Stellar atmospheres: an overview

M = 2x1033 g 50 Mo

R = 7x1010 cm 20 RoL = 4x1033 erg/s 106 Lo 104 (PN)  106 (HII) 1012 (QSO) Lo

∆R = 200 km ~ 3x10‐4 Ro 0.1 Ron = 1015 cm‐3 1014 cm‐3

T = 6000 K 40,000 K

∆R = 1000 km/1 Ro 100 Ro 105 Ro 0.1 (PN)  10 (HII)  1,000 (QSO) pcn = 1012/106 cm‐3 1011…108 cm‐3

T = 20,000/2x106 K 40,000…15,000 K

Core

Photosphere

Envelope Chromosphere/Corona

Spectral Analysis

Plasma phyics: diagnostics, line broadeningAtomic physics + quantum mechanics: light-matter interaction (micro)Thermodynamics: TE, LTE, non-LTEHydrodynamics: atmospheric structure, velocity fieldsRadiative transfer (macro)

Stellar properties: mass, radius, luminosity, temperature, chemical composition

Galactic structureStellar and galactic evolutionDistance scale

Spectral Analysis

Observed spectrum Synthetic spectrumcomparison

Theory of stellar atmospheres

•Geometry•Hydrodynamics•Thermodynamics•Radiative transfer•Atomic physics

ModelNumerical solution of theoretical

Equations

L, R, M, chemical composition

AWAP 05/19/05

complex atomic models

Non-Local Thermodynamic

Equilibrium (NLTE)

Kinetic equationsTransport TheoryThermodynamics

Quantum Mechanics

Photons11 TdV , 22 TdV ,

r

T

NLTE a non local problem!

Rate EquationsRate Equations

AWAP 05/19/05

NLTENLTE ModelsModels require atomic data atomic data forlines, collisions, ionization, recombination

Essential for occupation numbers, line blocking, line forceAccurate atomic models have been includedAccurate atomic models have been included

26 elements149 ionization stages5,000 levels ( + 100,000 )20,000 diel. rec. transitions4 106 b-b line transitions

Auger-ionization

recently improved models are based onrecently improved models are based on SuperstructureSuperstructureEisner et al., 1974, CPC 8,270

Rate EquationsRate Equations

Examples of models

HD 93129A (Milky Way)

O3 Iaf+

Taresch, Kudritzkiet al. 1997, A&A, 321, 531

consistent treatment of expanding atmospheres along withspectrum synthesis techniquesspectrum synthesis techniques allow the determination of

stellar parameters, wind parameters, andstellar parameters, wind parameters, and abundancesabundancesPauldrach, 2003, Reviews in Modern Astronomy, Vol. 16

AzV 232 (SMC)O7 Iaf+

Crowther et al. 2002, ApJ, 579, 774

non-LTE atmospheres with windsplus stellar evolution models

Synthetic spectra of galxies at high zas a function of Z, IMF, SFR

Population synthesis of highPopulation synthesis of high--z galaxiesz galaxies

Galaxy spectra

Stellar spectra

Stellar Population

Initial Mass Function

Star Formation History MetallicityStellar Evolution

Spectral diagnostics of high-z starburstsStarburst models - fully synthetic spectra based on model atmospheres

Rom

e 200

5

Rix, Pettini, Leitherer, Bresolin, Kudritzki, Steidel, 2004, ApJ 615, 98

Spectral diagnostics of high-z starburstsRo

me 2

005

cB58 @ z=2.7

fully synthetic spectravs. observationRix, Pettini, Leitherer,Bresolin, Kudritzki, Steidel2004, ApJ 615, 98

Starburst99 population synthesis models

+

UV stellar libraries at ~solar and ~0.25 solar (LMC,

SMC) abundance

NGC 5253

Leitherer et al. 2001, ApJ, 550, 724

OutlineIntroduction: Modern astronomy and the power of quantitative spectroscopy Basic assumptions for “classic” stellar atmospheres: geometry, hydrostatic equilibrium, conservation of momentum-mass-energy, LTE (Planck, Maxwell)Radiative transfer: definitions, opacity, emissivity, optical depth, exact and approximate solutions, moments of intensity, Lambda operator, diffusion (Eddington) approximation, limb darkening, grey atmosphere, solar modelsEnergy transport: Radiative equilibrium and convection, grey atmospheres,numerical solutions for model atmospheresAtomic radiation processes: Einstein coefficients, line broadening, continuous processes and scattering (Thomson, Rayleigh)Excitation and ionization (Boltzmann, Saha), partition functionExample: Stellar spectral typesNon-LTE: basic concept and examples2-level atom, formation of spectral lines, curves growthRecombination theory in stellar envelopes and gaseous nebulaeStellar winds: introduction to line transfer with velocity fields and radiation driven winds

ReadingsMihalas, D., “Stellar Atmospheres”, 2nd ed., Freeman & Co., San Francisco, 1978Gray, D.F., “The Observation and Analysis of Stellar Photospheres”, 2nd ed., Cambridge University Press, Cambridge, 1992Rutten, R.J., “Radiative Transfer in Stellar Atmospheres”, 7th ed., 2000 (http://www.astro.uu.nl/~rutten/tmr/)Rybicki, G.B. & Lightman, A., “Radiative Processes in Astrophysics”, New York, Wiley, 1979Osterbrock, D.E., “Astrophysics of Gaseous Nebulae and Active Galactic Nuclei”, University Science Books, Mill Valley, 1989our notes