Efficient Monte Carlo continuum radiative transfer with SKIRT
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Transcript of Efficient Monte Carlo continuum radiative transfer with SKIRT
Efficient Monte Carlo Efficient Monte Carlo continuum radiative transfer continuum radiative transfer
with SKIRTwith SKIRT
Maarten Baes
2nd East-Asia Numerical Astrophysics Meeting, Daejeon, Korea
3 November 2006
Brussels
Why continuum radiative transfer…
• the ISM is extremely dusty
Detailed continuum radiative transfer simulations are
necessary to investigate the effect of dust on observable
properties of all dusy systems…
• dust strongly affects the radiation field at all wavelengths
- X-ray: scattering - UV and optical: extinction - IR and submm: emission
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
• 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
• the radiation field is reconstructed by classifying the photons by position, propagation direction, wavelength…
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 !?
Monte Carlo radiative transfer
SKIRT
• Stellar Kinematics Including Radiative Transfer
• allows all geometries for sources and sinks: dust cells
• several dust cell geometries: spherical, cylindrical, cuboidal,…
Steinacker et al. 2003
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
- partly polychromatic 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
• LTE radiative transfer:
- radiative equilibrium: energy absorbed = energy emitted
- the absorbed energy determines the dust temperature• frequency distribution adjustment technique
Bjorkman & 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
Ivezić et al. (1997) benchmark tests
- star + spherical envelope
- V-band optical depths 1-1000
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
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
• “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
• question: how do the temperature distribution and the emerging radiation field depend on the structure of the circumstellar medium ?
Application 1: Circumstellar discs
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.
density temperature
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
Optical depth
Bu
lge lu
min
osity
Optical depth B
ulg
e lu
min
osity
Application 2: spiral galaxy atlas
R-band images Spitzer MIPS 160 μm
images
Conclusions
• 2 modes: LTE and KINE
• SKIRT = efficient 3D Monte Carlo radiative transfer code
• 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 2004Japan
EANAM 2006Korea
EANAM 2008China
EANAM 2010Iran ?
EANAM 2012Belgium!
See you there !