Ionization

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Ionization

• 24 March 2003
• Astronomy G9001 - Spring 2003
• Prof. Mordecai-Mark Mac Low

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Photoionization

• For hydrogenic ionic stages, the ionization
  cross-section (Dopita Sutherland, 2003)

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Recombination

• srec can be derived from si using detailed
  balance.
• For hydrogenic ions, the Gaunt factor gmf is
  evaluated approximately in PPISM

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Coronal Ionization Equilibrium

• collisional equilibrium between ion stages z and
  z1
• Ionization fraction fz nz/n depends only on T

coll. ioniz. rate (z to z1)
rad. recom. rate (z1 to z)
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Ion Distribution in Coronal Equilibrium
IX
VII
V
I
III
II
IV
VI
VIII
Dopita Sutherland 2003
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Nebular Ionization Equilibrium

• photoionization equilibrium between ion stages z
  and z1
• Ionization fraction depends on

photoioniz. rate (z to z1)
rad. recom. rate (z1 to z)
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10 keV X-ray synch. spectrum
Kallman McCray 1982
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H II Regions

• Observed H II regions limited
• ionization bounded all photons contained
• density bounded all atoms ionized
• The optical depth for ionization is tiny.
• At edge, s 6.3 ? 10-18 cm2, so if n 1 cm-3
• Thus, density bounded H II regions have sharp
  edges

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Strömgren Spheres

• Two components to local ionizing flux near a star
• direct ionizing flux
• diffuse flux from recombinations to ground state
• Calculate radius of ionized sphere in uniform ?
• balance flux of ionizing photons through sphere
  S(r) against recombinations to levels above
  ground a(2)
• Integrate over r until S(r) 0 at r rS

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Dynamical Solutions

• Temperature increases upon photoionization
• Resulting pressure differential can only be
  equalized by expansion of photoionized gas
• When pressures balance, photoionized gas far less
  dense than neutral gas
• Propagation of ionization front can be calculated
  by examining conservation equations, taking ?T
  across front into account.

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in the frame of the ionization front

• flux of photons at front
• mean mass per ion

Solving mass momentum conservation
for isothermal gas, we find
For this to have a real solution, either
R type ?i gt ?n
D type ?i lt ?n, shock precedes
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Stages of Growth

• Ultracompact
• less than 10, associated with young stars
• Compact
• more evolved, but still not nebular
• Standard
• single stars or groups, show structure
• Giant
• OB associations, early stages of superbubbles

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Ultracompact HII Regions

• Defined to be less than about 10 in size
• Should be rare if H II regions expand at roughly
  10 km/s
• Wood Churchwell (1989) found 10x more than
  expected.

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Confinement

• Three major mechanisms proposed
• Bow shocks (Van Buren et al. 1990) ram pressure
  of motion confines cometary regions.
• Disk photoevaporation (Hollenbach et al. 1994)
  dense disk provides mass source for core-halo
• Pressure confinement (García-Segura Franco,
  1996) self-gravity increases core pressure,
  confining very small regions

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Ionized Shell Instability
Garcia-Segura Franco 1996
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10,000 K
100 K
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Escape of Ionizing Radiation from Galaxy

• Direct measurements (Ha) require a screen
• High velocity clouds (Tufte et al. 1998,
  Bland-Hawthorn et al. 1998)
• Magellanic Stream gas (Weiner Williams 1996)
• Optical depth to ionizing radiation t 3
• about 4 escape fraction
• consistent with theoretical model of Domgörgen
  Mathis (1994)

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Molecular Cloud Ionization
Elmegreen 1979

• Cosmic-ray ionization in presence of charged
  grains (Elmegreen 1979) gives xne/n

depletion
grain-size factor (2-5)
cosmic-ray ioniz. rate
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Assignments

• Flashcode registration
• Read sections 1-4 (quick start), and start
  looking at rest of manual
• Read sections I, II, VII, and one other (to be
  summarized) of Hollenbach Tielens, Rev. Mod.
  Phys. 1999, 71, 173

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MHD Courant Condition

• Similarly, the time step must include the fastest
  signal speed in the problem either the flow
  velocity v or the fast magnetosonic speed vf2
  cs2 vA2

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Lorentz Forces

• Update pressure term during source step
• Tension term drives Alfvén waves
• Must be updated at same time as induction
  equation to ensure correct propagation speeds
• operator splitting of two terms

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Added Routines
Stone Norman 1992b
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Flashcode History

• Politics
• world historical
• political
• internal
• Components
• Coding philosophy
• spaghetti (Fortran IV/66)
• structured (Fortran 77)
• modular (Fortran 90)

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Flashcode Structure

• setup preprocessor script
• reads configuration files
• setups/ - problem specific
• Config file to generate Modules
• init_block.F90 to initialize one block
• source/sites/ - location specific
• sets up makefiles and code for a particular
  problem, and a particular site.
• Compilation with gmake (can be parallel)
• Runtime input parameters in flash.par