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)

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

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

Reaction networks -- stellar models

Observations Solar (system) abundances

Big bang nucleosynthesis (1 s) - 3 min - 20 min K K

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 William Fowler

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.

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

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

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

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

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

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

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 …

… 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!

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

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

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

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