We report abundance estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion AGB stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z = 38) to Pb (Z = 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron-capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a re-analysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure τ is estimated to be about 0.7mb −1 , significantly larger than that which best fits solar-system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed abundance pattern can be explained by a few recurrent neutron exposures, and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r ∼ 0.1).

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The dynamics and statistical mechanics of the quark-hadron phase transition are explored using the bag model and the known spectrum of hadronic states. We compute the maximum amplitude for isothermal baryon number density fluctuations to emerge from this phase transition and their effects on primordial nucleosynthesis, as a function of the coexistence temperature (or bag constant) and the fractional volume of the universe which will remain in quark-gluon plasma when the release of latent heat no longer compensates the cooling due to expansion. For values of the bag constant, B < (260 MeV) 4 , it is possible to find a pressure, temperature, and baryon-chemical-potential equilibrium between the quark-gluon phase and the hadron phase. We calculate the difference in baryon number concentration between these phases. This difference in baryon number concentration may lead to isothermal baryon density fluctuations. For B > (260 MeV) 4 phase equilibrium is not possible for temperatures below 300 MeV. Thus any differences in baryon concentration are small. In the extreme but interesting case of a Hagedorn limiting hadronic temperature and large bag constant we even find that the QCD vacuum energy can produce a mini-inflationary epoch. We discuss computations of the primordial nucleosynthesis yields corresponding to our estimates of the maximum baryon density fluctuations in a universe with Q = 1 in baryons. We find that 4 He, 3 He, and deuterium are within observed constraints for a large part of the parameter space. However, 7 Li is overproduced. Within the parameter space for an Q = 1 universe we'find only a very small abundance of elements heavier than mass 11 (total mass fraction X < 10" 8).

The Bardeen-Petterson e †ect around a rapidly rotating compact object causes a tilted accretion disk to warp into the equatorial plane of the rotating body. Viscous forces cause the accretion Ñow to divide into two distinct regionsÈan inner aligned accretion disk and an outer tilted accretion disk. The transition between these two occurs at a characteristic radius that depends on the mass and angular momentum of the central object and possibly on the accretion rate through the disk. We propose that accreting material passing through the transition region may generate quasi-periodic brightness oscillations such as have been observed in a number of X-ray binaries. We show that this e †ect may be present in the black hole X-ray binary GRO J1655[40. We also argue that the quasi-periodic oscillation (QPO) frequency range predicted by this model is consistent with observed QPO frequencies in both black hole and neutron star low-mass X-ray binaries.

We have constructed a series of non-rotating quasi-hydrostatic evolutionary models for the M2 Iab supergiant Betelgeuse (α Orionis). Our models are constrained by multiple observed values for the temperature, luminosity, surface composition and mass loss for this star, along with the parallax distance and high resolution imagery that determines its radius. We have then applied our best-fit models to analyze the observed variations in surface luminosity and the size of detected surface bright spots as the result of up-flowing convective material from regions of high temperature in the surface convective zone. We also attempt to explain the intermittently observed periodic variability in a simple radial linear adiabatic pulsation model. Based upon the best fit to all observed data, we suggest a best progenitor mass estimate of 20 +5 −3 M ⊙ and a current age from the start of the zero-age main sequence of 8.0 − 8.5 Myr based upon the observed ejected mass while on the giant branch.