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in subjects: "Star Evolution"

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Time-Dependent Two-Dimensional Radiation Hydrodynamics of Accreting Matter Onto Highly Magnetized Neutron Stars

We present for the first time, the self-consistent solution of the two-dimensional, time-dependent equations of radiation-hydrodynamics governing the accretion of matter onto the highly magnetized polar caps of luminous x-ray pulsars. The calculations show a structure in the accretion column very different from previous one-zone uniform models. We have included all the relevant magnetic field corrections to both the hydrodynamics and the radiative transport. We include a new theory for the diff… more

Date: November 24, 1989

Creator: Klein, R.I. (California Univ., Los Angeles, CA (USA). Dept. of Astronomy Lawrence Livermore National Lab., CA (USA) California Univ., Los Angeles, CA (USA). Inst. of Geophysics and Planetary Physics) & Arons, J. (California Univ., Berkeley, CA (USA). Dept. of Astronomy California Univ., Los Angeles, CA (USA). Inst. of Geophysics and Planetary Physics CEA Centre d'Etudes Nucleaires de Saclay, 91 -

Item Type: Article

Partner: UNT Libraries Government Documents Department

open access

Thermonuclear model for x-ray transients

The thermonuclear evolution of a 1.41 M sub solar neutron star accreting both solar and metal-deficient mixtures of hydrogen, helium, and heavy elements at rates ranging from about 10/sup -11/ to 10/sup -10/ M sub solar per year is examined using a one-dimensional numerical model. The metal deficient compositions may result either from placement of the neutron star in a binary system with a Population II red giant or from gravitational settling of heavy ions in the accreted material. For such a… more

Date: January 1, 1982

Creator: Wallace, R. K.; Woosley, S. E. & Weaver, T. A.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Gamow-Teller strength functions and neutrino problems

A quantitative understanding of spin strengths in nuclei is of vital importance in studies of nuclear double beta decay and in solar neutrino spectroscopy. The current status of these problems is outlined.

Date: January 1, 1982

Creator: Haxton, W. C.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Presupernova evolution of massive stars

Population I stars of 15 M/sub mass/ and 25 M/sub mass/ have been evolved from the zero-age main sequence through iron core collapse utilizing a numerical model that incorporates both implicit hydrodynamics and a detailed treatment of nuclear reactions. The stars end their presupernova evolution as red supergiants with photospheric radii of 3.9 x 10/sup 13/ cm and 6.7 x 10/sup 13/ cm, respectively, and density structures similar to those invoked to explain Type II supernova light curves on a st… more

Date: December 15, 1977

Creator: Weaver, T. A.; Zimmerman, G. B. & Woosley, S. E.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Review of the ultraviolet studies of galactic novae

The ultraviolet studies of galactic novae both in outburst and also in quiescence have provided new and fundamental data on these exploding stars. As a direct result of the IUE spectral studies, we have identified a new class of novae in which the outburst occurs on an ONeMg white dwarf. We have also been able to determine ejected masses, elemental abundances, and ultraviolet light curves for a wide variety of nova outbursts. 70 refs.

Date: January 1, 1986

Creator: Starrfield, S.

Item Type: Article

Partner: UNT Libraries Government Documents Department

open access

Constraints on stellar evolution from pulsations

Consideration of the many types of intrinsic variable stars, that is, those that pulsate, reveals that perhaps a dozen classes can indicate some constraints that affect the results of stellar evolution calculations, or some interpretations of observations. Many of these constraints are not very strong or may not even be well defined yet. In this review we discuss only the case for six classes: classical Cepheids with their measured Wesselink radii, the observed surface effective temperatures of… more

Date: January 1, 1983

Creator: Cox, A. N.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Fate of accreting white dwarfs: Type I supernovae vs collapse

The final fate of accreting C + O white dwarfs is either thermonuclear explosion or collapse, if the white dwarf mass grows to the Chandrasekhar mass. We discuss how the fate depends on the initial mass, age, composition of the white dwarf and the mass accretion rate. Relatively fast accretion leads to a carbon deflagration at low central density that gives rise to a Type Ia supernova. Slower accretion induces a helium detonation that could be observed as a Type Ib supernova. If the initial mas… more

Date: January 1, 1986

Creator: Nomoto, Ken'ichi

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Models for Type I supernovae

Two rather disjoint scenarios for Type I supernovae are presented. One is based upon mass accretion by a white dwarf in a binary system. The second involves a star having some 8 to 10 times the mass of the sun which may or may not be a solitary star. Despite the apparent dissimilarities in the models it may be that each occurs to some extent in nature for they both share the possibility of producing substantial quantities of /sup 56/Ni and explosions in stars devoid of hydrogen envelopes. These… more

Date: June 17, 1980

Creator: Woosley, S. E.; Weaver, T. A. & Taam, R. E.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Nuclear astrophysics away from stability

Explosive astrophysical environments invariably lead to the production of nuclei away from stability. An understanding of the dynamics and nucleosynthesis in such environments is inextricably coupled to an understanding of the properties of the synthesized nuclei. In this talk a review is presented of the basic explosive nucleosynthesis mechanisms (s-process, r-process, n-process, p-process, and rp-process). Specific stellar model calculations are discussed and a summary of the pertinent nuclea… more

Date: August 1, 1985

Creator: Mathews, G. J.; Howard, W. M.; Takahashi, K. & Ward, R. A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Stellar core collapse and supernova

Massive stars that end their stable evolution as their iron cores collapse to a neutron star or black hole long been considered good candidates for producing Type II supernovae. For many years the outward propagation of the shock wave produced by the bounce of these iron cores has been studied as a possible mechanism for the explosion. For the most part, the results of these studies have not been particularly encouraging, except, perhaps, in the case of very low mass iron cores or very soft nuc… more

Date: April 1, 1985

Creator: Wilson, J. R.; Mayle, R.; Woosley, S. E. & Weaver, T.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Neutron capture studies: 1, Multiple capture reactions and implications for calculated beta-delayed fission rates: 2, The nuclear level structure of 238Np

Astrophysical r-process calculations of transbismuth elements are of interest because certain actinide pairs can be treated as chronometers in determining the duration of nucleosynthesis. For one such calculation where a particularly long galactic age was derived, 21 + 2 - 4 Gyr, we present evidence that the effect of beta-delayed fission appears to be seriously overestimated in uranium decay chains with A = 252 to 257. With this conclusion, it follows that this estimate of the galactic age mus… more

Date: February 19, 1988

Creator: Hoff, R.W.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Optical pulsations in AM Her systems. Revision 1

The AM Her systems are widely believed to be mass transfer binaries containing a white dwarf primary accreting from a red dwarf secondary. The magnetic field of the white dwarf is so strong that it prevents the formation of an accretion disk and funnels the accretion flow into the polar caps of the white dwarf. The accreting matter is decelerated from free fall by passage through a standoff shock located somewhat above the surface of the white dwarf. The hot postshock gas radiates hard x-rays a… more

Date: June 1, 1985

Creator: Langer, S.H.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Theoretical models for Type I and Type II supernova

Recent theoretical progress in understanding the origin and nature of Type I and Type II supernovae is discussed. New Type II presupernova models characterized by a variety of iron core masses at the time of collapse are presented and the sensitivity to the reaction rate /sup 12/C(..cap alpha..,..gamma..)/sup 16/O explained. Stars heavier than about 20 M/sub solar/ must explode by a ''delayed'' mechanism not directly related to the hydrodynamical core bounce and a subset is likely to leave blac… more

Date: January 1, 1985

Creator: Woosley, S. E. & Weaver, T. A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Energy generation in convective shells of low mass, low metallicity stars

We report on the non-negligible energy generation from the {sup 13}C neutron source and neutron capture reactions in low mass, low metallicity AGB stars. About 10{sup 4} L{sub {circle dot}} are generated within the thermal pulse convective shell by the combination of the {sup 13}C({alpha}, n){sup 16}O rate and the sum of the Y(Z,A)(n,{gamma})Y(Z,A + 1) reactions and beta decays. The inclusion of this energy source in an AGB thermal pulse evolution is shown to alter the evolution of the convecti… more

Date: November 10, 1989

Creator: Bazan, G. (Illinois Univ., Urbana, IL (USA). Dept. of Astronomy) & Lattanzio, J.C. (Lawrence Livermore National Lab., CA (USA))

Item Type: Article

Partner: UNT Libraries Government Documents Department

open access

Nuclear data needs for studying the astrophysical r- and p-processes

Recent advances in understanding the astrophysical sites for the r-, p- and 3-processes has led to an increased understanding of the nuclear physics requires to calculate the thermonuclear origin of the heavy elements in nature. We review specific examples of where nuclear information obtained with Radioactive Nuclear Beams can greatly help our understanding of the thermonuclear origin of the elements in nature. 4 figs.

Date: September 1, 1991

Creator: Howard, W.M. (Lawrence Livermore National Lab., CA (United States)) & Meyer, B.S. (Clemson Univ., SC (United States). Dept. of Physics and Astronomy)

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Neutrino emission by the pair, plasma, and photo processes in the Weinberg-Salam model

The results of numerical integrations of the rates and emissivities of the photo, pair, and plasma neutrino emission mechanisms in the Weinberg-Salam theory of the weak interaction are presented. The range of densities 10 gm cm/sup -3/ less than or equal to rho < 10/sup 14/ gm cm/sup -3/ and the temperature range 10/sup 8/K less than or equal to T less than or equal to 10/sup 11/K are considered. Fitting formulae, similar to those provided by Beaudet, Petrosian, and Salpeter, which reproduce th… more

Date: June 1, 1986

Creator: Schinder, P. J.; Schramm, D. N.; Witta, P. J.; Margolis, S. H. & Tubbs, D. L.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Radiation-driven evolution of low-mass x-ray binaries and the formation of millisecond pulsars

Recent data on low-mass X-ray binaries (LMXBs) and millisecond pulsars (MSPs) pose a challenge to evolutionary theories which neglect the effects of disk and comparison irradiation. Here we discuss the main features of a radiation-driven (RD) evolutionary model that may be applicable to several LMXBs. According to this model, radiation from the accreting compact star in LMXBs vaporizes'' the accretion disk and the companion star by driving a self-sustained mass loss until a sudden accretion-tur… more

Date: August 8, 1991

Creator: Tavani, M. (Lawrence Livermore National Lab., CA (United States) California Univ., Berkeley, CA (United States). Dept. of Astronomy)

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Production and survival of /sup 99/Tc in He-shell recurrent thermal pulses

After a brief introduction to the present state of art of nuclear beta-decay studies in astrophysics, we report our recent work on the long-standing /sup 99/Tc problem. Having combined a detailed study of the recurrent He-shell thermal-pulse, third dredge-up episodes in a 2.25 M/sub solar/ star and an s-process network calculation, we show that a substantial amount of /sup 99/Tc can be produced by the s-process and can survive to be dredged up to the stellar surface. We stress that the factual … more

Date: May 30, 1985

Creator: Takahashi, K.; Mathews, G. J.; Ward, R. A. & Becker, S. A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Supernova Models and Light Curves

The current status of our understanding of Type II supernovae is reviewed with particular emphasis on the processes responsible for the emission of electromagnetic radiation. In addition, a relatively novel evolutionary scenario that appears to lead to a Type I supernova explosion is presented.

Date: January 1, 1980

Creator: Weaver, T. A. & Woosley, S. E.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Some nuclear data needs in astrophysics

In this paper we discuss a number of astrophysical environments and how improved nuclear data could facilitate a better understanding of them. One area of interest includes proton and alpha-particle reactions with unstable nuclei which are necessary for understanding the nucleosynthesis and energy generation in hot hydrogen-burning environments. Efforts underway at LLNL and elsewhere to develop the technology for the measurement of these reaction rates are discussed. Heavy-element nucleosynthes… more

Date: May 1, 1985

Creator: Mathews, G. J.; Bauer, R. W.; Bloom, S. D.; Haight, R. C.; Howard, W. M.; Takahashi, K. et al.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Some investigations about the Carson opacities

Accurate opacities for stellar composition mixtures are needed for all studies of stellar structure, evolution, stability, and pulsation. Three problem areas where increased opacities would be welcome are: the observed broadening of the upper main sequence that can be produced with larger opacities that tend to expand the stars; the existence of the double-mode Cepheids and their anomalously low period ratios which can be predicted to be lower, as observed, if opacities are larger; and the smal… more

Date: January 1, 1983

Creator: Cox, A. N. & Kidman, R. B.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Pre-main sequence sun: a dynamic approach

The classical pre-main sequence evolutionary behavior found by Hayashi and his coworkers for the Sun depends crucially on the choice of initial conditions. The Hayashi picture results from beginning the calculation with an already centrally condensed, highly Jeans unstable object not terribly far removed from the stellar state initially. The present calculation follows the work of Larson in investigating the hydrodynamic collapse and self-gravitational accretion of an initially uniform, just Je… more

Date: January 1, 1979

Creator: Newman, M.J. & Winkler, K.H.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Neon Novae, Recurrent Novae, and Type I Supernovae

Over the past few years, we have been investigating the effects of accretion onto massive white dwarfs and its implications for their growth in mass toward the Chandrasekhar limit, in attempts to identify a possible relationship between SN I and novae. In our studies we have considered accretion at various mass accretion rates onto a variety of different white dwarf masses. We have found that there is a critical white dwarf mass above which a significant fraction of the accreted mass can remain… more

Date: January 1, 1989

Creator: Starrfield, Sumner; Sparks, Warren M.; Truran, James W.; Shaviv, Giora; Illinois University, Urbana & Technion-Israel Institute of Technology, Haifa (Israel)

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Late time optical spectra from the Ni/sup 56/ model for Type I Supernovae

A numerical model has been created that produces self-consistent optical spectra, temperatures, and ionization states for a homologously expanding spherical shell which at t = 0 is assumed to consist of pure Nickel 56, and is further assumed to have density independent of radius within the shell. A brief summary is given of the model, followed by some results from the calculations. (GHT)

Date: April 1, 1980

Creator: Axelrod, T.S.

Item Type: Article

Partner: UNT Libraries Government Documents Department