Showing papers on "Stellar nucleosynthesis published in 1987"

Cosmological baryon diffusion and nucleosynthesis

Abstract: The diffusion rate of baryons through the big-bang plasma is calculated Fluctuations in baryon density in the early Universe lead to inhomogeneities in the neutron-proton ratio, due to the differential diffusion of these particles through the radiation plasma For certain types of nonlinear fluctuations, some nucleosynthesis would occur in very neutron-rich regions Nuclear products of homogeneous neutron-enriched regions are evaluated numerically using a standard reaction network and these results are used to estimate final abundances in an inhomogeneous universe Net deuterium and lithium abundances tend to increase and the net helium abundance tends to decrease compared to an unperturbed standard model It is suggested that pronounced nonlinear baryon-density fluctuations produced in QCD- or electroweak-epoch phase transitions could alter abundances sufficiently to make a closed baryonic universe consistent with current observations of these elements In such a model the abundance of heavier elements (C,N,O, etc) increases significantly and approaches observable levels Abundances can be used to place constraints on extreme scenarios for phase transitions at these epochs

Neutron capture nucleosynthesis during core helium burning in massive stars

Abstract: Neutron-capture nucleosynthesis during core He burning in massive (ZAMS mass = 50-100 solar mass) mass-losing stars, which are identified with Wolf-Rayet stars, is studied in the framework of recent stellar models based on the Roxburgh criterion for convection and on the latest nuclear data available. The nucleosynthesis is followed with the aid of a full nuclear reaction network incorporating up-to-date Maxwellian-averaged neutron-capture cross sections and new density- and temperature-dependent beta-decay rates. Numerical techniques are developed in order to integrate efficiently the set of coupled differential equations of the network. The resulting stellar core and surface abundances are presented, as well as the composition of the stellar winds ejected during the WC phase. Consideration is given to the implications of these results for the composition of OB associations and of the solar system, for the isotopic anomalies in meteorites and in the galactic cosmic rays, as well as for nuclear gamma-ray line astronomy. 114 references.

Current status of nuclear astrophysics

Abstract: The review describes the current status of nuclear astrophysics, where special emphasis is given to primordial and stellar nucleosynthesis of the chemical elements. The laboratory approaches in obtaining relevant reaction rates are discussed. The authors also focus attention on those processes where new knowledge of nuclear physics may have important astrophysical consequences. The major burning phases in stars and some critical reactions are discussed. Finally, miscellaneous topics of current interest are described.

A Reanalysis of Galactic Primary Nucleosynthesis

Abstract: Primary nucleosynthesis in the Galactic disk is reexamined in light of improved observational data for Mg in field stars and improved theoretical data in the form of stellar elemental yields and a higher C-12(alpha, gamma)O-16 reaction rate. It is concluded that the data for O, C, Fe, and Mg are consistent with a constant relative production rate for these elements over the lifetime of the disk in a standard infall model. The yield distribution is nonsolar. Additionally, any continuous mass function in which all stars over 12 solar masses explode both fails to meet the observational constraints on elemental production ratios and overproduces significant amounts of O for the most plausible mass function. Consideration of the probable uncertainties indicates that the discrepancy is real and casts doubt on the capability of current stellar nucleosynthesis theory to provide reliable constraints on stellar evolution, the history of the mass function, or models of galactic chemical evolution. 32 references.

Chemical Evolution, Stellar Nucleosynthesis, and A Variable Star Formation Rate

Abstract: The effects of a decreasing star formation rate (SFR) on the galactic abundances of elements produced in massive stars (M greater than or equal to 10 Msub solar). On the basis of a straightforward model of galactic evolution, a relation between the upper mass limit of type II supernovae (M/sub SN/) contributing to chemical evolution and the decline of the SFR (tau) is derived, when the oxygen abundance is determined only by massive stars. The additional requirement that all intermediate mass elements (Ne-Ti), which are also predominantly due to nucleosynthesis in massive stars, are produced in solar proportions leads to a unique value of M/sub SN/ and tau. The application of this method with abundance yields from Arnett (1978) and Woosley and Weaver (1986) resuults, however, in contradicting solutions: M/sub SN/ approx. = 45 Msub solar, tau = infinity, and M/sub SN/ approx. = 15 Msub solar, tau = 3 x 10/sup 9/ y. Thus, in order that this approach provide an effective probe of the SFR over the history of our galaxy it is essential that converging and more accurate predictions of the consequences of stellar and supernova nucleosynthesis will be forthcoming. 54 refs., 2 figs., 2 tabs.

Nucleosynthesis by 14N Depletion during Shell He Burning in Massive Stars

Abstract: New computations of He shell nucleosynthesis occurring during both radiative and convective phases are presented for stars in the range 8 – 25 M⊙, taking in account the recent changes proposed for many cross sections. While the internal zones, swept by the radiative shell, are shown to efficiently produce elements up to A = 70‒80 only in the most massive stellar models, the intermediate convective regions can be an important source for elements like 12C, 18O, 22Ne and Mg isotopes.

Advanced phases and nucleosynthesis in very massive stars

Abstract: A brief discussion of current problems in very massive star (M ≥ 60M⊙) evolution is presented. In particular we focus on those stars which encounter the electron- positron pair creation instability at oxygen ignition. A star of initial mass of 100 M⊙ is an important candidate for this kind of instability, and may be used to explore the role the pair instability in determining the fate of the known most luminous stars. Using up-dated input physics and a detailed nuclear reaction network, we present in this contribution results concerning the carbon and neon burning phases for this star. The ensuing dynamical evolution through explosive oxygen burning and the supernova stage are under way, and will be given elsewhere.