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Monte Carlo Photoionization Simulations of Diffuse Ionized Gas

Monte Carlo Photoionization Simulations of Diffuse Ionized Gas. Kenneth Wood University of St Andrews. In collaboration with John Mathis, Barbara Ercolano, Ron Reynolds, Torsten Elwert, Matt Haffner, Greg Madsen. Milky Way’s DIG: Recap. Wisconsin H a Mapper: DIG everywhere

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Monte Carlo Photoionization Simulations of Diffuse Ionized Gas

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  1. Monte Carlo Photoionization Simulations of Diffuse Ionized Gas Kenneth Wood University of St Andrews In collaboration with John Mathis, Barbara Ercolano, Ron Reynolds, Torsten Elwert, Matt Haffner, Greg Madsen

  2. Milky Way’s DIG: Recap • Wisconsin Ha Mapper: DIG everywhere • n(z) ~ 0.2 exp(-|z|/H) cm-3, ff ~ 0.2 • H ~ 1 kpc, if isothermal T ~ 8000 K Reynolds, Tufte, Haffner, Madsen,…

  3. Line Ratios: Physical Conditions [N II]/Ha Ha • [N II]/Ha increases with height => T increases • Problem for spherically averaged models • Extra heating (Reynolds et al. 1999) [S II]/[N II] [S II]/Ha Haffner et al. (1999)

  4. Scatter Plots • [N II]/Ha large where Ha faint • Note tightness of correlation [N II]/Ha [S II]/Ha [N II]/Ha Ha (R) Haffner et al. (1999)

  5. Need 3D Photoionization Codes

  6. Monte Carlo PhotoionizationWood, Mathis, & Ercolano (2004) • 3D density structure and radiation transfer • Ions: H, He, C, N, O, Ne, S • Stellar and diffuse photons in Cartesian grid • Input: ionizing spectrum from source(s) • Output: 3D temperature & ionization structure • Emissivities, emission line maps, line ratios • See also: Och, Lucy, Rosa (1998) Ercolano et al. (2003)

  7. Lexington H II Benchmarks • T*=40000K, Q(H)=4.26E49 s-1, n(H)=100 cm-3 + Monte Carlo ( CLOUDY

  8. 2D Ionization & Temperature • Point source, Q = 6 1049 s-1, n(z) ~ exp(-|z|/H) • Slices through grid in x-z plane • Temperature rises away from source z (kpc) Wood & Mathis (2004)

  9. 2D Models: Line Ratios z (kpc) • [N II]/Ha, [S II]/Ha increase with height • Highest energy photons penetrate to high z • Harder radiation field at large distances from source Wood & Mathis (2004)

  10. Scatter Plots • 1D models predict tight correlation: each sightline samples same temperature and ionization structure [N II]/Ha Ha (R) Elwert & Dettmar (2004)

  11. 2D models show scatter: sightlines probe different temperatures and ionization • Slope change in [S II]/Ha – [N II]/Ha: interfaces, not seen in Milky Way’s DIG Wood & Mathis (2004)

  12. Scatter Plots: 3D Structure? • Multiple sources with different spectra • 3D Density structure • Strategy: Planar emission at z = 0 Repeating boundaries in x, y Smooth and two-phase densities Vary Q, n(z) to fit Ha(z) • What is [N II]/Ha, extra heating?

  13. Two Phase Density • Dense grid cells with filling factor 0.01 < ff < 1 • Minimum “clump” size set by grid resolution

  14. Smooth Model • n(z) = 0.1 exp(-|z|/1.3) • Large Q to ionize grid: high ionization parameter • N mostly N++ at low z: [N II]/Ha too low at large Ha

  15. Clumpy Models ff =80% ff =40% • Decrease ff => lower U => less N++ => higher [N II]/Ha ff =10% ff =5%

  16. Summary • Photoionization heating explains most line ratios • Extra heating ~ 10-25ne erg cm-3 s-1 for largest line ratios, [N II]/Ha • Smooth models: too low [N II]/Ha at large Ha • Clumpy models with ff ~ 0.2 look good • Caution: 3D Toy Model!

  17. Future Work • More 3D models: lots of parameter space • Apply this to WHAM B star H II regions • Constrain models with additional WHAM data: [S II], [O I], [O II], [O III], He I • Need S dielectronic recombination rates • Merge 3D photoionization with MHD…

  18. Future Work • Take 3D density from MHD simulation • 3D ionization simulation log n Density from De Avillez & Berry (2001)

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