1 / 25

Lecture 2: Formation of the chemical elements

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

zenaida-joyce
Télécharger la présentation

Lecture 2: Formation of the chemical elements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 slowlyvaryying (if no resonances)

  4. Gamow peak: Note extreme T dependence!: E.g. x2 in T => x106 in yield G. Gamow (1904-68)

  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 • 1010 K -- 2 108 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), ”B2FH” William Fowler 1911-1995 Fred Hoyle 1915-2001

  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.

  11. Stellar nucleosynthesis pp chains The slow part T > 107 K

  12. CNO cycle Hans Bethe 1906-2005 T > 13 106 K Transforms 12C and 16O to 14N and 13C in addition to 41H 4He + energy

  13. Edwin Salpeter 1924-2008 Trippel  process T ~ 108 K 4He + 4He → 8Be (−92 keV) 8Be + 4He → 12C + e+ + e⁻ (+7.367 MeV) Resonance required here: Fred Hoyle’s prediction!

  14. For mass > 10 Msun, carbon burning starts at 600 MK Si burning at 2.7 GK, lasts ~5 days; ”explosive” Also O burning Then, no energy left => Rapid core contraction, Photodisintegration of nuclei => Neutron star

  15. The s-process Adding neutrons to heavy nuclei: ~105-1011 n per cm2 and s In red giants (Miras, S stars, C stars, maybe Ba II stars) Alistair Cameron 1925-2005

  16. r process n fluxes of typically ~1022 per cm2 and s In principle available in supernovae

  17. 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 … Supernovae Type II

  18. … 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!

  19. Supernovae type Ia 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.

  20. 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

  21. 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! Promote mixing in episodes up to the deep convective envelope

  22. 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)

More Related