AGB Stars and Massive Star Evolution
Iben
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..
Degeneracy occurs when there are not enough quantum states at low velocities, pushing electrons to higher momentum (i.e. velocity) states. From Bohm Vitense
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 7.275 Mev (compared 25 Mev for Hydrogen burning) When enough carbon accumulates: 12 CO + 4 He -> 16 O Releasing 7.162 Mev
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 50-100 Lsun.
Eddington Luminosity http://www.atlasoftheuniverse.com/hr.html http://stars.astro.illinois.edu/sow/hrd.html
Piotto et al. RR Lyrae gap 2002 Less Envelope More Envelope
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.
MS Sub-giant to Giant Horizontal Branch AGB Prialnik
Thin Shell Instabilities In Hydrostatic Equilibrium Prialnik
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
H -> He in convective region Dredge Up Prialnik
AGB stars pulsing Schwarzchild & Harm (1967)
The 9th Cycle Schwarzchild & Harm (1967)
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
When does degeneracy happen? Prialnik
Carbon Burning in Intermediate Mass AGB Stars Siess et al. 2006
Carbon Burning in Intermediate Mass AGB Stars Siess et al. 2006
Prumo & Siess 2007
The Supergiant Branch Relatively Constant Luminosity http://www.atlasoftheuniverse.com/hr.html http://stars.astro.illinois.edu/sow/hrd.html
Prialnik
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.
The Blue Loop Excursions
Massive Star Evolution Maeder et al. 1990 A&AS 84, 139
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.
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,