Nuclear phenomena are ubiquitous in the universe. The stars shine by nuclear energy, and the chemical compositions observed in the solar system and elsewhere are the results of nuclear processes that occurred in the big bang and inside the several generations of stars that have formed since then. Many astrophysical phenomena may only be understood by a combination of nuclear physics with methods more familiar to astrophysicists.
A particularly important problem is to determine rates for the nuclear reactions that occur in astrophysical environments. We have applied advances in the theoretical descriptions of light nuclei to compute cross sections important for big-bang nucleosynthesis and the solar neutrino flux. This work continues in close connection with our other work on light nuclei, and the main goals at present are to improve the wave functions and computational methods. There are many applications (for example, the rapid neutron capture process) where large contributions from theoretical nuclear physics -- particularly masses and cross sections -- will always be necessary as input, and we maintain research interests in these areas.
Understanding nucleosynthesis and energy generation in a particular astrophysical environment requires calculations of nuclear reaction networks. Even for cases in which the detailed astrophysical phenomena can only be understood from difficult calculations coupling a reaction network and hydrodynamics, simpler network calculations can identify the crucial reactions and other nuclear properties to be determined by more detailed theoretical and experimental work. Ongoing work in this area involves big-bang nucleosynthesis, nuclear burning in low-mass stars, and photon-nucleus reactions in high-energy cosmic rays.
A major goal of nucleosynthesis studies is to determine the specific physical conditions that gave rise to abundance patterns seen in nature: what mix of different kinds of stellar environments gave rise to observed chemical compositions? Large amounts of important new data on abundance patterns are now being collected, with important evidence arising from low-metallicity stars in our own galaxy, absorption-line systems backlit by distant quasars, and primitive inclusions and pre-solar grains embedded in meteorites. These data contain important clues about the nucleosynthetic history of the universe, both locally and globally, and the effort to disentangle the clues into information on stellar sources and galactic chemical evolution is necessarily coupled to our work on nucleosynthesis.
In addition, studies are underway of electroweak reaction rates relevant to astrophysical processes in dense nuclear matter. These are a part of our attempt to predict observable features of quark matter in compact astrophysical objects.
The subjects of nucleosynthesis and chemical evolution are today becoming increasingly important as probes of the star formation history of the Cosmos itself. The primordialcompositions of the first stars/first stellar generations reflect that of the Universe as it emerged from the cosmological Big Bang: hydrogen, deuterium, 3He, 4He, and 7Li. Within galaxies, stars and supernovae play the dominant role in synthesizing the elements from carbon through uranium and in returning heavy-element-enriched matter to the interstellar gas from which subsequent generations of stars are formed. This history is written in the compositions of the stars and gas in our Galaxy and other galaxies as a function of time (“metallicity”). The contributions both from massive starts (M > 10 Mo) and associated Type II supernovae and from thermonuclear (Type Ia) supernovae are particularly noteworthy. Observational studies with large aperture ground-based telescopes are now providing increasing amounts of information concerning both the compositions of the oldest stars in our Galaxy and nearby galaxies and the spectroscopic properties of gas clouds at high red shifts. Argonne researchers are involved with such projects as an outgrowth of their association with the Joint Institute of Nuclear Astrophysics (JINA). Our studies of the nuclear processes participating in nucleosynthesis, of the natures of the sites in which nucleosynthesis proceeds, and of the compositions of the stellar components of our Galaxy and other galaxies as a function of time (redshift) serve to inform us of the natures of the earliest stellar population of galaxies and the Cosmos.
Nuclear Dynamics with Subnucleonic Degrees of Freedom
Theoretical Physics Research