Stellar Structure and Evolution

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1 Stellar Structure and Evolution Achim Weiss Max-Planck-Institut für Astrophysik 01/2014 Stellar Structure p.1

2 Stellar evolution overview 01/2014 Stellar Structure p.2

3 Mass ranges Evolution of stars with M = M in the ρ c -T c diagram. Mass ranges are defined by ignition of nuclear burning stages. 01/2014 Stellar Structure p.3

4 ZAMS-structure: convection convective regions in ZAMS-stars as function of mass 01/2014 Stellar Structure p.4

5 HRD of low-mass stars M = M ; from main-sequence to helium ignition; notice brightness of tip of RGB, bump location, and maximum luminosity at upper end of mass range 01/2014 Stellar Structure p.5

6 HRD of intermediate-mass stars M = M ; from main-sequence to end of helium burning 01/2014 Stellar Structure p.6

7 HRD of massive stars M = 40 and 50M from main-sequence to end of helium burning with different assumptions about convection and mass loss 01/2014 Stellar Structure p.7

8 Evolution of low-mass stars 01/2014 Stellar Structure p.8

9 ZAMS-star properties The (zero-age) main sequence is the place of stars in core hydrogen burning with mass being the parameter along it. Radius increases with mass. Zero age = homogeneous composition (idealization) Homology relations: L M 3 µ 4 and R µ ν 4 ν 1 ν+3 M ν+3 pp-chain (for M 1.5M ), R µ 0.15 M 0.5 CNO-cycle R µ 0.6 M 0.8 central values T c M 4 ν+3 P c M 2(ν 5) ν+3 ρ c M 2(ν 3) ν+3 T c ρ Central temperature, but density with mass! 2 ν 3 c 01/2014 Stellar Structure p.9

10 Brown dwarfs stars that can never be stabilized by H-burning estimate: H-burning temperature T c K; from homology (ν 4): M BD < M (T c, /T c ) 2 0.1M numerical result: M BD 0.075M first detection on grounds of 7 Li-presence 01/2014 Stellar Structure p.10

11 Normal low-mass stars M 0.8M still on MS M 0.8M (at age of universe) leave MS (turn-off) found in Globular Clusters and Halo metal-poor stars: relative overabundance of O and other α-elements w.r.t. Fe of [α/fe] M 1.5M (a few Gyr) in disk and open clusters (around solar metallicity) convective envelope below 1.2 M 1 /M 1.4 convective core above 1.1 M 2 /M 1.3 (CNO-burning!) end of MS: transition to thick thin H-shell (always CNO) 01/2014 Stellar Structure p.11

12 Low-mass stars: Evolution MS and lower RGB evolution; influence of composition 01/2014 Stellar Structure p.12

13 Evolution up the Red Giant Branch H-shell develops within former core (X < X(t = 0)) continuously deepening outer convective envelope, reaching into former core first dredge-up brings H-burning products to surface C, N, 7 Li, 12 C/ 13 C 20 approaching H-shell finally pushes back convective envelope 01/2014 Stellar Structure p.13

14 Core evolution on RGB H-shell advances at M c = L X 0 q increasing He-core mass L M 7 c (shell homology) core degenerate isothermal at T c = T sh core contracting; shell and envelope expanding small heating effect due to contracting material from shell high core density plasma-ν-emission T-inversion; hottest point slightly below shell when T max 10 8 K He-ignition 01/2014 Stellar Structure p.14

15 Horizontal Branch after He-flash: same M c same L horizontal T eff determined by total (i.e. envelope) mass distribution due to mass loss, but not understood first HB parameter: composition (metal-poor blue HB) second parameter: unknown; age or helium content? 01/2014 Stellar Structure p.15

16 Past the HB... He-shell burning (2 shells) Asymptotic Giant Branch (see intermediate-mass stars) and/or extinction of H-shell crossing of HRD at constant L white dwarf The solar evolution from ZAMS to WD 01/2014 Stellar Structure p.16

17 Evolution of intermediate-mass stars 01/2014 Stellar Structure p.17

18 General features 2.5 M/M < 8: early evolution differs from M 1.3M stars mass range 1.3 < M/M < 2.5: properties of both groups convective core and radiative envelope on the MS (for M > 1.3M ); electron scattering opacity becoming important hydrogen-burning via CNO-cycle; ǫ ρxz CNO T 18 rapid transition from MS to RGB (so-called Hertzsprung gap in HRD) helium core remains non-degenerate non-violent ignition of He at center double-shell burning phase with degenerate C/O-core 01/2014 Stellar Structure p.18

19 Hertzsprung-gap Consequences: 1. after MS, cores of intermediate-mass stars contract on thermal timescale 2. envelope expands (why?) and gets cooler 3. fast crossing of HRD gap 4. envelope convection sets in 5. limited; Hayashi-line of fully convective stars approached 6. further expansion via radius increase at almost constant T eff 01/2014 Stellar Structure p.19

20 Evolution of5m -star 01/2014 Stellar Structure p.20

21 Helium-burning phase Helium ignites under non-degenerate conditions M 3 4M ignition during RGB ascent clump (and shorter RGB!) higher mass (M > 4M ): excursion to higher T eff loop through Cepheid-strip loops depend on detailed structure end of core helium burning: helium-burning shell around C/O core and double-shell phase asymptotic return to RGB: Asymptotic Giant Branch (AGB) second dredge-up event 01/2014 Stellar Structure p.21

22 AGB-phase Double-shell phase reached for M > 0.8M ; intermediate-mass stars are prototype special features: thermal pulses, nucleosynthesis of rare elements (s-process); strong mass-loss for 0.6 < M c /M < 0.9: L/L = (M c 0.495) Thermal pulses runaway events in helium shell duration: few hundred years interpulse time: few thousand years strong luminosity variations in shell variable convective zones dynamical phase possible 01/2014 Stellar Structure p.22

23 Luminosity during TPs Thermal pulses in a 2.5M star over the whole AGB evolution 01/2014 Stellar Structure p.23

24 The post-agb HRD AGB- and post-agb evolution of a 2M star. 01/2014 Stellar Structure p.24

25 Third dredge-up AGB-stars show s-process elements (rare earths) enriched process: thermal neutron captures (no Coulomb-barrier!); s for slow (compared to β-decays) needs large n-flux Two n-sources: 13 C(α,n) 16 O and 22 Ne(α,n) 25 Mg Mechanism: TP: outer convective zone reaches He-rich layers below H-shell during pulse (intershell convection) He-layers enriched in C 01/2014 Stellar Structure p.25

26 Evolution of massive stars 01/2014 Stellar Structure p.26

27 General features ignition of carbon in non-degenerate C/O-core M 8M at M 100M vibrational instability due to ǫ-mechanism (positive feedback from nuclear reaction T-dependence) disruption extended convective cores on MS; overshooting radiation pressure; electron scattering; relatively low ρ (20M : ρ c 6.5) mass loss strong and important (WR-stars: stellar winds uncovering core during MS-phase) nuclear burning stages from H- to Si-burning Fe-cores; γ-disintegration dynamical instability γ ad > 4/3) core collapse supernova 01/2014 Stellar Structure p.27

28 Onion-skin structure of evolved massive stars 01/2014 Stellar Structure p.28

29 HRD of massive stars - I Y = 0.285, Z = 0.02 (Limongi et al. 2000): no mass loss, no overshooting, semiconvection during Heburning 01/2014 Stellar Structure p.29

30 HRD of massive stars - II Y = 0.28, Z = 0.02 (Maeder, 1981): moderate mass loss, no overshooting 01/2014 Stellar Structure p.30

31 HRD of massive stars - III Y = 0.285, Z = 0.02 (de Loore & Doom): parametrized mass loss & overshooting 01/2014 Stellar Structure p.31

32 Mass loss Observational evidence: Wolf-Rayet stars CNO-burning products at surface no red supergiants envelope lost before red blue-to-red ratios mass loss explains spectroscopy Influence: total mass core evolution HRD-evolution Inclusion: parametrized or ab-initio-calculations Interaction with effect of overshooting 01/2014 Stellar Structure p.32

33 Mass loss on MS Stellar mass at end of MS (Mowlavi et al., 1998), mass loss according to de Jager et al. + Kudritzki (solid) or 2x the same (dashed) 01/2014 Stellar Structure p.33

34 Central evolution T c vs. ρ c for massive stars during subsequent nuclear burning stages (Limongi et al.) 01/2014 Stellar Structure p.34

35 Chemical structure at end of evolution 01/2014 Stellar Structure p.35

36 Onset of core collapse After end of Si-burning, no further burning, and core contracts. Then electron caption possible: e +(Z,N) (Z 1,N +1)+ν e This implies a loss of energy and insufficient pressure to halt the contraction. In addition photodisintegration is taking place at the highest temperatures and even more energy is lost. (Liberated protons capture e n+ν e.) Finally, free-fall collapse is started, with τ ff 0.1 s, matter from the outside follows, reaches supersonice speed and a shock develops, which ultimately leads to the explosion. 01/2014 Stellar Structure p.36

37 The upper mass limit radiative acceleration g rad = 1 ρ dp rad dr = a 3ρ T3dT dr Using the radiative flux equation we reformulate this as g rad = κf rad c = κl r 4πr 2 c A star can no longer be in hydrostatic equilibrium, once g rad > g, and this is the case for the Eddington luminosity L Edd = 4πcGM κ or L Edd L = κ for electron scattering opacity (X = 0.700). The Eddington limit is reached for about 200M M M = M M 01/2014 Stellar Structure p.37

38 T G i o 01/2014 Stellar Structure p.38

Evolution of Intermediate-Mass Stars

Evolution of Intermediate-Mass Stars General properties: mass range: 2.5 < M/M < 8 early evolution differs from M/M < 1.3 stars; for 1.3 < M/M < 2.5 properties of both mass ranges MS: convective core and