Nucleosynthesis Nucleosynthesis Big Bang Nucleosynthesis: First Three Minutes He4 and Deuterium No elements heavier than lithium Stellar Nucleosynthesis Heavier elements between lithium and iron. Particularly important is carbon. S-process which involves the slow absorption of neutrons. Supernova Nucleosynthesis Elements heavier than iron. R-process which are elements produced by rapid absorption of neutrons..

Synthesis Of Elements Its harder and harder to make nuclei with higher masses. So the most common substance in the Universe is hydrogen followed by helium, lithium, beryllium and boron (the first elements on the periodic table) Isotopes are formed, such as deuterium and tritium, but these elements are unstable and decay into free protons and neutrons.

Hotter fusion and heavier elements Could stars in principle live forever simply by contracting gravitationally and increasing their temperature to ignite the next heavier source of nuclear fuel whenever they run out? No Strong interaction range is smaller than diameters of smaller nuclei but coulomb interaction covers whole nucleus If nuclei get large enough the increase in electrostatic repulsion of protons becomes greater than increase in binding energy from strong interaction. The peak in binding energy per baryon vs atomic mass number relationship turns out to lie at iron

Hotter fusion and heavier elements (continued) Implication: Once a star’s core is composed completely of iron it can no longer replenish its energy losses by fusion and therefore stars must die.In otherwords,you get energy by fusion all the way up to production of iron but not beyond.

Theories The BBFH theory Hot Big Bang Stellar interiors or during supernova explosions Abundances of helium and deuterium in the universe couldn’t be explained Hot Big Bang Cooling from 10^32 Kelvin to approximately 10^9 Kelvin. Protons and Neutrons collided to produce deuterium (one proton bound to one neutron). Deuterium then collided with other protons and neutrons to produce helium and a small amount of tritium (one proton and two neutrons)

Test For Theories Isotope abundances Typically calculated by calculating the transition rates between isotopes in a network Few key reactions control the rate of other reactions. Abundances critically depends on the density of baryons (neutrons and protons) at the time of nucleosynthesis. one value of this baryon density can explain all the abundances at once

Historical Remarks Can Element Abundances be explained by the Big Bang Theory No significant Production of Nuclei more massive than Li7 and He4 expected to be most abundant after Hydrogen Computed n/p ratio following Big Bang

Neutrons and Protons

Neutrons and Protons X=n/(n+p) As T becomes large X=1/2 The conditions of thermal Equilibrium fixes the initial condition The results of Primeval Nucleosynthesis are predictions of the cosmological model

Expected abundances VS Density Gentle increase of abundance with density as bigbang starts H2 and He3 are burnt away by fusion thus their abundance decreases as density increases Li7 is destroyed by protons for low density with efficiency increasing with higher density and so competing effects create the valley

Stellar Nucleosynthesis (Triple Alpha reaction) How are elements heavier than Helium formed 2He4+2He4 4Be8 + 0γ0 4Be8+2He4 6C12 + 0 γ0 Requires temperature of ~108K

Stellar Nucleosynthesis The triple alpha reaction makes carbon Add a helium to Carbon and you get oxygen Two carbons can make magnesium To burn heavier elements generally require higher temperature Elements are burned at higher and higher temperatures in the core of a massive star until an Iron core forms If the stars cannot burn anything beyond iron than how are the remaining heavier elements formed?

Nucleosynthesis By Neutron Capture The construction of elements heavier than involves nucleosynthesis by neutron capture. A nuclei can capture or fuse with a neutron because the neutron is electrically neutral and, therefore, not repulsed like the proton. In everyday life, free neutrons are rare because they have short half life’s before they radioactively decay. Each neutron capture produces an isotope, some are stable, some are unstable. Unstable isotopes will decay by emitting a positron and a neutrino to make a new element 48Cd110+0n1 48Cd111 48Cd112+0n1 48Cd112 48Cd112+0n1 48Cd113 48Cd113+0n1 48Cd114 48Cd114+0n1 48Cd115 48Cd115+0n1 49In116 + e- + ν (Radio Active decay)

Slow Neutron Capture Neutron capture can happen by two methods, the s and r-processes, where s and r stand for slow and rapid. The s-process happens in the inert carbon core of a star, the slow capture of neutrons. The s-process works as long as the decay time for unstable isotopes is longer than the capture time. Up to the element bismuth (atomic number 83), the s-process works, but above this point the more massive nuclei that can be built from bismuth are unstable.

Rapid Neutron Capture The second process, the r-process, is what is used to produce very heavy, neutron rich nuclei. Here the capture of neutrons happens in such a dense environment that the unstable isotopes do not have time to decay. The high density of neutrons needed is only found during a supernova explosion and, thus, all the heavy elements in the Universe (radium, uranium and plutonium) are produced this way.

Recent Observations of Ongoing Nucleosynthesis: I Gamma-ray Lines from SNe remnants first proposed in 1969 (Clayton, Colgate, & Fishman, ApJ, 155, 75. 56Ni  56Co (t = 6.08 d) 56Co  56Fe (t = 77.3 d) Both transitions occur via β-decay. SN1987A first source in which these gamma-ray lines were observed. Represents the first direct measure of the synthesis of Fe, the sixth most abundant element in the universe (H, He, O, C, Ne, none of which are formed from radioactive progenitors) Later 57Co (and hence 57Ni and 57Fe) were discovered in SN1987A, and 44Ti was discovered in the CasA remnant

Recent Observations of Ongoing Nucleosynthesis: II COMPTEL on CGRO observed sky in 1.809 MeV line caused by decay of 26Al Mode of decay: β-decay 26Al  26Mg Half-life ~ 7 x 105 years Distribution of radiation matches distribution of massive stars (> 10 Msun) on the sky Massive stars can produce 26Al by burning H. The 26Al is then advected to the surface of the star and ejected through mass loss

Non standard Big Bang Model Non-standard BBN In addition to the standard BBN scenario there are numerous non-standard BBN scenarios .Insert additional physics in order to see how this affects elemental abundances. Removing the assumption of homogeneity or inserting new particles such as massive neutrinos. Done to resolve inconsistencies between BBN predictions and observations. Usefulness This has proved to be of limited usefulness in that in most cases trying to change BBN resulted in abundances that were more inconsistent with observations rather than less For example, standard BBN assumes that no exotic hypothetical particles were involved in BBN. One can insert a hypothetical particle (say a massive neutrino) and see what has to happen before BBN predicts abundances which are very different from observations. This has been usefully done to put limits on the mass of a stable tau neutrino.

Dark Matter This inconsistency between observations of deuterium and observations of the expansion rate of the universe, led to a large effort to find processes that could produce deuterium. After a decade of effort, the consensus was that these processes are unlikely, and the standard explanation now used for the abundance of deuterium is that the universe does not consist mostly of baryons and that dark matter makes up most of the mass of the universe.