Origin of the elements and Standard Abundance Distribution

Clementina Sasso

Lotfi Yelles Chaouche

Lecture on the Origins of the Solar Systems

Nucleosynthesis:Study of the nuclear processes responsible for the formation of the

elements which constitute the baryonic matter of the Universe

Contemporary nucleosynthesis theory associates the production of certain elements/isotopes or group of elements with:

The cosmological Big Bang

Stars

Supernovae

There are naturally occurring elements as heavy as Uranium.Some elements (e.g., Carbon, Nitrogen, Oxygen) are rather plentiful (1 atom in every 105 atoms).

Nucleosynthesis theory predicts that these elements were formed in the cores of stars

WHERE DO THE OTHER ELEMENTS COME FROM?

1 H H H e H e L i2 3 4 7, , , ,Cosmological Big Bang

BB

1,2H 3,4He 7Li

Intergalactic medium

Interstellar medium

Galaxy formation

Star formation

White Dwarfs

Stars

Neutron stars

SN

Mass loss

Black holes

Sites for nucleosynthesis

• Intermediate mass stars (2< M/M <10)

• Massive stars and associated type II supernovae (M/M >10)

• Exploding CO white dwarfs in binary stellar

sistems (type Ia supernovae)

.

.

Overview of nucleosynthesis mechanisms:(depending on the stellar mass)

• H-burning• He-burning• C-burning• O-burning• Si-burning

• ENS (Explosive NucleoSynthesis)

• Neutron capture

s-process

r-process• p-process

Fuel Process Products Tthreshold

H p-p He ~4

H CNO He 15

He 3α C, O 100

C C+C O, Ne, Na, Mg 600

O O+O Mg, S, Si, P 1000

Si Nuc. eq. Co, Fe, Ni 3000

( )106 K

H-burningT K1 0 7

*

* **

**

H-burning

The total abundance of C, N, O (and F) nuclei is constant in time

H-R Diagram

• Stars on the Main Sequence derive essentially all their energy from the conversion of H to He by nuclear fusion

in their core.

• In the course of this long phase the stellar configuration achieve both hydrostatic and thermal equilibrium.

Evolution to Red Giant• Once the star uses up all the H in its convective core, nuclear fusion

ceases, convection is quenched.

The star is no longer in hydrostatic equilibrium. – Gravity wins out over pressure, and the core

begins to collapse and heats up.– As the core shrinks, the energy of the inward

falling material is converted to heat.– Just outside the core, the hydrogen was

almost, but not quite, hot enough to undergo

fusion.– The added energy from the collapse of the

core heats the hydrogen surrounding the core to the point that it can undergo fusion. A shell of hydrogen begins to fuse to helium just outside the collapsing core.

Evolution to Red Giant

– As the surface expands, it cools down

and becomes redder in color.

– The luminosity increases.

– On the H-R diagram, the star leaves the

main sequence and moves to the upper

right, becoming a red giant.

• The envelope becomes convectively

unstable and H burning ashes move to the

surface (dredge-up).

After this point, the evolution depends

strongly on the mass of the star.

1) INTERMEDIATE MASS STARS 2) MASSIVE STARS

• The energy produced in this burning shell flows to the outer regions of the star, causing them to expand.

• Once the helium core reaches a T=108 K, the 3α reaction can take place:

Three helium atoms can fuse to

form one carbon atom releasing

energy in the process.

Evolution of Intermediate Mass Stars

4He +4He 8Be 4He + 8Be 12C

Evolution of Intermediate Mass Stars

A carbon atom reacts with a helium atom to produce oxygen:

4He16O

12C+ 4He 16O

He burning occurs in a convective core (inner part of the larger He core).The composition of the inner core is constantly mixed and turns gradually from He to C and O.When He is depleted, convection is quenched.

• Once the star has converted all its He to C, it can no longer maintain

hydrostatic equilibrium.

– Gravity begins to win, the core contracts

again.

• Heat released by the contracting core flows

into a shell of He just outside the core. The

heated He shell begins to fuse into C.

• Outside this He burning shell is a He shell,

and then a H burning shell.

Evolution of Intermediate Mass Stars

Evolution of Intermediate Mass Stars

• The C-O core heats up.

• The envelope expands and cools, and convection sets in again

throughout it.

The convective envelope overlaps the boundary of the new extinguished H burning shell and the processed material (Ni and

He), is once more dredged up and mixed in the envelope.

• Nuclear burning takes place in two shells.• The great difference between the two nuclear burning processes do not allow a steady state to develop.

The two shells do not supply energy concomitantly but in turn and the mass of the He layer changes periodically.

• Particularly noteworthy is the dredge-up

of processed material into the convective

envelope by the moving inner boundary of

the convective zone.

The lasting result of each cycle is the growth

of the C-O core.

Evolution of Intermediate Mass Stars

Evolution of Intermediate Mass Stars

• In these stars, the contracting carbon

core will not reach temperatures

sufficient to burn the C into heavier

elements.• No further nuclear reactions are

possible.• The inert carbon core that remains

(WD) will simply cool down over

billions of years.

The envelope mass decreases, mainly

because of mass loss at the surface (stellar

wind and superwind).

2) Evolution of Massive Stars

• Once the star exhausts the He in its core, the carbon-oxygen core begins to contract and heat up.

• The carbon core will become sufficiently hot to ignite the C.

• The carbon core can fuse into oxygen, neon, sodium, magnesium, silicon, etc.

Carbon burning

24Mg

20Ne

23Na

23Mg

+ p

+ n

+

T = 5 x 108 K

16O + 2

Oxygen burning

2α

31P

28Si

24Mg

16O

16O

32S

+

+

+

p

T ~ 109 K+

31S n

+

+

Silicon burning

28Si 7

PHOTODISINTEGRATION: Interaction between massive particles and energetic photons

T = 3 x 109 K

• In the late stages of the life of a massive star…– Helium converted into

heavier elements (carbon, oxygen, …, iron)

– The star has an onion-like structure

Evolution of High Mass Stars

What’s special about iron?

Binding energy per nucleon

Elements Heavier than Iron …

• Once iron is formed, it is no longer possible to create energy via fusion.

Elements heavier than iron are not created via nuclear fusion. (Iron is atomic number 26.)

•Elements heavier than iron are created by neutron capture

•The neutron is added to the nucleus and converted into a proton, increasing the atomic number to make the next element in the periodic table.•Proton capture can occur, but is less probable.

p-process: Proton capture:

s-process: Slow neutron capture:

Absorb n0, then … later … emit e- (-particle).Repeat. Progress up the valley of stability.

n

Ouf!

Fe5626 Fe57

26 X5727

p 56Fe

Ouf!

57X

r-process: Rapid neutron capture:

High n0 flux: absorb many n0s before emission.

nFe56

26 Fe6026 X61

27

n

n

Fe5926

….

n

These processes require energy. Occur only at high & T :

- Core & shell burning

- Supernovae

(,n) photodisintegration

Equilibrium favors“waiting point”

-decay

Temperature: ~1-2 GKDensity: 300 g/cm3

Neutron number

Pro

ton

num

ber

Seed

Neutroncapture

r-process and s-process

Speed Matters

r-process

departs from

valley of stability

s-process

Structure of an Evolved Massive Star

Evolution of High Mass Stars

• Without the ability to generate energy, the iron core begins to collapse.

• Initially, the electron degeneracy pressure can provide support for a short time.

• However, the silicon shell burning is continually adding more and more iron onto the core and eventually it will exceed the Chandrasekhar limit of 1.4 MSun.

Beginning to Collapse• Pressure and temperature rise as core collapses• Photodisintegration

– light begins to break apart nuclei• more energy loss

• Neutrino cooling is occurring• Electrons and protons combine to make neutrons

– p + e n

• Sources of energy to provide pressure are disappearing– core continues to collapse to

very dense matter.

Type II Supernova• Core collapses• Degenerate core

– nuclei get so close together the nuclear force repels them

• Core bounces– particles falling

inward sent back outward

– up to 30,000 km/s

• Type II supernova

One heck of an explosion

• On July 4, 1054 A.D. a supernova exploded in the

constellation Taurus. Today, we see the

remnant as the Crab Nebula.

SN 1987A

SN 1987A

Binary Star Systems

• Bigger star becomes a white dwarf• Smaller star eventually becomes a

red giant• Once smaller star fills its Roche

limit, it transfers mass to the white dwarf– if both are low mass, two white

dwarfs are formed– if more mass is present, more

interesting stuff happens…

Type Ia Supernova

• Chandrasekhar limit– a white dwarf must be less than 1.4

solar masses

• If a white dwarf reaches the Chandrasekhar limit, it starts burning carbon

• The whole dwarf burns in seconds!• More energy released than the

whole 10 billion years on main sequence!

• Glows very brightly for weeks/months and fades away

Type Ia supernovae occur about once a century in the Milky Way

Have a luminosity 10 billion times our Sun

Heavy Element Nucleosynthesis S(Slow)-process <ncap

R(Rapid)-process >ncap

Tests for these processes ?

• Typical abundances are calculated for each type of process, and compared with solar system abundances

• Comparison with processes in other stars

• Analysis of Isotopes in meteorites