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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

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

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

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

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