14 Matching Results

Search Results

Advanced search parameters have been applied.

Energetic Processing of Interstellar Silicate Grains by Cosmic Rays

Description: While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively 'low' energy, heavy ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg{sub 2}SiO{sub 4}) amorphizes when irradiated by 10 MeV Xe{sup ++} ions at large enough fluences. Using modeling, we extrapolate these results to show that 0.1-5.0 GeV heavy ion cosmic rays can rapidly ({approx}70 Million yrs) amorphize crystalline silicate grains ejected by stars into the interstellar medium.
Date: March 28, 2007
Creator: Bringa, E M; Kucheyev, S O; Loeffler, M J; Baragiola, R A; Tielens, A G Q M; Dai, Z R et al.
Partner: UNT Libraries Government Documents Department

HESS J1023-575: Non-Thermal Particle Acceleration Associated With the Young Stellar Cluster Westerlund 2

Description: The results from H.E.S.S. observations towards Westerlund 2 are presented. The detection of very-high-energy gamma-ray emission towards the young stellar cluster Westerlund 2 in the HII complex RCW49 by H.E.S.S. provides ample evidence that particle acceleration to extreme energies is associated with this region. A variety of possible emission scenarios is mentioned, ranging from high-energy gamma-ray production in the colliding wind zone of the massive Wolf-Rayet binary WR 20a, collective wind scenarios, diffusive shock acceleration at the boundaries of wind-blown bubbles in the stellar cluster, and outbreak phenomena from hot stellar winds into the interstellar medium. These scenarios are briefly compared to the characteristics of the associated new VHE gamma-ray source HESS J1023-575, and conclusions on the validity of the respective emission scenarios for high-energy gamma-ray production in the Westerlund 2 system are drawn.
Date: November 14, 2007
Creator: Reimer, O.; /Stanford U., HEPL /KIPAC, Menlo Park; Hinton, J.; U., /Leeds; Hofmann, W.; Hoppe, S. et al.
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

Massive Stars in Colliding Wind Systems: the GLAST Perspective

Description: Colliding winds of massive stars in binary systems are considered as candidate sites of high-energy non-thermal photon emission. They are already among the suggested counterparts for a few individual unidentified EGRET sources, but may constitute a detectable source population for the GLAST observatory. The present work investigates such population study of massive colliding wind systems at high-energy gamma-rays. Based on the recent detailed model (Reimer et al. 2006) for non-thermal photon production in prime candidate systems, we unveil the expected characteristics of this source class in the observables accessible at LAT energies. Combining the broadband emission model with the presently cataloged distribution of such systems and their individual parameters allows us to conclude on the expected maximum number of LAT-detections among massive stars in colliding wind binary systems.
Date: November 29, 2011
Creator: Reimer, Anita; Reimer, Olaf & /Stanford U., HEPL /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department

Old and new neutron stars

Description: The youngest known radiopulsar in the rapidly spinning magnetized neutron star which powers the Crab Nebula, the remnant of the historical supernova explosion of 1054 AD. Similar neutron stars are probably born at least every few hundred years, but are less frequent than Galactic supernova explosions. They are initially sources of extreme relativistic electron and/or positron winds (approx.10/sup 38/s/sup -1/ of 10/sup 12/ eV leptons) which greatly decrease as the neutron stars spin down to become mature pulsars. After several million years these neutron stars are no longer observed as radiopulsars, perhaps because of large magnetic field decay. However, a substantial fraction of the 10/sup 8/ old dead pulsars in the Galaxy are the most probable source for the isotropically distributed ..gamma..-ray burst detected several times per week at the earth. Some old neutron stars are spun-up by accretion from companions to be resurrected as rapidly spinning low magnetic field radiopulsars. 52 references, 6 figures, 3 tables.
Date: September 1, 1984
Creator: Ruderman, M.
Partner: UNT Libraries Government Documents Department

The origin and development of instabilities in radiatively-driven stellar winds

Description: The numerous observational indicators of instability in the radiatively-driven winds of hot stars are review briefly, followed by a summary of the present theoretical understanding of the linear instability of such winds. This provides the motivation for the hydrodynamic simulation, the major thrust of the paper. A serious approximation that must be made in order to reduce the cost of the simulations to a reasonable level--the absorption approximation for the radiation force--is discussed in some detail. The hydrodynamic methods are described briefly, and then the computational results for winds models computed in the absorption approximation are discussed. The most notable results pertain to the critical nature of the ratio v{sub th}/a of the intrinsic line width to the sound speed. When this ratio is large, only a negligible wind results; when the ratio is small, the wind executes permanent self-excited oscillations; in an intermediate range the wind is globally stable, but acts as a powerful wave amplifier. The morphology of the oscillations--strong rarefactions and reverse shocks--is described and related to Abbott's linear theory, and the possible connection to observations is mentioned. 30 refs.
Date: November 20, 1990
Creator: Castor, J.I.
Partner: UNT Libraries Government Documents Department

VHE Gamma-Rays From Westerlund 2 And Implications for the Inferred Energetics

Description: The H.E.S.S. collaboration recently reported the discovery of VHE {gamma}-ray emission coincident with the young stellar cluster Westerlund 2. This system is known to host a population of hot, massive stars, and, most particularly, the WR binary WR 20a. Particle acceleration to TeV energies in Westerlund 2 can be accomplished in several alternative scenarios, therefore we only discuss energetic constraints based on the total available kinetic energy in the system, the actual mass loss rates of respective cluster members, and implied gamma-ray production from processes such as inverse Compton scattering or neutral pion decay. From the inferred gamma-ray luminosity of the order of 10{sup 35} erg/s, implications for the efficiency of converting available kinetic energy into non-thermal radiation associated with stellar winds in the Westerlund 2 cluster are discussed under consideration of either the presence or absence of wind clumping.
Date: November 14, 2007
Creator: Reimer, O.; /Stanford U., HEPL /KIPAC, Menlo Park; Aharonian, F.; /Heidelberg, Max Planck Inst. /Dublin Inst.; Hinton, J.; U., /Leeds et al.
Partner: UNT Libraries Government Documents Department

The H II Region of a Primordial Star

Description: The concordance model of cosmology and structure formation predicts the formation of isolated very massive stars at high redshifts in dark matter dominated halos of 10{sup 5} to 10{sup 6} Msun. These stars photo-ionize their host primordial molecular clouds, expelling all the baryons from their halos. When the stars die, a relic H II region is formed within which large amounts of molecular hydrogen form which will allow the gas to cool efficiently when gravity assembles it into larger dark matter halos. The filaments surrounding the first star hosting halo are largely shielded and provide the pathway for gas to stream into the halo when the star has died. We present the first fully three dimensional cosmological radiation hydrodynamical simulations that follow all these effects. A novel adaptive ray casting technique incorporates the time dependent radiative transfer around point sources. This approach is fast enough so that radiation transport, kinetic rate equations, and hydrodynamics are solved self-consistently. It retains the time derivative of the transfer equation and is explicitly photon conserving. This method is integrated with the cosmological adaptive mesh refinement code enzo, and runs on distributed and shared memory parallel architectures. Where applicable the three dimensional calculation not only confirm expectations from earlier one dimensional results but also illustrate the multi-fold hydrodynamic complexities of H II regions. In the absence of stellar winds the circumstellar environments of the first supernovae and putative early gamma-ray bursts will be of low density {approx}1 cm{sup -3}. Albeit marginally resolved, ionization front instabilities lead to cometary and elephant trunk like small scale structures reminiscent of nearby star forming regions.
Date: June 7, 2006
Creator: Abel, Tom; Wise, John H.; /KIPAC, Menlo Park; Bryan, Greg L. & /Columbia U., Astron. Astrophys.
Partner: UNT Libraries Government Documents Department

X-Ray Spectral Study of the Photoionized Stellar Wind in Vela X-1

Description: We present results from quantitative modeling and spectral analysis of the high mass X-ray binary system Vela X-1 obtained with the Chandra High Energy Transmission Grating Spectrometer. The observations cover three orbital phase ranges within a single binary orbit. The spectra exhibit emission lines from H-like and He-like ions driven by photoionization, as well as fluorescent emission lines from several elements in lower charge states. The properties of these X-ray lines are measured with the highest accuracy to date. In order to interpret and make full use of the high-quality data, we have developed a simulator, which calculates the ionization and thermal structure of a stellar wind photoionized by an X-ray source, and performs Monte Carlo simulations of X-ray photons propagating through the wind. The emergent spectra are then computed as a function of the viewing angle accurately accounting for photon transport in three dimensions including dynamics. From comparisons of the observed spectra with results from the simulator, we are able to find the ionization structure and the geometrical distribution of material in the stellar wind of Vela X-1 that can reproduce the observed spectral line intensities and continuum shapes at different orbital phases remarkably well. We find that the stellar wind profile can be represented by a CAK-model with a star mass loss rate of (1.5-2.0) x 10{sup -6} M{sub {circle_dot}} yr{sup -1}, assuming a terminal velocity of 1100 km s{sup -1}. It is found that a large fraction of X-ray emission lines from highly ionized ions are formed in the region between the neutron star and the companion star. We also find that the fluorescent X-ray lines must be produced in at least three distinct regions: (1) the extended stellar wind, (2) reflection off the stellar photosphere, and (3) in a distribution of dense material partially covering and ...
Date: July 10, 2006
Creator: Watanabe, Shin; Sako, Masao; Ishida, Manabu; Ishisaki, Yoshitaka; Kahn, Steven M.; Kohmura, Takayoshi et al.
Partner: UNT Libraries Government Documents Department

A source of observational constraints on the structure of Wolf-Rayet winds

Description: In some WR + O-star binary systems the WR wind eclipses the O star. The profile changes as function of impact are reproduced with a simple model which gives information on both the radial dependence of the velocity and opacity. 2 refs.
Date: January 1, 1989
Creator: Auer, L.H.; Koenigsberger, G. (Los Alamos National Lab., NM (USA) & Universidad Nacional Autonoma de Mexico, Mexico City (Mexico). Inst. de Astronomia)
Partner: UNT Libraries Government Documents Department

Time-dependent mass loss from hot stars with and without radiative driving

Description: A numerical hydrodynamics code is used to investigate two aspects of the winds of hot stars. The first is the question of the instability of the massive radiatively-driven wind of an O star that is caused by the line shape mechanism: modulation of the radiation force by velocity fluctuations. The evolution of this instability is studied in a model O star wind, and is found, /ital modulo/ some numerical uncertainty, to lead to wave structures that are compatible with observations of wind instabilities. The other area of investigation is of main-sequence B star winds. Attempts were made to simulate a radiatively-driven and a pulsation-driven wind in a B star, but in each case the wind turned out to be very weak. It is argued that the pulsation-driven wind model is not likely to apply to B stars. 28 refs., 11 figs.
Date: January 29, 1988
Creator: Castor, J.I.; Owocki, S.P. & Rybicki, G.B.
Partner: UNT Libraries Government Documents Department

The EUV emission of cataclysmic variables

Description: Approximately half the luminosity of a typical cataclysmic variable may emerge as an optically thick component peaking in the EUV. Observations of this component are important for understanding the energetics and accretion rates of CV's in general, as well as for understanding the physics of the accretion process. The nature of the turbulent boundary layers and winds of disk accretors and the heating of the white dwarfs by accretion are among the problems which can be addressed by observations in the EUV. 46 refs., 2 figs.
Date: July 1, 1989
Creator: Raymond, J.C. & Mauche, C.W.
Partner: UNT Libraries Government Documents Department

Variable star research at Los Alamos

Description: Three major areas of variable star research at Los Alamos are carried out: (1) a study using improved Cepheid light curves in order to define more precisely the Hertzsprung sequence, in collaboration with John Castor and John Cox; (2) the suggestion by A. Cox that helium enrichment occurs in the stellar envelope, by a stellar wind, which may explain many of the mass anomalies, this work being with G. Michaud, D. King, R. Deupree, and S. Hodson; and (3) the study of Cepheid and RR Lyrae colors to compare directly to the observations. A brief discussion of the present status of each of these research programs will be given. 25 references.
Date: January 1, 1978
Creator: Davis, C.G.; Cox, A.N. & Adams, T.F.
Partner: UNT Libraries Government Documents Department

X-ray emission and the winds of cataclysmic variables

Description: X-ray and ultraviolet observations of cataclysmic variable stars reveal a variety of exotic behavior - pulsations, winds, and episodic outbursts - are these related, what do they tell us about the nature of the outburst, about the environment of the accreting white dwarf. The author summarizes the observed changes in the x-ray and uv continuum and spectral features through the outbursts of the dwarf novae. The author then discusses how the modeling of these data have refined our ideas about the location and nature of the emissions, and the source of the outbursts. The author shows how comparisons of the x-ray and uv properties of cataclysmic variables with similar phenomena in other astronomical systems - the solar corona, OB stars, and Be stars - suggest ways in which the x-ray and uv emissions in CVs may be related, and point to further, specific observations that would elucidate our understanding of the behavior and role of the white dwarf in the outburst. 26 references.
Date: January 1, 1985
Creator: Cordova, F.A.
Partner: UNT Libraries Government Documents Department