Evolution from the Main-Sequence

Transcription

1 9 Evolution from the Main-Sequence Lecture 9 Evolution from the Main-Sequence P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

2 9 Evolution from the Main-Sequence 1. Overview 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

3 9 Evolution from the Main-Sequence 1. Overview P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

4 9 Evolution from the Main-Sequence 1. Overview Different evolutions for different masses Different evolutionary paths according to the mass Low mass (< 2.3M ): e -degenerate core before He-burning Intermediate mass (2.3 < M < 8M ): no e -degenerate core before He-burning; e -degenerate core of C and O after He-burning P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

5 9 Evolution from the Main-Sequence 2. Life on the MS 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

6 9 Evolution from the Main-Sequence 2. Life on the MS Increasing µ P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

7 9 Evolution from the Main-Sequence 2. Life on the MS Hydrogen-shell burning via CNO cycle P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

8 9 Evolution from the Main-Sequence 2. Life on the MS L and R evolution consequence of nuclear fusion: µ increases Sun (Asplund et al. 2009) initially (entire Sun): X = , Y = , Z = , µ = 0.61 currently (photosphere): X = , Y = , Z = , µ = 0.60 final (H burnt): X = 0, Y = 0.98, µ = 1.34 Luminosity: L µ 4 for low-mass (Sun) stars, slower increase for higher mass stars Radius: also increases, greatest for most massive stars Consequence: T eff changes much for high-m ZAMS: for M = 1M, L/L = 0.74, R/R = 0.87 P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

9 9 Evolution from the Main-Sequence 2. Life on the MS H burning phase L increases more for lower M R increases regardless of M timescales: H He (from 1 3) P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

10 9 Evolution from the Main-Sequence 2. Life on the MS H burning phase dashed: locus of low-mass stars after 10 Gyr from Iben 1967 ARAA P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

11 9 Evolution from the Main-Sequence 2. Life on the MS Leaving the main sequence L M 3 (rather 3.5) nuclear timescale: t ms ɛmc 2 /L M 2 radiative cores: shell-burning, onion-like composition convective cores: shells are mixed, chemically homogeneous conv. zone convective core: all H in the core converted into He; convective core grows, hence t ms t nucl radiative core: H burnt only in the very center of the radiative core where nuclear reactions take place; t ms < t nucl ; Complication: mass loss P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

12 9 Evolution from the Main-Sequence 2. Life on the MS Leaving the main sequence Minimum for 1M : smallest convective zone P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

13 9 Evolution from the Main-Sequence 3. The red giant phase 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

14 9 Evolution from the Main-Sequence 3. The red giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

15 9 Evolution from the Main-Sequence 3. The red giant phase The onset of core collapse End of MS: H converted into He; inert He accumulates He core is isothermal and radiative (no nuclear energy prod.) Mass of the He core grows (H shell-burning) Upper limit of the He-core (Schönberg-Chandrasekhar limit): M c /M 0.37(µ env /µ core ) 2 Beyond the limit, hydrostatic eq. of the core is impossible µ env /µ core 0.6/1.34: M c /M 0.1 M c > 0.1M: core collapses Valid for M > 2M ; smaller mass stars become degenerate before it reaches the S-C limit P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

16 9 Evolution from the Main-Sequence 3. The red giant phase The sub-giant branch Collapse of core: no longer isothermal; heat from gravitational collapse; core contracts on KH timescale (time to restore hydro. eq.) T c increases and also in surroundings: H-burning accelerates (ind. of collapse) thermal equilibrium is lost (L nuc is too high) Virial for hydrost. eq.: Ω + 2U = 0 Moreover: Ω + U = L nuc L so that Ω/2 = L nuc L > 0 Ω GM 2 /R, U M T : Ω increases, U decreases R increases, T decreases Envelope expands Star moves to the right and up P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

17 9 Evolution from the Main-Sequence 3. The red giant phase The sub-giant branch R increases regardless of M T decreases faster for high M P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

18 9 Evolution from the Main-Sequence 3. The red giant phase The Red Giant branch from Iben 1967 ARAA T can not decrease below 4000 K (H -dominated opacity) Still expanding envelope (He burning) But L nuc > L: increase R ( Ω > 0) Ω and T decrease (energy conservation) L R 2 increases (L nuc L) Star becomes a Red Giant P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

19 9 Evolution from the Main-Sequence 3. The red giant phase End of Giant phase density in the envelope below Earth vacuum envelope is weakly bound: easily ejected Mass loss 10 7 M /yr (current Sun: M /yr) P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

20 9 Evolution from the Main-Sequence 3. The red giant phase Red Supergiants P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

21 9 Evolution from the Main-Sequence 4. Helium burning phase 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

22 9 Evolution from the Main-Sequence 4. Helium burning phase The helium burning phase (low mass) Temp. of isothermal core: T 3 c µ 4 M 2 c ρ env /µ env On the RG branch: ρ e decreases, M c increases, T c rises M c reaches the S-C limit: gravitational contraction of the core, heating up further During RG phase: T c increases P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

23 9 Evolution from the Main-Sequence 4. Helium burning phase High-mass: M T c reaches 10 8 K Triple α process ( 3-body collisions + resonance): 4 He + 4 He 8 Be 4 He + 8 Be 12 C + γ 12 C + 4 He 16 O + γ (if enough carbon) contraction halts Massive, hence lower central density: non-degenerate core H-burning shell slows down (lower grav. energy release) envelope shrinks, L decreases: down and left P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

24 9 Evolution from the Main-Sequence 4. Helium burning phase High-mass: M L stabilizes, L > L nuc envelope responds by contracting hence higher T eff Duration: He-burning + H-burning, all He into C, O 100 Myr horizontal branch Mass loss: oscillations (blue/red), Cepheid variables P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

25 9 Evolution from the Main-Sequence 4. Helium burning phase Low-mass: 1 1.8M core becomes e -degenerate before S-C limit T c increases (P e ind. of T ) Runaway: T c further increases until degeneracy is lost Core expands violently (few sec after He ignition): explosive Helium flash Outer layers contract and heat up Move down and left on KH timescale Star settles in a new state: He burning in non-degenerate core P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

26 9 Evolution from the Main-Sequence 4. Helium burning phase Mass below 1M T c does not rise enough to ignite He burning Envelope is lost (not clear how) End: degenerate He core Cools indefinitely: He white dwarf P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

27 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

28 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae Asymptotic Giant Branch He burning ends in the core: core contracts He burning shifts outwards: envelope expands Star moves to the right (lower T eff ) Stops at 4000 K (fully convective Hayashi limit): can not cool down further R increases and L rises Star goes up along the AGB (Second dredge-up: He and N at the surface) P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

29 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae Summary P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

30 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae Towards Planetary Nebulae Contracting core, expanding envelope: H-burning shell stops Core contraction stops when core becomes degenerate He-burning shell catches up with the silent H-burning shell H-burning turns on again, producing He He sinks into a layer below the H-burning shell, which contracts and heats up: He-burning in a thin layer Unstable: shell can not expand fast enough to mitigate temp. rise He burns explosively: core expands and cools rapidly H-burning stops Thermally pulsing phase: alternating He- and H-burning shells Matter is mixed and/or expelled Pulsing phase ends when outer layers have been expelled M < 8M : carbon-oxygen white dwarf P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

31 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

32 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

33 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

34 9 Evolution from the Main-Sequence 5. Asymptotic Giant Branch and Planetary Nebulae End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

35 9 Evolution from the Main-Sequence 6. End of Giant phase 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

36 9 Evolution from the Main-Sequence 6. End of Giant phase End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

37 9 Evolution from the Main-Sequence 6. End of Giant phase End of Giant Phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

38 9 Evolution from the Main-Sequence 6. End of Giant phase White dwarfs White dwarfs in M4. White dwarfs are 12 billion years old. P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

39 9 Evolution from the Main-Sequence 6. End of Giant phase Supernova explosion: energy budget t dyn 1sec E grav GM 2 c (1/R c 1/R nc ) M c 1.5M R c 10 4 km R nc 20 km E grav GM 2 c /R nc 3(53) erg Less energy in nuclear reactions (proton, elect. into neutro: 7 MeV): E nuc = 7 M c /m H 2(52) = 1/15 E grav E bind = GM c (M M c )/R c 5(51) erg, or E grav /60 Ejecta: v = km s 1 : E kin = 1/2(M M c )v erg, E grav /300 P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

40 9 Evolution from the Main-Sequence 6. End of Giant phase Supernova explosion: energy budget E rad erg P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

41 9 Evolution from the Main-Sequence 6. End of Giant phase SN 1987A P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

42 9 Evolution from the Main-Sequence 6. End of Giant phase Crab Nebula P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

43 9 Evolution from the Main-Sequence 6. End of Giant phase Crab Nebula P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

44 9 Evolution from the Main-Sequence 6. End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

45 9 Evolution from the Main-Sequence 6. End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

46 9 Evolution from the Main-Sequence 6. End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

47 9 Evolution from the Main-Sequence 6. End of Giant phase P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

48 9 Evolution from the Main-Sequence 7. Bibliography 9 Evolution from the Main-Sequence Overview Life on the MS The red giant phase Helium burning phase Asymptotic Giant Branch and Planetary Nebulae End of Giant phase Bibliography P. Hily-Blant (Master PFN) Stellar structure and evolution / 159

49 9 Evolution from the Main-Sequence 8. Bibliography Bibliography Alves, J. F., Lada, C. J., & Lada, E. A. 2001, Nature, 409, 159 Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481 Chandrasekhar, S. 1967, An introduction to the study of stellar structure Dame, T. M., Hartmann, D., & Thaddeus, P. 2001, ApJ, 547, 792 Iben, Jr., I. 1991, ApJS, 76, 55 Krumholz, M. R. 2014, MNRAS, 437, 1662 Larson, R. B. 1969, MNRAS, 145, 271 P. Hily-Blant (Master PFN) Stellar structure and evolution / 159