ScienceWeek

ASTROPHYSICS: ON STELLAR NUCLEOSYNTHESIS

The following points are made by J.J. Cowan and F-K. Thielemann (Physics Today 2004 October):

1) Almost all of the hydrogen and helium in the Cosmos, along with some of the lithium, was created in the first three minutes after the Big Bang. Two more light elements, beryllium and boron, are synthesized in interstellar space by collisions between cosmic rays and gas nuclei. All of the other elements in nature are formed by nuclear reactions inside stars.

2) Over the 14-billion-year history of the universe, elements made in stars have been ejected back into space to be incorporated into new stars and planets. Thus there is an intricate relationship between the life cycles of stars and the nucleosynthesis of the elements. Fusion reactions inside stellar cores are exothermic. They release the energy that powers stars and supports them against gravitational contraction. During most of a star's life, the principal fusion process is the burning of H to form He.

3) But binding energy per nucleon increases with nuclear mass only up to iron-56, the most tightly bound of all nuclei. The production of any heavier nucleus by direct fusion is endothermic. Another impediment to the production of heavy nuclei in stars is the growth of the Coulomb barrier with increasing proton number Z. At sufficiently high Z, the Coulomb barrier prevents all nuclear reactions induced by charged particles at stellar temperatures. Therefore, the isotopes of elements beyond Fe are almost exclusively formed in neutron-capture processes. The products are referred to as "n-capture elements".

4) The two main n-capture processes for astrophysical nucleosynthesis were originally identified in 1957 in pioneering work by Margaret and Geoffrey Burbidge, William Fowler, Fred Hoyle, and Alistair Cameron.(1) They are called the slow (s) and rapid (r) n-capture processes. After a nucleus has captured a neutron to become a heavier nucleus, the time scale t(sub-n) for it to capture an additional neutron is either slow or rapid on the competing time scale t(sub-beta) for it to undergo beta decay. Whereas t(sub-beta), the mean beta-decay lifetime, depends only on the nuclear species, t(sub-n) depends crucially on the ambient neutron flux.(2-5)

References (abridged):

1. E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Rev. Mod. Phys. 29, 547 (1957) ; A. G. W. Cameron, Chalk River rep. no. CRL-41, Chalk River Labs., Chalk River, Ontario (1957)

2. J. J. Cowan, F.-K. Thielemann, J. W. Truran, Phys. Rep. 208, 267 (1991)

3. C. Sneden, J. J. Cowan, Science 299, 70 (2003)

4. M. Busso, R. Gallino, G. J. Wasserburg, Annu. Rev. Astron. Astrophys. 37, 239 (1999)

5. D. L. Burris et al., Astrophys. J. 544, 302 (2000)

Physics Today http://www.physicstoday.org

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ON NUCLEAR FOSSILS IN STARDUST

The following points are made by Larry R. Nittler (Science 2004 303:636):

1) In 1952, P.W. Merrill reported that the radioactive element technetium had been observed in a special class of giant stars (1). Because this element has no stable isotopes, Merrill proposed that these stars "somehow produce technetium as they go along". This was direct evidence that elements are produced by nuclear reactions within stars, and it was only a few years later that a comprehensive theory of nucleosynthesis was laid out in largely modern form (2).

2) For decades, two lines of evidence have been used to test nucleosynthesis theories: chemical abundances measured spectroscopically in stars, and the bulk isotopic and elemental composition of the Solar System. Savina et al (3) have reported evidence for now-extinct technetium in microscopic grains of silicon carbide (SiC) extracted from a meteorite. These grains of circumstellar dust predate the solar system and provide a new and powerful way to investigate stellar evolution and nucleosynthesis with a level of detail and precision almost unheard of in nuclear astrophysics.

3) Since they were discovered in the late 1980s, stardust grains in meteorites have provided astrophysical information complementary to that obtained by astronomical observations (4,5). These rare and tiny (less than a few micrometers) grains of minerals such as SiC, graphite, and Al2O3 have isotopic compositions that establish their formation in stellar outflows and ejecta. They survived conditions in the interstellar medium and early solar system and became trapped in asteroids, pieces of which now fall to Earth as meteorites. Each individual grain is essentially a condensed piece of a single star, and each contains a record of a wide array of astrophysical processes.

4) The best studied type of presolar stardust in meteorites is SiC. More than 90% of presolar SiC grains (known as the "mainstream") are believed to have originated in asymptotic giant branch (AGB) stars, one of the last evolutionary stages of stars with mass up to a few times that of the Sun. AGB stars, including the S-type stars studied by Merrill, have a very interesting structure: An inert core is surrounded by thin helium- and hydrogen-burning shells, which are in turn surrounded by a large hydrogen-rich convective envelope. As they evolve, the helium and hydrogen shells alternately undergo nuclear burning. A key result is the release of neutrons in the region between the shells. These neutrons can be slowly captured by nuclei to produce heavier elements, which are then mixed into the stellar envelope during convective episodes. This so-called s-process (for "slow" neutron capture) is the source of many elements heavier than iron in the Universe. Freshly synthesized carbon is also convectively brought into the envelope, eventually resulting in a surface carbon/oxygen ratio greater than 1. At this point, SiC dust is observed to condense in strong stellar winds.

References (abridged):

1. P. W. Merrill, Science 115, 484 (1952)

2. E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Rev. Mod. Phys. 29, 547 (1957)

3. M. R. Savina et al., Science 303, 649 (2004)

4. E. Anders, E. Zinner, Meteoritics 28, 490 (1993)

5. L. R. Nittler, Earth Planet. Sci. Lett. 209, 259 (2003)

Science http://www.sciencemag.org

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GENESIS OF THE HEAVIEST ELEMENTS IN THE MILKY WAY GALAXY

The following points are made by C. Sneden and J. Cowan (Science 2003 299:70):

1) Most beginning chemistry students struggling with the complexities and underlying structure of the periodic table of the elements will simply accept the existence of the approximately 90 stable elements. Rarely does it occur to them that, somewhere and in some way, all of the elements had to be synthesized. Such element generation or nucleosynthesis, through transmutation of one element into another, is a crucial by-product of stellar energy generation. It has occurred since the birth of the first stars in the Galaxy, and without it life on Earth would not be possible. The authors review the observations and interpretations of heavy elements in the oldest stars, born 10 to 15 billion years ago (Ga). These old and metal-deficient stars exhibit surprisingly large variations in concentrations of heavy elements. Understanding these variations promises to improve our understanding of the origin of the elements.

2) The chemical composition of primordial solar system material is derived from analyses of the solar spectrum and carbonaceous chondrite meteorites [(1) and references therein]. This elemental abundance pattern is used as a reference standard to compare with data from other stars, and we find it repeated so often among stars of the galactic disk that it is often considered as the cosmic or universal chemical composition. In this abundance distribution, H and He account for nearly all of the ordinary matter; the rest of the periodic table provides only trace elements. Among the remaining elements, the distribution features rapidly declining abundances with increasing atomic number Z, an odd/even pattern with even-Z elements being more abundant than their immediate odd-Z neighbors, and relative abundance peaks that correspond to the more tightly bound atomic nuclei. H and He were created wholly or substantially in nuclear reactions that accompanied the Big Bang. Li also was made in the Big Bang and can sometimes be synthesized late in stellar lifetimes. The trio of Li, Be, and B are easily destroyed via proton capture in the interiors of ordinary stars, but can be made in high-energy cosmic ray or neutrino spallation reactions on C, N, and O target nuclei in the interstellar medium (ISM). All other elements owe their existence to nuclear reactions in stellar interiors.

3) Charged-particle fusion reactions account for the abundances of nearly all of the isotopes of elements through the Fe group. These reactions are generally exothermic and thus ultimately are the energy sources that power the luminosities of the stars. The most common reactions are those that fuse four H nuclei into a He nucleus and fuse three He nuclei into a C nucleus. Most stars such as the Sun are of relatively low mass and hence have interior temperatures and densities sufficient to create only the lighter elements. In the last stages and death throes of the rare, more massive stars (8M, where M is the mass of the Sun), the interior temperatures and pressures are high enough to form Fe-group nuclei, but the fusion process is essentially the same as that which turns H into He.

4) In summary: The authors review the origin and evolution of the heavy elements, those with atomic numbers greater than 30, in the early history of the Milky Way. There is a large star-to-star bulk scatter in the concentrations of heavy elements with respect to the lighter metals, which suggests an early chemically unmixed and inhomogeneous Galaxy. The relative abundance patterns among the heavy elements are often very different from the Solar System mix, revealing the characteristics of the first element donors in the Galaxy. Abundance comparisons among several halo stars show that the heaviest neutron-capture elements (including barium and heavier) are consistent with a scaled Solar System rapid neutron-capture abundance distribution, whereas the lighter such elements do not conform to the solar pattern. The stellar abundances indicate an increasing contribution from the slow neutron-capture process (s-process) at higher metallicities in the Galaxy. The detection of thorium in halo and globular cluster stars offers a promising, independent age-dating technique that can put lower limits on the age of the Galaxy.(2-5)

References (abridged):

1. N. Grevesse and A. J. Sauval, Space Sci. Rev. 85, 161 (1998)

2. The high temperatures that would be required (to overcome the electric Coulomb barriers) for fusion of nuclei beyond iron would also result in a large number of high-energy photons. These photons in turn result in photodisintegration of nuclei that suppress any possible charge-particle fusion reactions.

3. H fusion in stars occurs in the inner core (approximately the innermost 10% of the star by mass) and lasts for about 90% of the star's total life. After the exhaustion of H in the core, the star will then develop a thin (thousands of km) H fusion shell outside of the inert He core. Later, when the temperature rises to above 100 million K, the He core will fuse C and O. When the He is finally depleted in the core, a thin He fusion shell, outside of the now C/O core but interior to the H fusion shell, will ignite. These later fusion stages occur only during the last ~10% of a star's life.

4. Supernovae (SNe) are observationally categorized by the presence (type II) or absence (type I) of hydrogen spectral lines. Further, type II SNe are normally thought to result from the collapse and explosion of single massive, short-lived stars. Type I SNe are thought to be phenomena of lower-mass, longer-lived binary star systems, with the eventual explosion and complete destruction of the white dwarf member of the binary.

5. A. G. W. Cameron, in Essays in Nuclear Astrophysics, C. A. Barnes, D. D. Clayton, D. N. Schramm, Eds. (Cambridge Univ. Press, Cambridge, 1982), pp. 23-43.

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