ScienceWeek

ASTRONOMY: ON NUCLEOSYNTHESIS IN BINARY STARS

The following points are made by C.S. Jeffery et al (Science 2006 311:345):

1) The past decade has seen a revolution in the study of stellar evolution and nucleosynthesis, both in observations and theory. Despite this progress, however, the role of binary stars has been much neglected. Although their importance in iron production in some supernovae and in the production of rare isotopes of carbon, nitrogen, and oxygen in novae has been known for some time, binary stars have been treated only in isolation. Some effort to redress this situation was made in 1998 when Tout et al [1] considered a full population of binary stars and showed how they could systematically alter the chemical evolution of carbon from one generation of stars to the next. This is because the largest stars, the asymptotic giant branch (AGB) stars, are a major source of carbon and are also the stars most likely to interact in a binary system. Recently, a more complete accounting was given at the Lorentz Workshop on Nucleosynthesis in Binary Stars, held in April 2005 [2]. This international gathering of experts in the field and others interested in stellar evolution and nucleosynthesis featured presentations of data, models, and lengthy discussions on what problems should be tackled and how.

2) Stars are the cosmic factories that manufacture nearly all atoms heavier than helium. The mechanisms that dredge these nuclei from the stellar interior and distribute them through space are crucial to seeding the next generations of stars and planets. The main events are the explosions at the end of stars' active lives, whether in supernovae (massive stars) or ejection of planetary nebulae (less massive red giants). In a cosmic recycling exercise, this material forms new stars with an enriched chemical composition. A preliminary to quantifying the effects of binary stars on these processes is, of course, a detailed understanding of the processes operating in single stars. The Lorentz Workshop began with presentations by some of the main contributors to this area from recent years, including Norbert Langer, Roberto Gallino, Lionel Siess, and John Lattanzio. Specific talks on type Ia supernovae were given by Chris Tout and Sung-Chul Yoon.

3) The first generation of stars must have formed essentially from hydrogen and helium, the only species produced by the Big Bang. No observations have ever found these stars. Possibly this is because they were all relatively massive and all died out long ago. But when they died they ejected newly formed elements into space. A second generation of stars formed from these ejecta, and it is likely that these stars have been identified in recent surveys of the galactic halo. Astronomers measure the compositional age of a star by using the concept of "metallicity." Traditionally, but incorrectly, astronomers refer to all species heavier than helium as "metals" and the total mass fraction of such species is called the "metallicity" of a star. The metallicity of the Sun is about 0.015, for example. The lower this value, the older the material from which the star formed, because after the Big Bang the metallicity of the Universe was essentially zero. A surrogate for the metal abundance is usually taken to be the iron-to-hydrogen ratio, Fe/H, by number, compared to the solar value. The record so far is as low as 10^(-5), and there are several stars with less than 10^(-2.5). It is likely that these are indeed true second-generation stars. Possibly they formed from the ejecta of one nearby first-generation star, whether a supernova, a giant, or something else.

4) These putative second-generation stars show unexpected abundance patterns. Because the first stars were thought to be very massive (more than 40 solar masses), they ought to have produced far more iron than carbon. Yet about a quarter of the stars with very low metallicity show carbon and nitrogen enriched somewhat more than we expect. Carbon production is usually associated with AGB stars from 1 to 8 solar masses as well as stars of higher mass with strong winds, whereas nitrogen comes only from the AGB stars. One way around this would be to change the nature of the stars by interaction with a companion. If two stars are close enough, tides become so strong that material can flow from one to the other. This can strip off a star's envelope, exposing its processed interior, or can change its mass so much that it evolves in a different way. A recent study by Lucatello et al. [3] has found a very high occurrence of close binaries among the carbon-enriched metal-poor stars.[4,5]

References (abridged):

1. C. A. Tout, A. I. Karakas, J. C. Lattanzio, J. R. Hurley, O. R. Pols, in Asymptotic Giant Branch Stars, T. Le Bertre, A. Lebre, C. Waelkens, Eds. (Astronomical Society of the Pacific, San Francisco, 1999), pp. 447-452

2. Lorentz Workshop on Nucleosynthesis in Binary Stars, Lorentz Center, Leiden, Netherlands, 4 to 15 April 2005. [Workshop]

3. S. Lucatello et al., Astrophys J. 625, 825 (2005)

4. M. A. Lugaro et al., Astrophys. J. 593, 486 (2003)

5. D. S. Dearborn, J. C. Lattanzio, P. Eggleton, Astrophys J., in press

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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)

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