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Lecture on the Origins of the Solar Systems Origin of the elements and Standard Abundance Distribution Clementina Sasso Lotfi Yelles Chaouche 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 Cosmological Big Bang 1 H, 2 H, 3 He,4 He,7 Li 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). WHERE DO THE OTHER ELEMENTS COME FROM? Nucleosynthesis theory predicts that these elements were formed in the cores of stars BB 1,2H 3,4He 7Li Intergalactic medium Galaxy formation Interstellar medium Star formation Mass loss Stars White Dwarfs Neutron stars SN 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 H H He C O Si Process Products p-p He CNO He 3α C, O C+C O, Ne, Na, Mg O+O Mg, S, Si, P Nuc. eq. Co, Fe, Ni Tthreshold (10 ~4 15 100 600 1000 3000 6 K) H-burning T 10 7 K * * ** ** 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 • The energy produced in this burning shell flows to the outer regions of the star, causing them to expand. – 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 Evolution of Intermediate Mass Stars • Once the helium core reaches a T=108 K, the 3α reaction can take place: +4He 8Be 4He + 8Be 12C 4He Three helium atoms can fuse to form one carbon atom releasing energy in the process. Evolution of Intermediate Mass Stars A carbon atom reacts with a helium atom to produce oxygen: 12C+ 4He 16O 16O 4He Evolution of Intermediate Mass Stars 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 • 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. Evolution of Intermediate Mass Stars • • 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 • 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 T = 5 x 108 K 20Ne + 23Mg +n 24Mg 23Na +p 16O + 2 Oxygen burning T ~ 109 K 32S + + 31S + n 31P + p 16O 16O 28Si 24Mg + + 2α Silicon burning T = 3 x 109 K 28Si 7 PHOTODISINTEGRATION: Interaction between massive particles and energetic photons Evolution of High Mass Stars • 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 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: Ouf! p s-process: 56Fe 57X Slow neutron capture: b Ouf! n 56 26 Fe 57 26 Fe 57 27 X Absorb n0, then … later … emit e- (b-particle). Repeat. Progress up the valley of stability. r-process: Rapid neutron capture: b n n 56 26 Fe 59 26 Fe 60 26 Fe n 61 27 n X …. High n0 flux: absorb many n0s before b emission. These processes require energy. Occur only at high r & T : - Core & shell burning - Supernovae r-process and s-process Temperature: ~1-2 GK Density: 300 g/cm3 Neutron capture b-decay Seed (,n) photodisintegration Neutron number Equilibrium favors “waiting point” 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+en • 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 tb<tncap R(Rapid)-process tb>tncap Tests for these processes ? • •• Typical Comparison abundances with Analysis of Isotopes are processes calculated in other for in meteorites each starstype of process, and compared with solar system abundances