axisymmetric envelope

    • V-band optical depths 0.1-100


SKIRT 2D

benchmark

SKIRT 3D vs benchmark


Efficiency of monte carlo rt
Efficiency of Monte Carlo RT

  • “common wisdom” about Monte Carlo RT: numerically demanding

  • comparison between SKIRT and other codes used in Pascucci et al.

SKIRT 2D: 2.5 MBySKIRT 3D: 58 MBy

MC RT codes can be very efficient when modern optimization techniques are used.

Limited memory usage is extra advantage when moving to 3D

Baes 2006, MNRAS, submitted


Application 1: Circumstellar discs

  • large homogeneous survey of post-AGB stars - they all seem to be binary systems - they have a MIR excess due to dust starting at the sublimation temperature - MIR-submm SED and VLTI data suggest circumbinary discs


Application 1: Circumstellar discs

  • question: how do the temperature distribution and the emerging radiation field depend on the structure of the circumstellar medium ?

density temperature

We can see some systematic effects, but in general the structure of the dust temperature distribution is rather insensitive to the structure of the ISM.


Application 2: spiral galaxy atlas

  • simulation of a large set of spiral galaxy models

    • - images at various inclinations and passbands - global and spatially resolved spectral energy distributions- attenuation maps- dust temperature distributions

  • scientific goals

  • - investigate the systematic effects of physical parameters on the observables (luminosity, dust content, bulge-to-disc ratio, inclination…)

  • - construct an optimized galaxy dust mass estimator for IRAS, Spitzer, Akari,… data

  • - provide a database for statistical / cosmological applications


Application 2: spiral galaxy atlas

Optical depth

Bulge luminosity

Bulge luminosity

Optical depth


R-band images

Spitzer MIPS 160 μm images


Conclusions
Conclusions

  • SKIRT = efficient 3D Monte Carlo radiative transfer code

  • 2 modes: LTE and KINE

  • uses efficient optimization techniques

  • reproduces the 1D and 2D benchmark test easily

  • ready to go….

    • - models for circumbinary discs around post-AGB stars - atlas of dusty spiral galaxy models - simulations of accretion discs in the centre of AGNs - kinematics of dusty galaxies and galactic nuclei

- your radiative transfer problem ???


Thank you…

EANAM 2008China

EANAM 2004Japan

EANAM 2012Belgium!

EANAM 2006Korea

EANAM 2010Iran ?

See you there !


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