3D-PDR3D-PDR: A new three-dimensional radiative : A new three-dimensional radiative transfer and transfer and astrochemistry code for treating astrochemistry code for treating
Photodissociation RegionsPhotodissociation Regions
Thomas G. BisbasThomas G. Bisbas
University College LondonUniversity College LondonHarvard-Smithsonian Centre for AstrophysicsHarvard-Smithsonian Centre for Astrophysics
With thanks to “Friends for
Astronomy Group”, Thessaloniki
Greece
http://ww.ofa.gr
© Kallias Ioannidis
Diffusive Nebulae
© Markos Aspridis
The Orion Nebula (M42)
Diffusive Nebulae
© Kallias Ioannidis
The California Nebula (NGC 1499)
Planetary Nebulae
© Vlachos Paul
© Vlachos Paul
The Dumbbell Nebula (M27)
Ring Nebula (M57)
Supernova Remnants
© Giaourtsis Theodoros
The Crab Nebula (M1)
Supernova Remnants
© Giaourtsis Theodoros
The Jellyfish Nebula (IC 443)
Structure of ionized regions
© Kallias IoannidisThe Elephant Trunk Nebula
Telescope: AT65QCamera: QHY-9Filters: Hα, OIII, SII, R, G, B.Exp.Time: 12h
Structure of ionized regions
© Kallias Ioannidis
Ionized gas
Massive Stars
Ionization Front
Molecular Gas
Molecular Gas
Ionized region
© Kallias Ioannidis
Emission of ionizing photons (hν > 13.6eV)
Increase the temperature to T ~ 104 K.
Free e- and p+.
MOCASSIN (Ercolano et al. 2003; 2005; 2008) is a three-dimensional algorithm to calculate the chemistry in the interior of an HII region.
Uses Monte-Carlo method.
Telescope: AT65QCamera: QHY-9Filters: Ha,OIII,SIIExp. Time: 390min
Photodissociation Regions (PDR)Neutral regions of the interstellar medium in which the FUV photons strongly influence the gas chemistry and act as the most important source of heat.
PDRs occur in any region of the ISM that is dense and cold enough to remain neutral but has too low column density to prevent the penetration of FUV photons.
PDRs are located in the edge of the HII regions, where the temperature drops very abruptly from T ~ 104 – 10 K.
As the binding energy of the H2 molecule is lower than that of the hydrogen atom, HII
regions are enveloped by a region of atomic hydrogen.
In this region UV is great enough to photodissociate H2 but the recombination rate is
high enough to keep the ionized fraction low.
Deeper in the cloud, UV has been sufficiently attenuated, such that most hydrogen is bound to H
2.
Photodissociation Regions (PDR)
Orion Bar
© Vlachos Paul
© Kallias Ioannidis
Red: emission in CO 1-0 transition
Green: emission in 1-0 S(1) H
2 line
Blue: PAH emission
Tielens et al. (1993)
Photodissociation Regions (PDR)
PDRs are the regions where star formation occurs.
Theoretical studies of PDRs have been done using one-dimensional codes. The chemical structure is very complicated. Effort has been made in understanding the PDRs using one-dimensional astrochemical codes which are able to treat such complicated chemical networks.
However the interstellar medium has been observed to be irregular and to contain many clouds. Therefore, a three-dimensional astrochemistry code is needed in order to examine this arbitrary density distribution.
We have worked in this direction and we have implemented the first three-dimensional code (3D-PDR) which is able to handle such irregular structures.
© Kallias Ioannidis
Dynamical evolution
© Kallias Ioannidis
© Kallias Ioannidis
© Kallias Ioannidis
Offset2.aviBisbas et al. (2009) A&A, 497, 649
Smoothed Particle Hydrodynamics
Triggered Star Formation
Proplyds. Molecular gas surviving UV radiation.
Shock - compressed molecular gas
EGGs
© Kallias Ioannidis
Sequence of Star Formation in the NGC 281 (Pacman Nebula).
Triggered Star Formation – Radiation Driven Implosion
MolecularClump
UV radiation
Cometary tail formed due to compression and re-expansion
Bisbas et al. (2011) ApJ, 736, 142sim2.avi
Bisbas et al. (2011) ApJ, 736, 142
Triggered Star Formation – Radiation Driven Implosion
Modelling Photodissociation Regions: The 3D-PDR code
Overview of the code
The 3D-PDR code uses the chemical model features of the fully benchmarked one-dimensional code UCL_PDR (Bell et al. 2006).
It solves the chemistry and the thermal balance self-consistently within a given three-dimensional cloud of arbitrary density distribution.
The code uses a ray-tracing scheme based on the HEALPix package to calculate the total column densities and thus to evaluate the attenuation of the far-ultraviolet (FUV) radiation into the region, and the propagation of the FIR/submm line emission out of the region.
An iterative cycle is used to calculate the cooling rates using a three-dimensional escape probability method, and heating rates.
At each element within the cloud, it performs a depth- and time- dependent calculation of the abundances for a given chemical network to obtain the column densities associated with each individual species.
The iteration cycle terminates when the PDR has obtained thermodynamical equilibrium, in which the thermal balance criterion is satisfied i.e. the heating and cooling rates are equal to within a user-defined tolerance parameter.
Rays Level of refinement
HEALpix (Gorski et al. 2005, ApJ, 622, 759)
Modelling Photodissociation Regions: The 3D-PDR code
Modelling Photodissociation Regions: The 3D-PDR code
Treatment of the UV radiation field
A realistic treatment of the UV radiation field is needed in order to account for the complicated irregular structures in a three-dimensional HII region. This can be accomplished using the MOCASSIN code to calculate the interior of the ionized region, to obtain a realistic temperature profile and the abundances of species of the given chemical network.
However, we will simplify the calculations by adopting an exponential factor to scale the UV field, neglecting the contribution due to the diffusive radiation and backscattering.
The interstellar radiation field for a given element p(x,y,z) will thus be calculated using the equations:
Modelling Photodissociation Regions: The 3D-PDR code
Gas heating
We account for photoelectric ejections of electrons from dust grains; PAHs; Collisional de-excitation of vibrationally excited H
2 following
FUV pumping; Photoionization of neutral carbon and the energy liberated by the grain surface formation of H
2; Cosmic rays; Turbulence.
Gas Cooling
The gas cooling occurs primarily by the collisional excitation and subsequent emission of a number of key atomic and molecular species. We account for CII, CI, and OI emission, and the rotational transitions of CO
© Vlachos Paul
Modelling Photodissociation Regions: The 3D-PDR code
3D escape probability method
The escape probability method (de Jong, Dalgarno, & Chu 1975, ApJ, 199, 69) describes the probability that a photon of frequency ν
ij escapes from the element
p(x,y,z) without interacting with the rest of the cloud.
An escaping electron cools down the given cloud element since it carries energy.
The three-dimensional approach uses the HEALPix algorithm to calculate each individual escape probability per direction.
Analytical Numerical
Modelling Photodissociation Regions: The 3D-PDR code
Chemistry involved
The code determines the relative abundances of a limited number of atomic and molecular species at each cloud element, by solving the time-dependent chemistry of a self-contained network of formation and destruction reactions.
We use the UMIST database containing 33 species (including e-) and 320 reactions.
We solve for steady-state chemistry (chemical evolution time set to t=100Myr), although the code is able to follow the full time dependent evolution of chemistry within the cloud.
It is thus a powerful tool when one moves to dynamically evolved simulations.
© Giaourtsis Theodoros
3D-PDR: Benchmarking
3D-PDR: Application 1
Uniform density sphere
UV
Very good agreement with the 1D codes for all models examined
n=103 cm-3, χ=10 Draine
3D-PDR: Application 2
Multi-UV field application
Uniform density sphere
UV
n=5x103 cm-3, χISO
=150 Draine, χUNI
=2000 Draine Cross section of the surface temperature
K k
m /
s
Emission maps for [CII] 158 μm, [CI] 610 μm, [OI] 63 μm, and CO(1-0)
3D-PDR: Application 2Multi-UV field application
RGB composite image for CO(1-0), [CI], [CII] emission maps. The values correspond to the [CII] emission map.RGB colour bar ratios of 5:1:10 for CO(1-0):[CI]:[CII].
3D-PDR: Application 2Multi-UV field application
Simulation using Smoothed Particle Hydrodynamics
RDC.avi
Snapshot at t=0.12Myr from the SPH simulation (see the movie)
3D-PDR: Application 3Radiation Driven Implosion
Emission maps for [CII] 158 μm, [CI] 610 μm, [OI] 63 μm, and CO(1-0)
K k
m /
s
3D-PDR: Application 3Radiation Driven Implosion
3D-PDR: Application 3Radiation Driven Implosion
RGB composite image for CO(1-0), [CI], [CII] emission maps. The values correspond to the [CII] emission map.RGB colour bar ratios of 8:1:2 for CO(1-0):[CI]:[CII].
3D-PDR: Projects we are currently developing(with J. Drake, N. Wright & B. Ercolano)
Column density plotRGB synthetic image (CII, CI, CO)
Simulating the PDR of pillar-like structures (see Ercolano et al. 2012, MNRAS, 420, 141).
MOCASSIN synthetic (Hα, OIII)
Simulating PDR and XDR (X-ray dominated region) of disks.
3D-PDR: Projects we are currently developing (with J. Drake, N. Wright & B. Ercolano)
Conclusions
Structure of nebulae (ionized medium, Photodissociation regions)
Triggered star formation in ionized nebulae
Radiation Driven Implosion
3D-PDR code: modelling PDRs in three-dimensions for the first time HEALPix based escape probability method Benchmarking tests agree with the rest of 1D codes Applications in 3D show that 1D codes might not able to simulate properly the clumpy ISM. Test using an SPH snapshot
Future tasks include the coupling of 3D-PDR with MOCASSIN to treat as realistically as possible the irregular structures observed in the Interstellar Medium.