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Stationary neutrino radiation transport by maximum entropy closure

Description: The authors obtain the angular distributions that maximize the entropy functional for Maxwell-Boltzmann (classical), Bose-Einstein, and Fermi-Dirac radiation. In the low and high occupancy limits, the maximum entropy closure is bounded by previously known variable Eddington factors that depend only on the flux. For intermediate occupancy, the maximum entropy closure depends on both the occupation density and the flux. The Fermi-Dirac maximum entropy variable Eddington factor shows a scale invariance, which leads to a simple, exact analytic closure for fermions. This two-dimensional variable Eddington factor gives results that agree well with exact (Monte Carlo) neutrino transport calculations out of a collapse residue during early phases of hydrostatic neutron star formation.
Date: November 1994
Creator: Bludman, S. A. & Cernohorsky, J.
Partner: UNT Libraries Government Documents Department

Induced star formation and morphological evolution in very high redshift radio galaxies

Description: Near-infrared, sub-arcsecond seeing images obtained with the W M Keck I Telescope of show strong evolution at rest-frame optical wavelengths in the morphologies of high redshift radio galaxies (HzRGs) with 1 9 < z < 4 4 The structures change from large-scale low surface brightness regions surrounding bright, multiple component and often radio-aligned features at z > 3, to much more compact and symmetrical shapes at z < 3 The linear sizes ({approximately} 10 kpc) and luminosities (M{sub B} {approximately} -20 to -22) of the individual components in the z > 3 HzRGs are similar to the total sizes and luminosities of normal, radio-quiet, star forming galaxies seen at z = 3 - 4 `R`-band, 0 1`` resolution images with the Hubble Space Telescope of one of these HzRGs, 4C41 17 at z = 3 800, show that at rest-frame UV wavelengths the galaxy morphology breaks up in even smaller, {approximately} 1 kpc-sized components embedded in a large halo of low suface brightness emission The brightest UV emission is from a radio-aligned, edge-brightened feature (4C41 17.North) downstream from a bright radio knot A narrow-band Ly-{alpha} image, also obtained with HST, shows an arc-shaped Ly-{alpha} feature at this same location, suggestive of a strong jet/cloud collision Deep spectropolarimetric observations with the W M Keck II Telescope of 4C41 17 show that the radio-aligned UV continuum is unpolarized Instead the total light spectrum shows ahsorption lines and P-Cygni type features that are similar to the radio-quiet z = 3 - 4 star forming galaxies This shows that the rest-frame UV light in 4C41 17 is dominated by starlight, not scattered light from a hidden AGN The combined HST and Keck data suggest that the radio--aligned rest-frame UV continuum is probably caused by jet-induced star formation The strong morphological evolution suggests that we ...
Date: October 1, 1997
Creator: van Breugel, W. J. M.
Partner: UNT Libraries Government Documents Department

MACHO observations of Type II cepheids and RV Tauri Stars in the LMC

Description: We report the of the existence of RV Tauri stars in the Large Magellanic Cloud (LMC). This class of variable star has hitherto been unidentified in the Magellanic Clouds. In light and color curve behavior the RV Tauri stars appear to be an extension of the Type II Cepheids to longer periods. A single period-luminosity-color relationship is seen to describe both the Type II Cepheids and the RV Tauri stars in the LMC.
Date: July 1, 1996
Creator: Alcock, C.; Pollard, K.A. & Alisman, R.A.
Partner: UNT Libraries Government Documents Department

GRBs from the First Stars

Description: We present an estimate of the Gamma Ray Bursts which should be expected from metal-free, elusive first generation of stars known as PopulationIII (PopIII). We derive the GRB rate from these stars from the Stellar Formation Rate obtained in several Reionization scenarios available in the literature. In all of the analyzed models we find that GRBs from PopIII are subdominant with respect to the ''standard'' (PopII) ones up to z {approx} 10.
Date: April 16, 2007
Creator: Iocco, Fabio & /Naples U. /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department

Evolutionary Processes in Multiple Systems

Description: There are several ways in which triple stars can evolve in somewhat unusual ways. They discuss two situations where Case A Roche-lobe overflow, followed by a merger, can produce anomalous wide binaries such as {gamma} Per; and Kozai cycles in triples with non-parallel orbits, which can produce merged rapidly-rotating stars like AB Dor, and which can also lead to the delayed ejection of one component of a multiple, as may have been observed in T Tau in 1998.
Date: February 14, 2006
Creator: Eggleton, P P & Kisseleva-Eggleton, L
Partner: UNT Libraries Government Documents Department

On the quasihydrostatic flows of radiatively cooling self-gravitating gas clouds

Description: Two model problems are considered, illustrating the dynamics of quasihydrostatic flows of radiatively cooling, optically thin self-gravitating gas clouds. In the first problem, spherically symmetric flows in an unmagnetized plasma are considered. For a power-law dependence of the radiative loss function on the temperature, a one-parameter family of self-similar solutions is found. The authors concentrate on a constant-mass cloud, one of the cases, when the self-similarity indices are uniquely selected. In this case, the self-similar flow problem can be formally reduced to the classical Lane-Emden equation and therefore solved analytically. The cloud is shown to undergo radiative condensation, if the gas specific heat ratio {gamma} > 4/3. The condensation proceeds either gradually, or in the form of (quasihydrostatic) collapse. For {gamma} < 4/3, the cloud is shown to expand. The second problem addresses a magnetized plasma slab that undergoes quasihydrostatic radiative cooling and condensation. The problem is solved analytically, employing the Lagrangian mass coordinate.
Date: March 1, 1995
Creator: Meerson, B.; Megged, E. & Tajima, T.
Partner: UNT Libraries Government Documents Department

Linear pulsations of strange modes in LBVs

Description: Outbursts of the luminous blue variables have been studied for a long time, but a detailed understanding of the mechanism has eluded astronomers. In the last few years it has been recognized that the dramatic increase in outburst brightness is due almost entirely to the luminosity being shifted into the visual band, rather than a true luminosity increase. Some ideas about how these very massive and very luminous stars might display their dramatic increase of visual brightness have been given by many. A sampling is given here. The strange modes we consider in this paper have been studied by the G{umlt o}ttingen group under Fricke and Glatzel. We are interested in strange modes because some are very rapidly growing when conditions are right, and amplitudes reach large radial velocity (200 km/s) and luminosity (0.1 mag). Then the radiative luminosity in deep layers can surpass the Eddington limit during each pulsation cycle, and outbursts occur.
Date: December 31, 1996
Creator: Cox, A.N.; Guzik, J.A. & Soukup, M.S.
Partner: UNT Libraries Government Documents Department

The effects of metallicity on {delta} Scuti star asteroseismology

Description: In {delta} Scuti star seismology, most researchers use evolution and pulsation models assuming a solar element mixture and Z = 0.02 for preliminary determinations of stellar masses or evolutionary state from observed frequencies. Here the authors investigate the consequences of this assumption by considering the effects of metallicity changes in the models on their inferences of the internal structure of {delta} Scuti stars. They use the main-sequence {delta} Scuti star FG Vir, and the more evolved shell hydrogen burning star {delta} Scuti to illustrate their results.
Date: March 1, 1998
Creator: Guzik, J.A.; Bradley, P.A. & Templeton, M.R.
Partner: UNT Libraries Government Documents Department

A nonlinear study of luminous blue variables and possible outbursts

Description: Linear pulsation analysis of luminous blue variable models shows instability to pulsations in multiple radial and nonradial strange modes (see Glatzel, these proceedings). These modes have large linear growth rates, sometimes exceeding several hundred percent per period, which prompted us to investigate the nonlinear behavior of envelope models. While the nonradial modes are predicted in the linear analysis to have higher growth rates than the radial modes, nonlinear nonradial pulsations are beyond the capabilities of pulsation hydrodynamics codes developed to date. As for relevant radial nonlinear calculations, Stothers & Chin (1993) report briefly on nonlinear hydrodynamic calculations of one dynamically unstable massive star envelope model. Aikawa & Sreenivasan (1996) have done nonlinear oscillation modeling of strange modes in low-mass AGB stars. Kiriakidis et al. (these proceedings) present nonlinear models (not including convection) of two types of strange-mode pulsators, massive stars and Wolf-Rayet stars. They find periodic or irregular pulsations, and suggest that pulsation drives mass loss. Here we present new nonlinear hydrodynamic calculations to explore the link between strange-mode pulsations and LBV outbursts.
Date: December 31, 1996
Creator: Guzik, J.A.; Cox, A.N.; Despain, K.M. & Soukup, M.S.
Partner: UNT Libraries Government Documents Department

Aspen Winter Conference Series

Description: (B204) The meeting will bring together observers and theorists in a highly interactive format, to further connect the local and cosmological star formation communities. Forward looking talks, aimed at the other communities, will survey terminology, achievements, problems and aspirations. Discussion will focus on the definition of the key questions, how the different communities can help each other, and preparations for the incorporation of realistic star formation into cosmological simulations.
Date: January 1, 1999
Partner: UNT Libraries Government Documents Department

GRAVITATIONAL WAVES FROM STELLAR COLLAPSE

Description: Stellar core-collapse plays an important role in nearly all facets of astronomy: cosmology (as standard candles), formation of compact objects, nucleosynthesis and energy deposition in galaxies. In addition, they release energy in powerful explosions of light over a range of energies, neutrinos, and the subject of this meeting, gravitational waves. Because of this broad range of importance, astronomers have discovered a number of constraints which can be used to help them understand the importance of stellar core-collapse as gravitational wave sources.
Date: January 1, 2001
Creator: FRYER, C. L.
Partner: UNT Libraries Government Documents Department

Bulge Delta Scuti Stars in the MACHO database

Description: The authors describe the search for {delta} Scuti stars in the MACHO database of bulge fields. Concentrating on a sample of high amplitude {delta} Scutis, they examine the light curves and pulsation modes. They also discuss their spatial distribution and evolutionary status using mean colors and absolute magnitudes.
Date: July 29, 1997
Creator: Minnniti, D; Alcock, C; Alves,D R; Axelrod, T S; Becker, A; Bennett, D P et al.
Partner: UNT Libraries Government Documents Department

New developments in the mechanism for core-collapse supernovae

Description: Recent results indicate that the standard type-2 supernova scenario in which the shock wave stagnates but is reenergized by neutrino heating fails to consistently produce supernova explosions having the required characteristics. The authors review the theory of convection and survey some recent calculations indicating the importance of convection operating on millisecond timescales in the protoneutron star. These calculations suggest that such convection is probably generic to the type-2 scenario, that this produces a violet overturn of material below the stalled shock, and that this overturn could lead to significant alterations in the neutrino luminosity and energy. This provides a mechanism that could be effective in reenergizing the stalled shock and producing supernovae explosions having the quantitative characteristics demands by observations. This mechanism implies, in turn, that the convection cannot be adequately described by the 1-dimensional hydrodynamics employed in most simulations. Thus, a full understanding of the supernova mechanism and the resulting heavy element production is likely to require 3-dimensional relativistic hydrodynamics and a comprehensive description of neutrino transport. The prospects for implementing such calculations using a new generation of massively parallel supercomputers and modern scalable algorithms are discussed.
Date: December 1994
Creator: Guidry, M.
Partner: UNT Libraries Government Documents Department

The evolution and explosion of massive Stars II: Explosive hydrodynamics and nucleosynthesis

Description: The nucleosynthetic yield of isotopes lighter than A = 66 (zinc) is determined for a grid of stellar masses and metallicities including stars of 11, 12, 13, 15, 18, 19, 20, 22, 25, 30, 35, and 40 M{sub {circle_dot}} and metallicities Z = 0, 10{sup {minus}4}, 0.01, 0.1, and 1 times solar (a slightly reduced mass grid is employed for non-solar metallicities). Altogether 78 different model supernova explosions are calculated. In each case nucleosynthesis has already been determined for 200 isotopes in each of 600 to 1200 zones of the presupernova star, including the effects of time dependent convection. Here each star is exploded using a piston to give a specified final kinetic energy at infinity (typically 1.2 {times} 10{sup 51} erg), and the explosive modifications to the nucleosynthesis, including the effects of neutrino irradiation, determined. A single value of the critical {sup 12}C({sub {alpha},{gamma}}){sup 16}O reaction rate corresponding to S(300 keV) = 170 keV barns is used in all calculations. The synthesis of each isotope is discussed along with its sensitivity to model parameters. In each case, the final mass of the collapsed remnant is also determined and often found not to correspond to the location of the piston (typically the edge of the iron core), but to a ``mass cut`` farther out. This mass cut is sensitive not only to the explosion energy, but also to the presupernova structure, stellar mass, and the metallicity. Unless the explosion mechanism, for unknown reasons, provides a much larger characteristic energy in more massive stars, it appears likely that stars larger than about 30 M{sub {center_dot}} will experience considerable reimplosion of heavy elements following the initial launch of a successful shock. While such explosions will produce a viable, bright Type II supernova light curve, lacking the radioactive tail, they will have dramatically ...
Date: August 30, 1995
Creator: Woosley, S.E. & Weaver, T.A.
Partner: UNT Libraries Government Documents Department

The DBV stars: Progress and problems

Description: The DB white dwarfs comprise the majority of the 20% or so of non-DA white dwarfs, and have effective temperatures between 11,000 K and 30,000 K. The hottest DBs define the cool end of the so called ``DB gap`` that lies between 30,000 K and 45,000 K; in this region, no helium atmosphere white dwarf is known. The existence of this gap presents a great puzzle concerning the origin and evolution of helium atmosphere white dwarfs. Asteroseismology of the DBV stars as a class will tell scientists what DBs just below the red edge of the DB gap are like. This, coupled with structural understanding of the pulsating PG 1159 stars, the interacting binary white dwarfs (IBWDs), and white dwarf evolution calculations should fill in the gaps of knowledge about the DB white dwarfs and their origins. Here, the author describes the current status of understanding of DBV white dwarf structure via asteroseismology, with an emphasis on what was learned through Whole Earth Telescope data.
Date: September 1995
Creator: Bradley, P. A.
Partner: UNT Libraries Government Documents Department

Science & Technology Review May/June 2008

Description: This month's issue has the following articles: (1) Biomedical Technology Has a Home at Livermore--Commentary by Cherry A. Murray; (2) Shaping the Future of Aneurysm Treatments--Livermore foam devices may offer significant advantages for treating some forms of aneurysms; (3) Ring around a Stellar Shell: A Tale of Scientific Serendipity--Using a three-dimensional model, Livermore scientists have solved a long-standing puzzle of stellar evolution; and (4) On Assignment in Washington, DC--Livermore personnel in Washington, DC, support federal sponsors and become valuable assets to Laboratory programs.
Date: March 19, 2008
Creator: Chinn, D J
Partner: UNT Libraries Government Documents Department

Comparing ligo merger rate observations with theory: distribution of star-forming conditions

Description: Within the next decade, ground based gravitational wave detectors are in principle capable of determining the compact object merger rate per unit volume of the local universe to better than 20% with more than 30 detections. Though these measurements can constrain our models of stellar, binary, and cluster evolution in the nearby present-day and ancient universe, we argue that the universe is sufficiently heterogeneous (in age and metallicity distribution at least) and that merger rates predicted by these models can be sufficiently sensitive to those heterogeneities so that a fair comparison of models per unit similar star forming mass necessarily introduces at least an additional 30%--50% systematic error into any constraints on compact binary evolution models. Without adding new electromagnetic constraints on massive binary evolution or relying on more information from each merger (e.g. , binary masses and spins), as few as the {approx_equal}5 merger detections could exhaust the information available in a naive comparison to merger rate predictions. As a concrete example immediately relevant to analysis of initial and enhanced LIGO results, we use a nearby-universe catalog to demonstrate that no one tracer of stellar content can be consistently used to constrain merger rates without introducing a systematic error of order 0(30%) at 90% confidence (depending on the type of binary involved). For example, though binary black holes typically take many Gyr to merge, binary neutron stars often merge rapidly; different tracers of stellar content are required for these two types. More generally, we argue that theoretical binary evolution can depend sufficiently sensitively on star-forming conditions -- even assuming no uncertainty in binary evolution model -- that the distribution of star forming conditions must be incorporated to reduce the systematic error in merger rate predictions below roughly 40%. We emphasize that the degree of sensitivity to star-forming conditions depends ...
Date: January 1, 2008
Creator: Belczynski, Kryzysztof; Kopparapu, R & O' Shaughnessy, R
Partner: UNT Libraries Government Documents Department

Helium-induced reactions in astrophysics

Description: Helium-induced reactions play a crucial role in stellar nucleosynthesis. Carbon and oxygen are produced mainly during the helium-burning phase by the chain of reactions {sup 8}Be({alpha}, {gamma} + e{sup +}e{sup {minus}}){sup 12}C({alpha}, {gamma}){sup 16}O. The first step, often called triple-{alpha} capture, was proposed by Hoyle to bypass the mass stability gap at {sup 8}Be. The second step gives rise to the largest uncertainty in most of the calculated stellar abundances. Later {alpha}-captures on {sup 13}C are believed to be a major source of s-process neutrons. The status of each of these important reactions is reviewed here.
Date: November 1, 1997
Creator: Hale, G.M.
Partner: UNT Libraries Government Documents Department

Pulsations and outbursts of luminous blue variables

Description: We propose an outburst mechanism for the most luminous stars in our and other galaxies. These million solar luminosity stars, with masses (after earlier mass loss) of between 20 and maybe 70 solar masses, are pulsationally unstable for both radial and low-degree nonradial modes. Some of these modes are ``strange,`` meaning mostly that the pulsations are concentrated near the stellar surface and have very rapid growth rates in linear theory. The pulsation driving is by both the high iron line opacity (near 150,000 K) and the helium opacity (near 30,000 K) kappa effects. Periods range from 5 to 40 days. Depending on the composition, pulsations periodically produce luminosities above the Eddington limit for deep layers. The radiative luminosity creates an outward push that readily eases the very low gamma envelope to very large outburst radii. A key point is that a super-Eddington luminosity cannot be taken up by the sluggish convection rapidly enough to prevent an outward acceleration of much of the envelope. As the helium abundance in the envelope stellar material increases by ordinary wind mass loss and the luminous blue variable outbursts, the opacity in the deep pulsation driving layers decreases. This makes the current Eddington luminosity even higher so that pulsations can then no longer give radiative luminosities exceeding the limit. For the lower mass and luminosity luminous blue variables there is considerably less iron line opacity driving, and pulsations are almost all caused by the helium ionization kappa effect.
Date: June 1, 1997
Creator: Cox, A.N.; Guzik, J.A.; Soukup, M.S. & Despain, K.M.
Partner: UNT Libraries Government Documents Department

Neutron star evolution and emission

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The authors investigated the evolution and radiation characteristics of individual neutron stars and stellar systems. The work concentrated on phenomena where new techniques and observations are dramatically enlarging the understanding of stellar phenomena. Part of this project was a study of x-ray and gamma-ray emission from neutron stars and other compact objects. This effort included calculating the thermal x-ray emission from young neutron stars, deriving the radio and gamma-ray emission from active pulsars and modeling intense gamma-ray bursts in distant galaxies. They also measured periodic optical and infrared fluctuations from rotating neutron stars and search for high-energy TeV gamma rays from discrete celestial sources.
Date: August 1, 1997
Creator: Epstein, R.I.; Edwards, B.C. & Haines, T.J.
Partner: UNT Libraries Government Documents Department

Deriving the structure of pre-supernovae and delta Scuti stars using nonradial oscillations

Description: This is the final report of a three-year Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The objective is to learn more about the internal structure of two classes of variable stars, by using the observational data afforded by their pulsation properties. The authors updated the one-dimensional computer codes to calculate the evolution and pulsation frequencies of representative delta Scuti and LBV models. They compared the observed pulsation properties with model predictions in an iterative process to find a model (or models) with interior structures that matched the observational constraints for several delta Scuti stars. They carried out nonlinear hydrodynamic modeling of LBV envelopes and proposed a mechanism for their periodic outbursts. Finally, they began validation of a two-dimensional stellar evolution code that will be used to investigate the effects of rotation and hydrodynamic instabilities on the interior structure of these stars.
Date: November 1, 1998
Creator: Guzik, J.A.; Bradley, P.A.; Cox, A.N.; Swenson, F.J.; Deupree, R.G.; Soukup, M.S. et al.
Partner: UNT Libraries Government Documents Department

Possible new class of dense white dwarfs

Description: If the strange quark matter hypothesis is true, then a new class of white dwarfs can exist whose nuclear material in their deep interiors can have a density as high as the neutron drip density, a few hundred times the density in maximum-mass white dwarfs and 4 {times} 10{sup 4} the density in dwarfs of typical mass, M {approximately} 0.6M{sub {circle_dot}}. Their masses fall in the approximate range 10{sup {minus}4} to 1M{sub {circle_dot}}. They are stable against acoustical modes of vibration. A strange quark core stabilizes these stars, which otherwise would have central densities that would place them in the unstable region of the sequence between white dwarfs and neutron stars.
Date: January 10, 1995
Creator: Glendenning, N.K.; Kettner, C. & Weber, F.
Partner: UNT Libraries Government Documents Department

Properties of LMC planetary nebulae and parent populations in the MACHO database

Description: The MACHO microlensing experiment's time-sampled photometry database contains blue and red lightcurves for nearly 9 million stars in the central bar region of the Large Magellanic Cloud (LMC). We have identified known LMC Planetary Nebulae (PN) in the database and find one, Jacoby 5, to be variable. We additionally present data on the ``parent populations`` of LMC PN, and discuss the star formation history of the LMC bar. 14 refs., 1 fig.
Date: October 1, 1996
Creator: Alves, D.R.; Alcock, C. & Cook, K.H.
Partner: UNT Libraries Government Documents Department