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

-50

-25

0

25

50

z / p

c

t = 0MC2 t = 1 Myr t = 2 Myr

-50 -25 0 25y / pc

-50

-25

0

25

z / p

c

t = 3 Myr

-50 -25 0 25y / pc

t = 4 Myr

-50 -25 0 25 50y / pc

t = 5 Myr

-50

-25

0

25

50

z / p

c

t = 0MC1 t = 1 Myr t = 2 Myr

-50

-25

0

25

z / p

c

t = 3 Myr t = 4 Myr

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-1

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

-50 -25 0 25y / pc

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-25

0

25

50

z / p

c

H H2 CO (scaled by 100)Ytot

-50 -25 0 25y / pc

H H2 CO (scaled by 100)

-50 -25 0 25y / pc

H H2 CO (scaled by 100)

-50 -25 0 25 50y / pc

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-1

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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