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Model Atmosphere Results (Kurucz 1979, ApJS, 40, 1)

Model Atmosphere Results (Kurucz 1979, ApJS, 40, 1). Kurucz ATLAS LTE code Line Blanketing Models, Spectra Observational Diagnostics. ATLAS by Robert Kurucz (SAO). Original paper and updated materials (kurucz.cfa.harvard.edu) have had huge impact on stellar astrophysics

brent-vinson
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Model Atmosphere Results (Kurucz 1979, ApJS, 40, 1)

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  1. Model Atmosphere Results(Kurucz 1979, ApJS, 40, 1) Kurucz ATLAS LTE codeLine BlanketingModels, SpectraObservational Diagnostics

  2. ATLAS by Robert Kurucz (SAO) • Original paper and updated materials (kurucz.cfa.harvard.edu) have had huge impact on stellar astrophysics • LTE code that includes important continuum opacity sources plus a statistical method to deal with cumulative effects of line opacity (“line blanketing”) • Other codes summarized in Gray

  3. ATLAS Grid • Teff = 5500 to 50000 KNo cooler models since molecular opacities largely ignored.Models for Teff > 30000 K need non-LTE treatment (also in supergiants) • log g from main sequence to lower limit set by radiation pressure (see Fitzpatrick 1987, ApJ, 312, 596 for extensions) • Abundances 1, 1/10, 1/100 solar

  4. Line Blanketing and Opacity Distribution Functions • Radiative terms depend on integrals • Rearrange opacity over interval:DF = fraction of interval with line opacity < ℓν • Same form even with many lines in the interval

  5. ODF Assumptions • Line absorption coefficient has same shape with depth (probably OK) • Lines of different strength uniform over interval with near constant continuum opacity (select freq. regions carefully)

  6. ODF Representation • DF as step functions • Pre-computed for grid over range in: temperatureelectron densityabundancemicroturbulent velocity(range in line opacity) T = 9120 K

  7. Line Opacity in Radiation Moments

  8. Atmospheric Model Listings • Tables of physical and radiation quantities as a function of depth • All logarithms except T and 0 (c.g.s.)

  9. Emergent Fluxes (+ Intensities)

  10. Temperature Relation with Line Blanketing • With increased line opacity, emergent flux comes from higher in the atmosphere where gas is cooler in general; lower Iν, Jν • Radiative equilibrium: lower Jν → lower T • Result: surface cooling relative to models without line blanketing

  11. Temperature Relation with Line Blanketing • To maintain total flux need to increase T in optically thick part to get same as gray case • Result: backwarming

  12. Flux Redistribution (UV→optical):opt. Fν ~ hotter unblanketed model

  13. Temperature Relation with Convection • Convection: • Reduces T gradient in deeper layers of cool stars

  14. Geometric Depth Scale • Physical extent large in low density cases (supergiants)

  15. Observational Parameters • Colors: Johnson UBVRI, Strömgren ubvy (Lester et al. 1986, ApJS, 61, 509) • Balmer line profiles (Hα through Hδ)

  16. Flux Distributions • Wien peak • Slope of Paschen continuum (3650-8205) • Lyman jump at 912 (n=1)Balmer jump at 3650 (n=2)Paschen jump at 8205 (n=3) • Strength of Balmer lines

  17. H 912 He I 504 He II 227

  18. Comparison to Vega

  19. IDL Quick Look • IDL> kurucz,teff,logg,logab,wave,flam,fcontINPUT: • teff = effective temperature (K, grid value) • logg = log gravity (c.g.s, grid value) • logab = log abundance (0,-1,-2)OUTPUT: • wave = wavelength grid (Angstroms) • flam = flux with lines (erg cm-2 s-1 Angstrom-1) • fcont = flux without lines • IDL> plot,wave,flam,xrange=[3300,8000],xstyle=1

  20. Limb DarkeningEddington-Barbier Relationship S=B(τ=0) S=B(τ=1)

  21. How Deep Do We See At μ=1? Answer Depends on Opacity T(τ=1)low opacity T(τ=1)high opacity T(τ=0) Limb darkening depends on the contrast between B(T(τ=0)) and B(T(τ=1))

  22. Limb Darkening versus Teff and λ • Heyrovský 2007, ApJ, 656, 483, Fig.2 • u increases with lower λ, lower Teff • Both cases have lower opacity→ see deeper, greater contrast between T at τ=0 and τ=1 Linear limb-darkening coefficient vs Teff for bands B (crosses), V (circles), R (plus signs), and I (triangles)

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