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Ionization

Ionization. 24 March 2003 Astronomy G9001 - Spring 2003 Prof. Mordecai-Mark Mac Low. Photoionization. For hydrogenic ionic stages, the ionization cross-section ( Dopita & Sutherland, 2003). Recombination. σ rec can be derived from σ i using detailed balance.

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Ionization

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

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

  3. Recombination • σrec can be derived from σi using detailed balance. • For hydrogenic ions, the Gaunt factor gmf is evaluated approximately in PPISM

  4. Coronal Ionization Equilibrium • collisional equilibrium between ion stages z and z+1 • Ionization fraction fz = nz/n depends only on T coll. ioniz. rate (z to z+1) rad. recom. rate (z+1 to z)

  5. Ion Distribution in Coronal Equilibrium V VII IX I III II IV VI VIII Dopita & Sutherland 2003

  6. Nebular Ionization Equilibrium • photoionization equilibrium between ion stages z and z+1 • Ionization fraction depends on: photoioniz. rate (z to z+1) rad. recom. rate (z+1 to z)

  7. 10 keV X-ray synch. spectrum Kallman & McCray 1982

  8. 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, σ = 6.3  10-18 cm2, so if n = 1 cm-3 • Thus, density bounded H II regions have sharp edges

  9. 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 ρ: • balance flux of ionizing photons through sphere S(r) against recombinations to levels above ground α(2): • Integrate over r until S(r) = 0 at r = rS

  10. 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 ΔT across front into account.

  11. 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: ρi > ρn D type: ρi < ρn, shock precedes

  12. 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

  13. 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.

  14. 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

  15. Ionized Shell Instability Garcia-Segura & Franco 1996

  16. 10,000 K 100 K

  17. Escape of Ionizing Radiation from Galaxy • Direct measurements (Hα) 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 τ = 3 • about 4% escape fraction • consistent with theoretical model of Domgörgen & Mathis (1994)

  18. Molecular Cloud Ionization Elmegreen 1979 • Cosmic-ray ionization in presence of charged grains (Elmegreen 1979) gives x=ne/n depletion grain-size factor (2-5) cosmic-ray ioniz. rate

  19. 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

  20. 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

  21. 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

  22. Added Routines Stone & Norman 1992b

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

  24. 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

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