RADIATIVE TRANSFER AND STELLAR ATMOSPHERES. Institute for Astronomy Fall Semester Rolf Kudritzki

Transcription

1 RADIATIVE TRANSFER AND STELLAR ATMOSPHERES Institute for Astronomy Fall Semester 2007 Rolf Kudritzki

2 Fall 2007 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, hydrodynamics of radiation driven winds

3 1. Introduction Astrophysics is based on the collection of photons from cosmic objects from the whole electromagnetic spectrum. The majority of these photons originates in stellar objects (photospheres or envelopes), constituents of galaxies. The quantitative analysis of spectra is necessary for the physical understanding of most astronomical objects in the universe.

4

5 The birthplace of stellar spectroscopy Munich University Observatory 1820

6 Spectrum of the sun Joseph von Frauenhofer 1820 Spectrum of Arcturus, α CrB Lamont, 1836

7 Munich solar eclipse, 1999

8 Munich University Observatory solar eclipse, 1999

9 Fall 2007 Examples of spectra The visible solar spectrum NOAO/AURA/NSF

10 Fall 2007 Examples of spectra

11 Fall 2007 Examples of spectra O4 supergiant ζ Puppis Pauldrach, Puls, Kudritzki et al. 1994, SSRev, 66, 105 UV spectrum Stellar winds

12 Spectral diagnostics of massive stars diagnostic problem: high luminosity enormous energy and momentum density of radiation field NLTE stellar winds Model atmospheres and radiative transfer detailed NLTE treatment radiation-hydrodynamics of line-driven winds spherical extension

13 Fall 2007 LTE LTE vs NLTE each volume element separately in thermodynamic equilibrium at temperature T(r) 1. f(v) dv = Maxwellian with T = T(r) 2. Saha: (n p n e )/n 1 / T 3/2 exp(-hν 1 /kt) 3. Boltzmann: n i / n 1 = g i / g 1 exp(-hν 1i /kt) However: volume elements not closed systems, interactions by photons LTE non-valid if absorption of photons disrupts equilibrium

14 Fall 2007 NLTE 1. f(v) dv remains Maxwellian 2. Boltzmann Saha replaced by dn i / dt = 0 (statistical equilibrium) for a given level i the rate of transitions out = rate of transitions in rate out = rate in i n i X j6=i P ij = X j6=i n j P ji rate equations P i,j transition probabilities

15 Fall 2007 complex atomic models for O-stars (Pauldrach et al., 2001)

16 NLTE Atomic Models in modern model atmosphere codes 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 ) 20,000diel. 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

17 Basic equations T eff g R? [Z] Input M =4πr 2 ρv g rad = g cont + const ρ Rate equations v dv dr = dp dr ρ 1 + g rad g P ) lines f lug l ( n l g l n u g u ) R 0 n i P j 6=i (Sν Iν)κν = μ I ν r + 1 r μ2 energy equation R +1 1 I ν(μ)φ(ν)μdμdν (R ij + C ij )+n i (R ik + C ik ) = P j 6=i Iν μ Hydrodyn. n j (R ji + C ji )+n + 1 (R Ki + C Ki ) Radiative transfer v de dr + pv d dr ( 1 ρ ) = 1 ρ R 0 4πκ ν (J ν S ν )dν Non-linear coupling complex iteration!!!

18 Fall 2007 HD 93129A O3Ia Taresch, Kudritzki et al. 1997, A&A, 321, 531

19 Fall 2007 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

20 Fall 2007 AV SMC Crowther et al. 2002, ApJ, 579, 774

21 Fall 2007 Examples of spectra Supernovae (photospheric phase) Filippenko 1997, ARA&A, 35, 309

22 Fall 2007 Examples of spectra Quasar composite Vanden Berk et al. 2001, AJ, 122, 549 (Sloan)

23 Fall 2007 Examples of spectra Quasar + Damped Lyman a system Carlton & Churchill 2000

24 Fall 2007 Examples of spectra Seyfert 1 & 2 Osterbrock 1978, Physica Scripta, 17, 137

25 Fall 2007 Bresolin & Kennicutt 2002, ApJ, 572, 838 Examples of spectra HII regions in M83 disk element abundances stellar content ionizing flux, stellar atmospheres Hot spot

26 PN Fall 2007

27 Fall 2007 Planetary Nebula Examples of spectra Zhang & Liu 2002, MNRAS, 337, 499

28 Fall 2007 Examples of spectra Galactic halo star Stephens & Boesgaard 2002, AJ, 123, 1647

29 Fall 2007 SEDs of massive stars in star forming regions Spectral Analysis heavy extinction IR spectroscopy O-star SED (intrinsic) IR-excess stellar wind A V = 30 mag

30 Arches cluster in GC

31 Fall 2007 Galactic Center Arches cluster: quantitative IR spectroscopy NIII NIII NIII Najarro, Figer, Hillier, Kudritzki, 2004, ApJ Letters

32 Extragalactic stellar astronomy with the brightest stars in the universe Rolf Kudritzki, Fabio Bresolin, Miguel Urbaneja

33 Properties of stellar populations Evolution of galaxies Chemical abundance and abundance pattern gradients Interstellar extinction Distances Dark matter content Quantitative stellar spectroscopy of individual stars in galaxies beyond the Local Group Extragalactic stellar astronomy

34 TMT 2007 A supergiants objects in transition Brightest normal stars at visual light: -7 M V -10 mag B8 A4 t ev ~ 10 3 yrs L, M ~ const. ideal to determine chemical compos. abundance grad. SF history extinction extinction laws distances of galaxies

35 Kudritzki, Urbaneja, Bresolin, 2007, ApJ, in prep. pilot study W. Gieren, G. Pietrzynski, raucaria Project: F. Bresolin, M. Urbaneja, RPK TMT 2007 NGC 300 Sculptor Group (2 Mpc) 70 blue supergiant spectra 117 cepheids

36 TMT 2007 Bresolin, Gieren, Kudritzki et al ApJ 567, 277

37 TMT 2007 NGC 300: spectral classification Galactic template NGC 300 A2 supergiant V = 19.0 Galactic template Bresolin, Gieren, Kudritzki et al. 2002, ApJ 567, 277

38 Study of metallicities A supergiants

39 TMT 2007 Metallicity: spectral window

40 TMT 2007 Spectral window Å

41 TMT 2007 Spectral window Å χ 2 i = S N 2 1 n pix n pix j=1 (O j C j ) 2

42 TMT 2007 χ i spectral window Å

43 TMT 2007 another spectral window

44 TMT 2007 Spectral window Å

45 TMT 2007 χ i spectral window Å

46 TMT 2007 Χ i all windows [Z] = -0.4±0.1

47 TMT 2007 WN 11 star

48 TMT 2007 Wolf-Rayet star in NGC 300 WN11 star emission line diagnostics: first detailed abundance pattern outside Local Group Bresolin, Kudritzki, Najarro et al. 2002, ApJ Letters 577, L107

49 TMT 2007 NGC 300 WN11 star non-lte line-blanketed hydrodynamic model atmospheres with stellar winds stellar parameters wind parameters H, He, CNO, Al, Si, Fe abundances Bresolin, Kudritzki, Najarro et al. 2002, ApJ Letters 577, L107

50 TMT 2007 NGC 300 WN11 star

51 Stellar metallicity gradient in NGC300 Rome 2005 B0 B3 supergiants B8 A4 supergiants --- [Z] = ρ/ρ 0 = d/kpc ρ 0 = 9.75 arcmin 5.7kpc [Z] = log(z/z_sun) Kudritzki, Urbaneja, Bresolin, Przybilla, Gieren, Pietrzynski, 2007, in prep.

52 AAS 2007 NGC 3621: 7 Mpc HST/ACS Bresolin, Kudritzki, Mendez & Przybilla 2001 ~19 blue supergiant candidates (VLT/FORS) 4 analyzed

53 AAS 2007 Galactic template NGC 3621 A0 supergiant NGC 3621: Bresolin, Kudritzki, Mendez & Przybilla 2001 ~19 blue supergiant candidates (VLT/FORS) 4 analyzed Galactic template Bresolin, Kudritzki, Mendez, Przybilla 2001, ApJ Letters 548, L159

54 AAS & 0.5 solar metallicity models A0 Ia star V = 20.5 M V = -9 Bresolin, Kudritzki, Mendez, Przybilla 2001, ApJ Letters 548, L159

55 TMT 2007 Blue supergiants as distance indicators

56 TMT 2007 Flux weighted Gravity Luminosity Relationship (FGLR) Kudritzki, Bresolin, Przybilla, ApJ Letters, 582, L83 (2003) L,M ~ const. B1-A4 M ~ g R 2 ~ L (g/t 4 ) = const. const. with L ~ M x ~ L x (g/t 4 ) x, x ~ 3 L 1-x ~ (g/t 4 ) x or with M bol ~ -2.5log L M bol = a log(g/t 4 ) + b FGLR a =2.5 x/(1-x) ~ 3.75

57 TMT 2007 Kudritzki, Bresolin & Przybilla, 2003,ApJL, 582, L83 FGLR Local Group, NGC300 & NGC3621 Kudritzki, Urbaneja, Bresolin et al., ApJ, 2007, in prep. M bol = 3.75 log(g/t 4 eff,4 ) σ= 0.24

58 TMT 2007 Application to TMT (30m telescope) WFOS quantitative spectroscopy possible down to m V ~ 24.5 mag with objects M V -8 mag m M ~ 32.5 mag ~ 30 Mpc possible chemical evolution studies SF ISM, extinction, extinction laws distances 10 objects per galaxy Δ(m-M) ~ 0.1 mag

59 Mauna Kea Observatories Mauna Kea The best in the world $ 1 billion science endeavor

60 Operating at 14,200 ft.

61 Adaptive Optics

62 TMT

63 TMT.PMO.PRE REL03

64 TMT Design

65 Site on Mauna Kea Northern Plateau - below summit - less visibility - less cultural and economic impact - foreseen in year 2000 Master Plan of UH

66

67 TMT- 13 North test site

68 Planets around other stars Brown Dwarf orbiting a star Gemini/Keck AO detection by Michael Liu (IfA) Problem: Planets much fainter than Brown Dwarfs 30m telescope needed!! TMT!!

69 TMT will allow for the first time To image giant planets surrounding many hundred stars To determine masses and radii To analyze atmospheric structure and chemical composition The power of TMT

70 Exploring other solar systems Artist conception of planetary system orbiting around 55 Cancri using results of radial velocity Keck observations

71 55 Cancri physical characterization by spectroscopy Sudarsky, Burrows & Lunine, 2003

72 Predicted spectra of a 5 Gyr Jupiter-like planet Hubeny, 2007, priv. comm.

73 Predicted spectra of some interesting planets Hubeny, 2007, priv. comm.

74 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 ΔR = 200 km ~ 3x10 4 R o 0.1 R o n = cm cm 3 T = 6000 K 40,000 K ΔR = 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

75 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

76 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

77 Basic equations T eff g R? [Z] Input M =4πr 2 ρv g rad = g cont + const ρ Rate equations v dv dr = dp dr ρ 1 + g rad g P ) lines f lug l ( n l g l n u g u ) R 0 n i P j 6=i (Sν Iν)κν = μ I ν r + 1 r μ2 energy equation R +1 1 I ν(μ)φ(ν)μdμdν (R ij + C ij )+n i (R ik + C ik ) = P j 6=i Iν μ Hydrodyn. n j (R ji + C ji )+n + 1 (R Ki + C Ki ) Radiative transfer v de dr + pv d dr ( 1 ρ ) = 1 ρ R 0 4πκ ν (J ν S ν )dν Non-linear coupling complex iteration!!!

78 Fall 2007 complex atomic models for O-stars (Pauldrach et al., 2001)

79 Fall 2007 HD 93129A O3Ia Taresch, Kudritzki et al. 1997, A&A, 321, 531

80 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

81 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

82 Fall 2007 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

83 Fall 2007Spectral diagnostics of high-z starbursts z=2.7 fully synthetic spectra vs. observation Rix, Pettini, Leitherer, Bresolin, Kudritzki, Steidel 2004, ApJ 615, 98

84 Fall 2007 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

85 Fall 2007 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, hydrodynamics of radiation driven winds

86 Fall 2007 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 these notes