Stellar atmospheres: an overview

Transcription

1 Stellar atmospheres: an overview Core M = 2x10 33 g R = 7x10 10 cm 50 M o 20 R o L = 4x10 33 erg/s 10 6 L o 10 4 (PN) 10 6 (HII) (QSO) L o Photosphere Envelope Chromosphere/Corona R = 200 km ~ 3x10 4 R o 0.1 R o n = cm cm 3 T = 6000 K 40,000 K R = 1000 km/1 R o 100 R o 10 5 R o 0.1 (PN) 10 (HII) 1,000 (QSO) pc n = /10 6 cm cm 3 T = 20,000/2x10 6 K 40,000 15,000 K

2 Spectral Analysis Plasma phyics: diagnostics, line broadening Atomic physics + quantum mechanics: light-matter interaction (micro) Thermodynamics: TE, LTE, non- LTE Hydrodynamics: atmospheric structure, velocity fields Radiative transfer (macro) Stellar properties: mass, radius, luminosity, temperature, chemical composition Galactic structure Stellar and galactic evolution Distance scale

3 Spectral Analysis Observed spectrum comparison Synthetic spectrum Theory of stellar atmospheres Geometry Hydrodynamics Thermodynamics Radiative transfer Atomic physics Model Numerical solution of theoretical Equations L, R, M, chemical composition

4 complex atomic models NLTE a non local problem! dv1, T1 dv2, T2 Photons T r Non-Local Thermodynamic Equilibrium (NLTE) Kinetic equations Transport Theory Thermodynamics Quantum Mechanics Rate Equations AWAP 05/19/05

5 Rate Equations NLTE Models require atomic data for lines, collisions, ionization, recombination Essential for occupation numbers, line blocking, line force Accurate atomic models have been included 26 elements 149 ionization stages 5,000 levels ( + 100,000 ) 20,000 diel. rec. transitions b-b line transitions Auger-ionization recently improved models are based on Superstructure Eisner et al., 1974, CPC 8,270 AWAP 05/19/05

6 Examples of models HD 93129A (Milky Way) O3 Iaf + Taresch, Kudritzki et al. 1997, A&A, 321, 531

7 Pauldrach, 2003, Reviews in Modern Astronomy, Vol. 16 consistent treatment of expanding atmospheres along with spectrum synthesis techniques allow the determination of stellar parameters, wind parameters, and abundances

8 AzV 232 (SMC) O7 Iaf + Crowther et al. 2002, ApJ, 579, 774

9 Population synthesis of high-z z galaxies Stellar spectra Stellar Population Initial Mass Function Star Formation History Metallicity Stellar Evolution Galaxy spectra non-lte atmospheres with winds plus stellar evolution models Synthetic spectra of galxies at high z as a function of Z, IMF, SFR

10 Rome 2005 Spectral diagnostics of high-z starbursts Starburst models - fully synthetic spectra based on model atmospheres Rix, Pettini, Leitherer, Bresolin, Kudritzki, Steidel, 2004, ApJ 615, 98

11 Rome 2005 Spectral diagnostics of high-z starbursts z=2.7 fully synthetic spectra vs. observation Rix, Pettini, Leitherer, Bresolin, Kudritzki, Steidel 2004, ApJ 615, 98

12 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

13 Outline Introduction: 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 models Energy transport: Radiative equilibrium and convection, grey atmospheres, numerical solutions for model atmospheres Atomic radiation processes: Einstein coefficients, line broadening, continuous processes and scattering (Thomson, Rayleigh) Excitation and ionization (Boltzmann, Saha), partition function Example: Stellar spectral types Non-LTE: basic concept and examples 2-level atom, formation of spectral lines, curves growth Recombination theory in stellar envelopes and gaseous nebulae Stellar winds: introduction to line transfer with velocity fields and radiation driven winds

14 Readings Mihalas, D., Stellar Atmospheres, 2 nd ed., Freeman & Co., San Francisco, 1978 Gray, D.F., The Observation and Analysis of Stellar Photospheres, 2 nd ed., Cambridge University Press, Cambridge, 1992 Rutten, R.J., Radiative Transfer in Stellar Atmospheres, 7 th ed., 2000 ( Rybicki, G.B. & Lightman, A., Radiative Processes in Astrophysics, New York, Wiley, 1979 Osterbrock, D.E., Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, University Science Books, Mill Valley, 1989 our notes