I. Introduction. First suspicion of existence of continuous stellar winds: Optical spectrum of Wolf-Rayet stars: widths of lines

Transcription

1 8. Stellar Winds History of Stellar Winds Spectroscopic Signatures of Stellar Winds Stellar Winds in Astronomy: Extragalactic supergiants, Mass loss, Galaxy evolution 1

2 I. Introduction First suspicion of existence of continuous stellar winds: 1929 C. Beals MNRAS 90, 202; 91, 966 (1931) 1934 S. Chandrasekhar MNRAS 94, 522 Optical spectrum of P Cygni B2 hypergiant and LBV: broad H, HeI, metal lines with blue-shifted absorption red-shifted emission Optical spectrum of Wolf-Rayet stars: broad He, C, N emission lines no hydrogen lines widths of lines Doppler-shifts of out-flowing atmospheres V expansion 200 km/s to 3000 km/s! 2

3 H and HeI lines of P Cygni Note different y-scale of plots Emission in H α is huge H δ is much weaker How can we use this information to learn about wind velocities? Najarro, Kudritzki et al

4 Same Figure as before, but now replaced by Can use this to estimate of outflow velocities! V expansion 200 km/s to 3000 km/s! 4

5 1934 Adams and McCormack ApJ 81, 119 optical spectra of M supergiants α Ori M2 Ib α 1 Sco M1 Ib α 1 Her M5 II blue-shifted absorption cores of groundstate lines of MnI, CaI, CrI etc. ( 0.00 ev lines ) modern spectra (high res. & S/N) narrow P Cygni profile superimposed to photospheric line core (Bernat & Lambert, 1976, ApJ 204, 830) v exp 10 to 20 km/s but v esc = [2GM/R] 1/2 escape velocity is of same order! Is the flow able to escape the gravitational potential? 5

6 α Ori MnI λ Å λ Å small P Cygni profile superimposed to photosperic line core Bernat & Lambert, 1976 ApJ 204, 830 6

7 1956 Armin J. Deutsch ApJ 123, 210 Mt. Palomar 5m Coude spectra with very high resolution of M supergiants with earlier spectral-type companions α 1 Sco M1 Ib α 2 Sco B2 V CaII, TiII etc. lines very unsual for B2 α 1 Her M5 II α 1 Her G0 III MNI, CrI etc. very unusual for G0 What can explain these unusual spectral features? 7

8 wind envelope extension 1000 R star v exp 20 km/s >> v esc = [2GM/r] 1/2 1/30 v esc-photosphere V exp >> V esc (r) circumstellar lines produced by wind of M supergiant! 8

9 Solar wind 1951 Ludwig Biermann Zeitschrift fuer Astrophysik 29, 274 all comet plasma tails point away from sun solar wind with v 400 km/s 1962 Marcia Neugebauer & C.W. Snyder Science,138, 1169 Mariner 2 probe to Venus finds fast solar wind present all times v 500 ± 300 km/s n wind = 5 cm -3 = some M sun /yr at Earth orbit highly variable 1958 Eugene Parker ApJ 128, 664 first hydrodynamic theory of solar wind 9

10 II. Spectroscopic Evidence for Stellar Winds 1. Spectral lines in hydrostatic atmospheres Hydrostatic -> barometric formula thin, plane-parallel atmosphere with temperature gradient 10

11 symmetric absorption line around central wavelength λ 0 Width of line determined by a) thermal motion of gas b) stellar rotation c) Pressure broadening, stellar gravity 11

12 12

13 in front of stellar disk: blue-shifted scattering by gas moving towards observer 13

14 in front of stellar disk: blue-shifted scattering by gas moving towards observer remaining envelope: red- & blue-shifted scattering by gas moving towards and away from observer 14

15 in front of stellar disk: blue-shifted scattering by gas moving towards observer remaining envelope: red- & blue-shifted scattering by gas moving towards and away from observer P Cygni profile width determined by v max 15

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17 Recap from last lecture: Theoretical & Empirical Curve of Growth 17

18 Recap from last lecture: Stellar Winds & P Cygni Profiles Escape Velocity: 1/2 2GM v esc = ( r ) Max. Wind Velocity: v max = Δλ λ c 18

19 Recap Quiz You are comparing equivalent widths of an empirical and theoretical curve of growth in the linear regime and determine an abundance of log(fe)=-5.3. What would be the abundance determined from the damped regime of the curve of growth? 19

20 Recap Quiz You are comparing equivalent widths of an empirical and theoretical curve of growth in the linear regime and determine an abundance of log(fe)=-5.3. What would be the abundance determined from the damped regime of the curve of growth? You are observing a P Cygni profile for a line with lambda=1000nm and determine that the blueshifted component meets the continuum at 999nm. What is the terminal velocity of the wind? 20

21 Line Scattering photon is absorbed and re-emitted by spontaneous emission in same line transition. Source function will be proportional geometric dilution of the radiation field -> absorption trough de facto scattering number of photons is conserved N abs = N em typical process for resonance lines 21

22 2. Thermal or recombination emission in expanding atmospheres If τ spont > τ coll absorbed photon not immediately re-emitted. Remission ocurrs e.g. through recombination or collisional excitation re-emission coupled to local T(r) since wind envelope can a huge volume, an emission line might occur pure emission profile Not all winds produce a P Cygni profile! width determined by v max 22

23 If ionization of the atoms with subsequent recombination dominates, then the population of the upper level is controlled from electron transitions cascading from above ionization from ground or excited level cascade of subsequent spontaneous emissions pure emission line 23

24 3. Stellar winds across different wavelengths Stellar winds are ubiquitous in all wavelength domains! Radio: hot stars: free-free emission of winds radio- excess cool stars: maser emission of winds OH, H 2 O, SiO, NH 3 IR: ground-based & satellite telescope (IRAS, ISO, Spitzer, Herschel) hot stars: free-free emission of winds weaker than radio- excess rich emission line spectra H, He, metals cool stars: dust emission in winds, PAHs 24

25 stellar winds are ionized plasma ff/bf radiation of extended stellar wind envelope additional contribution to photospheric SED photosperic SED excess from wind IR/radio from cool wind (T=T eff ) X-rays from very hot (T ~ K) shocks in wind 25

26 P Cygni mid IR (ISO) Najarro,

27 Optical: line diagnostics of mass-loss rates wind velocities, chemical composition A supergiant B supergiant O supergiant Variation of Mdot by ± 20% Kudritzki et al. 1999, A&A 350, 970 Kudritzki & Puls, 2000, AARA 38,

28 NGC

29 WN11 star (Wolf-Rayet subtype) emission line diagnostics: first detailed abundance pattern outside Local Group! Bresolin, Kudritzki, Najarro et al. 2002, ApJ Letters 577, L107 29

30 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 30

31 UV: very rich stellar wind spectra of hot and cool stars UV spectrum of O4 supergiant z Puppis Pauldrach, Puls, Kudritzki et al. 1994, SSRev, 66,

32 Taresch, Kudritzki et al. 1997, A&A, 321,

33 X-ray: hot stars: emit X-rays through shocks in their winds strong X-ray emitters ζ Pup, O4 If ι Ori O9 III 15 Mon O7 V Feldmeier, Kudritzki et al., 1997 theory convolved with ROSAT FWHM ROSAT 33

34 III. Stellar Winds and Astronomy 1. Stellar winds and galaxies Massive stars dominate light of star forming galaxies Strong and broad stellar wind lines easily detectable in spectra of integrated stellar populations! Example: starburst galaxies at high z, UV shifted to optical/ir population synthesis, metallicities, energy and momentum input galactic winds, star formation Nuclear burned material input chemical evolution of galaxies 34

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36 Population Synthesis of High-z galaxies Stellar spectra + Stellar Population Initial Mass Function Star Formation History Metallicity Stellar Evolution non-lte atmospheres with winds plus stellar evolution models Synthetic spectra of galxies at high z as a function of Z, IMF, SFR Galaxy spectra 36

37 Starburst99 population synthesis models + UV stellar libraries at ~solar and ~0.25 solar (LMC, SMC) abundance NGC 5253 Leitherer et al. 2001, ApJ, 550,

38 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 38

39 Spectral diagnostics of high-z starbursts z=2.7 fully synthetic spectra vs. observation favors metallicity of Z~0.4-1Z Rix, Pettini, Leitherer, Bresolin, Kudritzki, Steidel 2004, ApJ 615, 98 39

40 2. Winds and stellar evolution Winds affect stellar evolution significantly mass-loss mass is decisive parameter for stellar structure/evolution; mass loss can alter evolutionary path! 40

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42 Low-mass stars mass loss significant during red giant stage Good approximation: Reimers formula: M = η (L /L ) 1.5 M /M Teff 2 3 M /yr Reimers, 1975; Kudritzki & Reimers, 1978, A&A 70, 227 η typically a free parameter in stellar models! Note: tremendous mass-loss in RGB and AGB. Star with 8 M sun on main sequence ends up with 1 M sun 7 Msun re-cycled to ISM ejection of Planetary Nebula at tip of AGB! 42

43 RGB Mass Loss from Asteroseismology Direct mass loss constraint from observations of p-modes (asteroseismology). Implies Reimers η ~

44 Late stages of low-mass stellar evolution: CSPN depends on L small, but mass of hydrogen burning shell t evol strongly modified by winds for review of CSPN winds see Kudritzki, Mendez et al., 1997, Proc. IAU Symp. 180, 64 Kudritzki et al., 2006, Proc. IAU Symp. 234,

45 White Dwarfs: extremely narrow mass distribution from the HRDs of open clusters we know that all stars with have formed WDs The mass spectrum observed can be explained by IMF and stellar evolution with mass-loss applying Reimers - formula 45

46 High-mass stars Stellar winds observable during whole evolution Kudritzki & Puls, 2000, AARA 38, 683 strong for no evolution towards red supergiants stars can lose up to 90% of their mass until He-ZAMS Wolf-Rayet stars: massive stars on He-ZAMS with no H very strong winds!!! 46

47 3. Winds and galaxy evolution stellar winds Energy and momentum input into ISM affects star formation causes galactic winds recycles nuclear burned matter into ISM affects chemical evolution 47

48 Recap from last lecture: Line Emission: Recombination Emission: N abs = N em (P Cygni Profile) τ spont > τ coll (Pure Emission) Reimers Mass Loss Law: M = η (L /L )1.5 M /M T 2 eff M /yr 48

49 Recap Quiz Consider two evolutionary models with a Reimers mass loss parameter η = 0.1 and η = 0.3. Which model will reach the white dwarf stage at a younger age? By which factor do the mass-loss rates differ? 49