1. Lecture 2: Formation of the chemical elements Bengt Gustafsson: Current problems in Astrophysics Ångström Laboratory, Spring 2010

2. Outline Basic physics Observation methods Big Bang Nucleosynthesis, just a few words Stellar Nucleosynthesis Inter-stellar Nucleosynthesis, just a few words

3. 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)

4. Gamow peak: G. Gamow (1904-68) Note extreme T dependence!: E.g. x2 in T => x10 6 in yield

5. Cross sections, resonances Note, reactions in stars occur at relatively low energies (~ 100 keV) Resonances still a major problem for many important reactions !

6. Reaction networks -- stellar models

7. Observations Solar (system) abundances

8. Big bang nucleosynthesis (1 s) - 3 min - 20 min 10 10 K -- 2 10 8 K

9. 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), ”B 2 FH” Fred Hoyle 1915-2001 William Fowler 1911-1995

10. 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.

12. Stellar nucleosynthesis pp chains The slow part T > 10 7 K

13. CNO cycle T > 13 10 6 K Transforms 12 C and 16 O to 14 N and 13 C in addition to 4 1 H 4 He + energy Hans Bethe 1906-2005

14. Trippel  process Edwin Salpeter 1924-2008 4 He + 4 He → 8 Be (−92 keV) 8 Be + 4 He → 12 C + e+ + e ⁻ (+7.367 MeV) Resonance required here: Fred Hoyle’s prediction! T ~ 10 8 K

15. For mass > 10 M sun, 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

16. The s- process Adding neutrons to heavy nuclei: ~10 5 -10 11 n per cm 2 and s Alistair Cameron 1925-2005 In red giants (Miras, S stars, C stars, maybe Ba II stars)

17. r process n fluxes of typically ~10 22 per cm 2 and s In principle available in supernovae

19. Supernovae Type II SN 1987a in LMC of Type IIp How do they explode? Collapse - Bounce - - Shock wave - - Neutrinos from p+n (seen) But models do not explode …

20. … until seemingly very recently H. Th. Janka et al. MPI München 0.4 s, 11.2 M sun 0.7 s, 15 M sun Complex 3D instabilities!

22. 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 ~ 3M sun => come after Type II. Supernovae type Ia

23. Bensby & Feltzing (2008): Galactic disk stars Major Fe source (SN Ia) came after major O source (SN II). Note two different populations with different age and different kinematics

24. But most elements (C, N, F, s-elements) probably come from red giant stars 12 C from trippel , 14 N from C N O s-elements from 13 C+  16 O + n Problem to get them up: He shell flashes! Promote mixing in episodes up to the deep convective envelope

25. And then to get it up from the star! Mass loss observed -- for stars high up on AGB of 10 -5 Msun/year or even more! How does it work? Pulsations, dust formation, radiative pressure in interaction Höfner and Freytag (2008)