Modelling the chemistry and dynamics of molecular clouds ...ism2017/Download... · Modelling...
Transcript of Modelling the chemistry and dynamics of molecular clouds ...ism2017/Download... · Modelling...
Modelling the chemistry and dynamics of molecular clouds:
!What is needed?
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ISM 2017 conference Cologne, 16.2.2017
!Daniel Seifried
I. Physikalisches Institut, University of Cologne
Modelling molecular clouds
■ Numerous work on molecular cloud (MC) evolution
■ However:
■ often clouds in isolation, pre-assembled spheres
■ lack of galactic environment
■ lack of self-consistent formation
■ Chemical networks not yet always included
!■ Recently, more and more focus on chemical modeling of MC1
■ What spatial resolution is needed to model H2, CO properly?
■ What resolution is needed to reproduced dynamics (linewidth) and structure (filaments) reliable? (remember the discussion after Mordecai’s talk)
!■ Many studies lack a detailed investigates of these resolution effect?!
Modelling molecular clouds
■ Aims of this work:
!■ Model formation of MCs out of the diffuse ISM
■ include galactic environment
■ include chemical network for H2 and CO (Nelson & Langer 1997, Glover et al. 2010)
■ high (<< 1 pc) spatial resolution
■ explore resolution effects on ■ structural
■ dynamical and
■ chemical properties
Numerics
■ Simulations based on the SILCC simulations (Walch et al. 2015, Girichidis et al. 2016)
■ part of a galactic disk
!■ FLASH code including:
■ Network for H2 and CO chemistry
■ Supernova driving
■ „base grid“ resolution: 4 pc
Zoom-in procedure
■ We select regions in which MCs are about to form (about 50-100 pc in size)
■ We increase the resolution from 4 pc to 0.12 pc
■ Increase occur progressively over time (1.5 Myr)
Zoom-in procedure
■ We select regions in which MCs are about to form (about 50-100 pc in size)
■ We increase the resolution from 4 pc to 0.12 pc
■ Increase occur progressively over time (1.5 Myr)
Zoom-in procedure
■ We select regions in which MCs are about to form (about 50-100 pc in size)
■ We increase the resolution from 4 pc to 0.12 pc
■ Increase occur progressively over time (1.5 Myr)
Overview
■ 2 molecular cloud complexes
!■ Complex shaped molecular clouds
■ Highly fragmented
■ Filamentary structure
■ Large cavities created by supernovae
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z / p
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t = 0MC2 t = 1 Myr t = 2 Myr
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t = 3 Myr
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t = 4 Myr
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t = 5 Myr
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t = 0MC1 t = 1 Myr t = 2 Myr
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t = 3 Myr t = 4 Myr
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0log(Y / g cm-2)
t = 5 Myr
Chemical composition
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!■ Extended envelope of atomic H
■ H2 surrounding CO
■ CO in densest gas with n > 300 cm-3
■ H2 content extended into low-density regions
■ Significant amount of CO-dark gas
H H2 CO (scaled by 100)Ytot
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H H2 CO (scaled by 100)Ytot
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H H2 CO (scaled by 100)
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H H2 CO (scaled by 100)
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0log(Y / g cm-2)
H2 content
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■ H2 (red line) present in lower density gas n < 30 cm-3 (blue line) ( see talk V. Valdivia this morning)
■ H2 formation time scale ~ 30 Myr >> simulated time (a few Myr) ■ Turbulent mixing from dense regions into low-density gas
■ Chemical postprocessing NOT possible for H2
■ H2 shielding also affects CO formation (Glover et al. 2010) -> postprocessing for CO?
Resolution effects I. - Structure
■ H2 (red line) present in lower density gas n < 30 cm-3
■ Evolution time of a few Myr << H2 formation time scale (about 30 Myr)
■ Turbulent mixing from dense regions into low-density gas
■ Chemical postprocessing not possible for H2
■ H2 shielding also affects CO formation -> postprocessing for CO
■ we
!■ Filamentary structure only visible for dx ≤ 0.24 pc
■ For dx ≥ 0.5 pc, clouds rather clumpy and filamentary structure barely resolved
■ Degree of fragmentation increases with resolution
Resolution effects II. - Dynamics
■ dx = 4 pc ■ resolution insufficient in all
aspects
■ dx = 1 pc ■ significant deviations
■ dx = 0.5 pc ■ velocity dispersion off by
about 30%
■ dx ≤ 0.24 pc ■ good agreement (deviations <
5%)
Resolution effects III. - Chemistry
■ CO fraction below H2 fraction ■ CO-dark gas
!■ dx = 1 - 4 pc
■ H2 and CO underrepresented by up to 40%
■ dx = 0.5 pc ■ abundances not converged
■ deviations ~ 10%
■ dx ≤ 0.24 pc ■ good agreement
(deviations < 5%)
Resolution effects
■ Intermediate summary:
■ dx ≥ 1 pc (in SPH: ≥ 3 Msun): not sufficient in any aspect
■ dx ≤ 0.25 pc (in SPH: ≤ 0.6 Msun): Convergence of structural, dynamical, and chemical properties
Resolution effects VI.
!!!!!!!
!!!■ Two simulations (identical conditions)
■ in the left the resolution is increased slower
■ final resolution identical
Giant disks?■ Giant (5 - 10 pc) disk in the low-
resolution run
■ Not present in the high-resolution run
■ Observations rare
■ ⇒ disk unphysical?
!■ ⟺ Seen in a couple recent
simulations of molecular cloud formation, galactic disk evolution
■ Unavoidable due to limited computing power/resolution
■ Careful when interpreting
■ avoided by resolving turbulent colliding flows
Zoom-in time■ So far progressive increase of resolution over
1.5 Myr
!■ If too slow:
■ large, rotating disks
■ delayed collapse (less mass, less H2, less CO)
Zoom-in time■ So far progressive increase of resolution over
1.5 Myr
!■ If too slow (green line):
■ large, rotating disks
■ delayed collapse (less mass, less H2, less CO)
■ If too fast: ■ grid artefacts
■ cells need time to adapt to higher resolution
!■ As a rule of thumb:
■ faster than the free-fall time
■ a few 100 timestep between each refinement
Conclusions
■ Simulations for formation of MCs out of diffuse ISM ■ including galactic environment
■ combining chemistry and very high spatial resolution
■ Zoom-in on selected regions with dx = 0.12 pc
!■ Complex shaped, filamentary, fragmented clouds
■ H2 extended into low-density gas by turbulent mixing ■ Chemical post processing questionable
!■ Resolution of ≥ 1 pc insufficient to model structural, dynamical, and chemical
cloud properties
■ Resolution of ≤ 0.25 pc or better required for convergence
■ Effect of zoom-in (refinement time) has to be explored carefully
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Thank you for your attention!
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