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Non-LTE Models for Hot Stars

Added ComplicationsComplete Linearization

Line Blanketed, Non-LTE Models

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Massive Hot Starswww.ster.kuleuven.ac.be/~coralie/ghost3_bouret.pdf

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Interesting Complications

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Complete Linearization (CL)(Auer & Mihalas 1969)

• Linearized versions of - transfer equation- radiative equlilibrium- hydrostatic equilibrium- conservation of particle number- statistical equilibrium

• Use matrix operations in a Newton – Raphson correction scheme (iterative)

• Used for H + He models (Mihalas + …)

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Complete Linearization (Auer & Mihalas 1969)

• Always works but expensive in computer time …varies as(NF+NL+NC)3 x ND x Niter

• NF = # frequency points (~106)

• NL = # atomic energy levels

• NC = # constraint equations (~3)

• ND = # depth points

• Niter = # iterations to convergence

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Model Atmospheres for Hot Stars

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TLUSTY/SYNSPEC

• OSTAR2002:Lanz & Hubeny 2003, ApJS, 146, 417

• BSTAR2006:Lanz & Hubeny 2007, ApJS, 169, 83

• Web site:http://nova.astro.umd.edu/

• TLUSTY – atmosphereSYNSPEC – detailed spectrum

• Versions available for accretion disks

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Line Blanketed Non-LTE Models for Hot Stars by Hubeny & Lanz

(1995, ApJ, 439, 875)

• Uses hybrid CL + ALI scheme(Accelerated Lambda Iteration:solve for J = Λ[S] using approximate Λ-operator plus a correction term from prior iteration)

• Divide frequency points into groups ofcrucial – full CL treatment andALI – use fast ALI treatment

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Non-LTE Opacity Distribution Functions

• Group all transitions:parity energy

• Make superlevels for each group (~30 per ion)

• Assign single NLTE departure coefficient to each superlevel

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Non-LTE Opacity Distribution Functions

• For each pair of superlevel transitions, get total line opacity in set frequency intervals

• Represent in model as an ODF

• Alternatively use Opacity Sampling(Monte Carlo sampling of superline cross sections)

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Line Blanketing: OSTAR2002

• Low tau: top curves are for an H-He model, and the temperature is progressively lower when increasing the metallicity

• Large tau:reverse is true at deeper layers

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NLTE populations: OSTAR2002 • He (left), C (right)

ionization vs. tau for Teff = 30, 40, 50 kK(top to bottom)

• LTE = dashed lines• NLTE: numbers tend to be

lower in lower stages (overionized by the strong radiation field that originates in deep, hot layers) and conversely higher in higher stages

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OSTAR2002: Lyman Jump & Teff

• Top to bottom:Teff = 55, 50, 45, 40, 35, and 30 kK

• Lyman jump gradually weakens with increasing temperature and disappears at 50 kK

• Weakening and disappearance of Lyα, Si IV 1400, C IV 1550, etc. at hot end

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OSTAR2002: Lyman Jump & g

• Top to bottom, > 912 Å:log g = 4.5, 4.25, 4.0, 3.75, 3.5

• Order reversed for < 912 Å

• Saha eqtn.: low ne, low neutral H, less b-f opacity

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Lyman Jump & metallicity

• Z / ZSUN = 2, 1, 1/2, 1/5, 1/10 (bold line)

• Strong absorption 1000 – 1600 Å balanced by higher flux < 912 Å in metal rich cases(flux constancy)

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NLTE (TLUSTY) vs. LTE (ATLAS)

• (Teff, log g) = (40 kK, 4.5), (35 kK, 4.0), (30 kK, 4.0) (thick lines), compared to Kurucz models with the same parameters (thin histograms)

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OSTAR2002 &

BSTAR2006• grad/g vs.Teff and log g

Thick and dashed line = Eddington limitfor solar and zero metallicity

• BSTAR2006 grid (filled) and OSTAR2002 grid (open)

• Evolutionary tracks (Schaller et al. 1992) are shown for initial masses of 120, 85, 60, 40, 25, 20, 15, 12, 9, 7, 5, and 4 MSUN (left to right)

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BSTAR2006 vs. ATLAS• (Teff, log g) =

(25 kK, 3.0), (20 kK, 3.0), (15 kK, 3.0) (black lines); compared to Kurucz models, same parameters (red histograms)

• In near UV, LTE fluxes are 10% higher than NLTE

• Lower NLTE fluxes result from the overpopulation of the H I n = 2 level at the depth of formation of the continuum flux, hence implying a larger Balmer continuum opacity

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BSTAR2006 vs. ATLAS

• NLTE effects most important for analysis of specific lines(NLTE – black,LTE – red)