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
Jan 01, 2016
Monte Carlo Photoionization Simulations of
Diffuse Ionized Gas
Kenneth Wood
University of St Andrews
In collaboration withJohn Mathis, Barbara Ercolano, Ron Reynolds,
Torsten Elwert, Matt Haffner, Greg Madsen
Milky Way’s DIG: Recap
• Wisconsin H 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,…
Line Ratios: Physical Conditions
• [N II]/H increases with height => T increases
• Problem for spherically averaged models
• Extra heating (Reynolds et al. 1999)Haffner et al. (1999)
H [N II]/H
[S II]/H [S II]/[N II]
Scatter Plots
• [N II]/H large where Hfaint
• Note tightness of correlation
Haffner et al. (1999)
[N II]/H H(R)
[S I
I]/H
[N I
I]/H
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)
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 Wood & Mathis (2004)
z (k
pc)
2D Models: Line Ratios
• [N II]/H, [S II]/H increase with height
• Highest energy photons penetrate to high z
• Harder radiation field at large distances from source
Wood & Mathis (2004)
z (k
pc)
Scatter Plots
• 1D models predict tight correlation: each sightline samples same temperature and ionization structure
Elwert & Dettmar (2004)
[N I
I]/H
H(R)
• 2D models show scatter: sightlines probe different temperatures and ionization
• Slope change in [S II]/H – [N II]/H: interfaces, not seen in Milky Way’s DIG
Wood & Mathis (2004)
Scatter Plots: 3D Structure?
• Multiple sources with different spectra
• 3D Density structure
• Strategy: Planar emission at z = 0Repeating boundaries in x, ySmooth and two-phase densitiesVary Q, n(z) to fit H(z)
• What is [N II]/H, extra heating?
Two Phase Density
• Dense grid cells with filling factor 0.01 < ff < 1
• Minimum “clump” size set by grid resolution
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]/H too low at large H
Clumpy Models
• Decrease ff => lower U => less N++ => higher [N II]/H
ff =80% ff =40%
ff =10% ff =5%
Summary
• Photoionization heating explains most line ratios
• Extra heating ~ 10-25 ne erg cm-3 s-1 for largest line ratios, [N II]/H
• Smooth models: too low [N II]/Hat large H• Clumpy models with ff ~ 0.2 look good• Caution: 3D Toy Model!
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…