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The high-entropy α-rich freeze-out of neutron-rich nuclei suggested by Woosley and Hoffman and the subsequent formation of r-nuclides are studied in the framework of a recent supernova model. The calculations are performed with the help of a single reaction network that is able to describe the establishment of the nuclear statistical equilibrium, the charged-particle reaction freeze-out, and the subsequent neutron captures and β-decays that produce the r-nuclides. We analyze the effect of using various models for the nuclear binding energies, and in particular the new Extended Thomas-Fermi plus Strutinsky Integral method and the finite-range droplet model

We show that the high-temperature, high-entropy evacuated region outside the recent neutron star in a core-collapse supernova may be an ideal r-process site. In this high-entropy environment it is possible that most nucleons are in the form of free neutrons or bound into alpha particles. Thus, there can be many neutrons per seed nucleus even though the material is not particularly neutron rich. The predicted amount of r-process material ejected per event from this environment agrees well with that required by simple galactic evolution arguments. When averaged over regions of different neutron excess in the supernova ejecta, the calculated r-process abundance curve can give a good representation of the solar-system r-process abundances as long as the entropy per baryon is sufficiently high. Neutrino irradiation may aid in smoothing the final abundance distribution.

The study suggests that the requisite thermodynamic conditions may occur when carbon-oxygen white dwarfs explode by deflagration or detonation. When these stars undergo such explosive disruption, there will be a region near the surface where the burning temperature lies in the 2.4-3.2 range. To examine this astrophysical site, calculations are performed for an s-process nucleosynthesis during helium shell flashes and the nuclear transmission taking place when such mass zones are heated by the deflagration or detonation wave, and the results are compared with the solar-system distribution of the p-isotopes. It is demonstrated that Type Ia supernovas provide a viable site for the gamma process, and that the same thermodynamic conditions would also exist in Type II-p powered supernovas, provided that they are powered by detonation.

We calculate a zero impact parameter collision of a 5 x 10/sup 17/ gasteroid with a 1.51 M/sub sun/ neutron star using a two-dimensional Lagrangian-Eulerian hydrodynamics code. The radiation transfer is followed with a Planckian LTE diffusion model, and allowed to couple to the matter through bremsstrahlung and Compton processes. The effects self-gravity on the asteroid, relativity, and magnetic fields are not included. The kinetic energy of impact is converted into radiant energy within 1 ms of impact. However, the neutron star is rapidly (

Resultats de 17 atmospheres modeles d'etoiles neutroniques ayant des temperatures effectives de 0,25 a 3 keV, des gravites de 10 14 a 10 15 cm s −2 , et des abondances en helium et en fer

An attempt is made to understand the physical and kinematic conditions in the M16 active star formation site, by means of mapping observations of neutral and ionized hydrogen and observations of (C-12)O from selected positions within the H II regions. The 21-cm maps suggest that at least two neutral clouds are present in the M16 complex, one of which is likely to be the remains of the cloud in which star formation initially occurred, and the other is probably the cold gas remaining from a disrupted cloud edge where the ionization front has crossed the cloud-intercloud boundary. Multislit echelle spectra of the H-alpha line are obtained to determine the physical conditions within the ionized gas, and H-alpha maps show that the ionized gas is systematically blueshifted in the east and splits into two streams in the southwest.

We present self-consistent atmospheric models for X-ray bursting neutron stars which include the effects of inelastic Compton scattering and free-free and bound-free emission and absorption. We show that the calculated spectra are harder than blackbodies of the same effective temperature. This spectra hardening effect is attributable to the reduction of the radiation source function by electron scattering, and the strong frequency dependence of the free-free opacity. Using the calculated spectra to interpret the observations results in lower bolometric surface fluxes than has been inferred from the common assumption of blackbody emission at the effective temperature. The apparent super-Eddington fluxes inferred from observed spectral temperatures can thus be misleading unless accurate distance determinations are available. The expected strength of iron absorption features and their use as diagnostics for the gravitational redshift at the neutron star surface are also discussed.

We investigate the consequences for nucleosynthsis of the neutron release from the nuclear reaction /sup 22/Ne(..cap alpha..,n)/sup 25/ Mg when it occurs in the convective helium-burning cores of massive stars. We follow the neutron-induced buildup from an original solar system distribution for the cores of 9, 15, and 25 M/sub sun/stars which have an initial composition Y=0.28, Z=0.02, and show that significant enhancements in nuclear abundances up to atomic mass number Aapprox. =90 can be achieved in all but the lightest of the three models. In particular, the s-process nuclei in the mass range 60< or =A< or =70 are produced. For the 9 M/sub sun/star, we find that only nuclei in the mass range 269 or approx. =A9 or approx. =58 are significantly enhanced, but in all cases we obtain substantial concentrations of some nuclei not readily produced in other environments (notably /sup 36/S, /sup 40/Ar, /sup 40/K, and /sup 58/Fe). Our results may be important for considerations of the origin of galactic cosmic rays and as input to detailed supernova nucleosynthesis calculations.