Download Consumed in He burning

In the beginning…
The composition of the solar system and earth
Interior of the Genesis sample collection module
The Genesis sample collection module after ‘landing’
Picking through the pieces
Features that demand an explanation:
• H and He are by far most abundant elements
• Li, Be and B are anomalously low in abundance
• Overall ~ exponential drop in abundance with increasing Z
• Even Z > odd Z
• Fe and neighbors are anomalously abundant
“Hydrogen as food’ hypothesis: Burbidge et al., 1957
(built on ideas of Gamow re. nucleosynthesis in big bang)
I. H burning
positron
(rxn. discovered by H. Bethe, 1939)
H + H = D + + +  + 
D +H = 3He + …
3He
3He
neutrino
photons
+ 3He = 4He + 2H + …
+ 4He = 7Be + …
(and similar reactions to make Li and B)
Products quickly decay:
Timescale ~ 10-16 s
{
7Be
+ e- = 7Li
7Li + P = 8Be
8Be = 2.4He
Stuck; no way to elements
heavier than B
“We do not argue with the critic who urges that stars are not hot enough for this process; we tell him to go and find a hotter place.”
A. Eddington
Willie Fowler, Salpeter and Hoyle
Show the solution is the following reaction in red giant stars:
4He
+ 4He + 4He = 12C
Opens possibility of many similar reactions:
12C
+ 4He = 16O
16O + 4He = 20Ne
20Ne + 4He = 24Mg
Collectively referred to as ‘He burning’
“Would you not say to yourself, 'Some super- calculating intellect must have designed the properties of the carbon atom,
otherwise the chance of my finding such an atom through the blind forces of nature would be utterly minuscule.' Of course
you would . . .. A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well
as with chemistry and biology, and that there are no blind forces worth speaking about in nature. The numbers one calculates
from the facts seem to me so overwhelming as to put this conclusion almost beyond question.”
F. Hoyle
Advanced burning:
origin of the 2nd quartile of the mass range
12C
+ 12C = 23Na + H
16O
+ 16O = 28Si + 4He
CNO cycle
12C
+ P = 13N = 13C
13C + P = 14N
14N + P = 15O = 15N
15N + P = 12C + 4He
The E process (for ‘Equilibrium’): why the cores of planets are Fe-rich
A quasi-equilibrium between proton+neutron addition + photo-degradation
Promotes nuclei with high binding energy per nucleon
Neutron capture as a mode of synthesizing heavy elements
Occurs in environments rich in high-energy neutrons, such as super-novae
Features that demand an explanation:
• H and He are by far most abundant elements
H primordial; He consequence of 1˚ generation H burning
• Li, Be and B are anomalously low in abundance
Consumed in He burning
• Overall ~ exponential drop in abundance with increasing Z
Drop in bonding energy per nucleon w/ increasing Z
• Even Z > odd Z
Memory of He burning
• Fe and neighbors are anomalously abundant
Maximum in bonding energy per nucleon at Fe
These factors are directly responsible for the fact that terrestrial
planets are made of silicates and oxides (‘rocks’) with magnetic Fe
cores.
Primitive meteorites look a lot like the sun
(minus the gas and all the hotness)
N
II. Accretion of the Earth
(and inheritance of interstellar dust)
But primitive meteorites are diverse; how are we to know which
is most like the earth?
letters indicate compositional fields of various
types of primitive meteorites
Earth is somewhere near here
Much of the diversity in meteorite composition reflects
variations in oxidation state of solar nebula (H2O/CO ratio)
Infer the earth based on ‘geochemical’ vs. ‘cosmochemical’
diversity in composition
Broad groupings of elements in geochemical processes
The earth’s mantle is mostly chondritic, but depleted in moderately
volatile elements (K, Na)
1
Silicate earth
CI chondrites
Are they simply missing, or hiding somewhere in the earth?
We’ll revisit this question in two lectures
The earth’s mantle is also depleted in siderophile elements (Ni, Cu, Au)
Silicate earth
CI chondrites
0.1
Are they simply missing, or hiding somewhere in the earth?
We’ll revisit this question next lecture
Stratospheric IDPs
2µm- 25µm et particles
Collected by NASA
U2/ER2/WB-57F aircraft
Entry speed estimated by
thermally stepped
He release method
>18 km/s
probable comet origin
<14 km/s
probable asteroid origin
1 ng IDP >105 components
Contents: anhydrous silicate minerals
amorphous silicate (glass)
Fe, Ni sulfides
oxides
Fe, Ni metal grains
organic materials
bulk composition ~ solar
1 µm
The ‘Stardust’ mission
Samples from the Kuiper Belt
(Neptune and beyond)
“Kuiper Belt” around HR 4796A
Stardust’s Wild Ride - 3 loops around the Sun
Earth
gravity
Assist
1/15/2001
Earth
return
1/15/2006
Loop #1
Loops #2 & 3
Launch
2/7/99
Interstellar dust
Collection periods
Wild 2 encounter
1/2/2004
direction of
interstellar flux
January 15, 2006
A piece of ‘star dust’