Sunday, May 1, AGB Stars and Massive Star Evolution

Transcription

1 AGB Stars and Massive Star Evolution

2 Iben

3 The Helium Flash (< 2 Msun) The central core becomes dense enough, that degeneracy pressure dominates over thermal pressure. Still, the core radiates energy and continues to contract, until Helium fusion occurs. Helium burning Once He fusion stars, temperature goes up, but core doesn t expand immediately. This creates a brief burst of energy, much of which goes into inflating the core and star..

4 Degeneracy occurs when there are not enough quantum states at low velocities, pushing electrons to higher momentum (i.e. velocity) states. From Bohm Vitense

5 Nuclear Burning: the triple α process 4 He + 4 He -> 8 Be 8 Be+ 4 He -> 12 C q = k ρ 2 T 40 Releasing Mev (compared 25 Mev for Hydrogen burning) When enough carbon accumulates: 12 CO + 4 He -> 16 O Releasing Mev

6 The Horizontal Branch The core expands and nuclear reaction rates and luminosity may decrease. The star then enters a helium burning main sequence. Helium burning Hydrogen burning At this point the star is relatively steady for 10 8 years. However, pulsations may occur. Luminosities of Lsun.

7 Eddington Luminosity

8 Piotto et al. RR Lyrae gap 2002 Less Envelope More Envelope

9 Asymptotic Giant Branch Now the central carbon/oxgen core becomes unstable and starts to contract. There is now Helium and Hydrogen burning in shells. Helium burning Hydrogen burning The star may become a supergiant. However, pulsations and dust formation in the envelope may lead to the ejection of the envelope, leaving a white dwarf.

10 MS Sub-giant to Giant Horizontal Branch AGB Prialnik

11 Thin Shell Instabilities In Hydrostatic Equilibrium Prialnik

12 Imagine layers expands. If pressure from surrounding gas (i.e. hydrostatic equilibrium) drops faster than the pressure in expansion, layer continues to expand and cool. If not, temperature rises and we get thermal instability. Prialnik

13 H -> He in convective region Dredge Up Prialnik

14 AGB stars pulsing Schwarzchild & Harm (1967)

15 The 9th Cycle Schwarzchild & Harm (1967)

16 Carbon Burning 12 C + 12 C -> 24 Mg + γ 12 C + 12 C -> 23 Mg + n 12 C + 12 C -> 23 Na + p ~13 ΜeV per reaction 12 C + 12 C -> 20 Ne + α 12 C + 12 C -> 16 Ο + 4α

17 When does degeneracy happen? Prialnik

18 Carbon Burning in Intermediate Mass AGB Stars Siess et al. 2006

19 Carbon Burning in Intermediate Mass AGB Stars Siess et al. 2006

20 Prumo & Siess 2007

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23 The Supergiant Branch Relatively Constant Luminosity

24 Prialnik

25 The Blue Loop Excursions Contracting core and Hydrogen shell burning lead to large envelope and red colors. Helium burning in core implies expanded core and shell, smaller envelope and bluer colors.

26 The Blue Loop Excursions

27 Massive Star Evolution Maeder et al A&AS 84, 139

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30 Semi-Convection Opacity per mass higher for Hydrogen then Helium (more electrons per mass) Imagine core of 40% Helium surrounded by an envelope layer of mostly 10% Helium. High opacity leads to convection, which mixes Helium into envelope, lowering opacity. Thus, such regions may have slow convection just to keep material mixed.

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32 Summary Asymptotic Giant Branch phase occurs after Helium core burning. Thin shell instability in helium burning leads to pulsations. These pulsations lead to significant change in luminosity. Helium and Hydrogen shell burning alternate. Intermediate mass AGB stars may have carbon burning phase. Massive stars shown mainly evolution in temperature (close to Eddington limit). Mass loss important. Contains nested shells of nuclear burning, up to Iron Semi-convection mixes hydrogen and helium in core,