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 (1904-68)

Note extreme T dependence!: E.g. x2 in T => x106 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

• 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.

Stellar nucleosynthesis

pp chains The slow part

T > 107 K

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)

r process

n fluxes of typically ~1022 per cm2 and s

In principle available in supernovae

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!

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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

And then to get it up from the star!

Mass loss observed -- for stars high up on AGBof 10-5 Msun/year or even more!

How does it work?Pulsations, dust formation, radiative pressurein interaction

Höfner and Freytag (2008)