Lecture 2: Formation of the chemical elements Bengt Gustafsson: Current problems in Astrophysics...
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Transcript of Lecture 2: Formation of the chemical elements Bengt Gustafsson: Current problems in Astrophysics...
Lecture 2: Formation of the chemical elements
Bengt Gustafsson:
Current problems in Astrophysics
Ångström Laboratory, Spring 2010
Outline
• Basic physics
• Observation methods
• Big Bang Nucleosynthesis, just a few words
• Stellar Nucleosynthesis
• Inter-stellar Nucleosynthesis, just a few words
Basic Physics: thermo-nuclear reactions
• At LTE: Maxwell distribution
• Coulomb barrier:
• Cross section:
or tunneling probablity
= Sommerfeld parameter.
S(E) is slowly varyying (if no resonances)
Cross sections, resonances
• Note, reactions in stars occur at relatively low energies (~ 100 keV)
• Resonances still a major problem for many important
reactions!
Big bang nucleosynthesis
• (1 s) - 3 min - 20 min
• 1010 K -- 2 108 KQuickTime™ and a
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Chemical elements
- were there from the beginning?
- formed in the Big Bang (Alpher, Bethe, Gamow 1948)
- formed in stars (Fred Hoyle 1946)
- Burbidge, Burbidge, Fowler & Hoyle (1957), ”B2FH”
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Fred Hoyle 1915-2001
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William Fowler 1911-1995
Discoveries in 1950:ies of key significance:
Tc in Mira stars (Merrill 1952) -- half life 4.2 Myear
Subdwarfs (Pop II) very metal-poor (Chamberlain & Aller 1951)
Ba II stars (rich in s elements)
Since then stellar spectroscopy developed:
Spectra in high resolution, high S/N
Model atmospheres to model the spectra
Abundances to better than 10%-30% today.
CNO cycle
T > 13 106 K
Transforms 12C and 16O to 14N and 13C
in addition to
41H 4He + energy
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Hans Bethe 1906-2005
Trippel process
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Edwin Salpeter 1924-2008
4He + 4He → 8Be (−92 keV)8Be + 4He → 12C + e+ + e (+7.367 MeV)⁻
Resonance required here: Fred Hoyle’s prediction!
T ~ 108 K
For mass > 10 Msun, carbon burning starts at 600 MK
Si burning at 2.7 GK, lasts ~5 days;”explosive”
Then, no energy left =>Rapid core contraction,Photodisintegration of nuclei
=> Neutron star
Also O burning
The s-process
Adding neutrons to heavy nuclei:
~105-1011 n per cm2 and s
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Alistair Cameron 1925-2005
In red giants (Miras, S stars, C stars, maybe Ba II stars)
Supernovae Type II
SN 1987a in LMCof Type IIp
How do they explode?
Collapse - Bounce -- Shock wave - - Neutrinos from p+n
(seen)But models do not
explode …
… until seemingly very recently
• H. Th. Janka et al. MPI München
0.4 s, 11.2 Msun
0.7 s, 15 Msun
Complex 3D instabilities!
http://www.ci.uchicago.edu/flashviz/gallery/main.php?g2_itemId=4827
• HD 3D Simulations by R. Fischer et al.
• A deflagrating white dwarf (A SN type Ia)
• A few seconds event
• => Fe, Ni, Si, Ca
• Original mass ~ 3Msun
=> come after Type II.
Supernovae type Ia
Bensby & Feltzing (2008): Galactic disk starsMajor Fe source (SN Ia) came after major O source (SN
II). Note two different populations with different age and
different kinematics
But most elements (C, N, F, s-elements) probably come from red giant stars
• 12C from trippel , • 14N from C N O• s-elements from 13C+ 16O + n• Problem to get them up: He shell flashes!
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Promote mixingin episodes up to the deep convective envelope