Astrophysical Simulations Research Articles

Nonparametric estimation of density profiles

view Abstract Citations (127) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS NonParametric Estimation of Density Profiles Merritt, David ; Tremblay, Benoit Abstract We present two nonparametric algorithms for estimating the surface and space density profiles of a spherical stellar system directly from a set of measured positions on the plane of the sky. The algorithms require no binning and are able to cope with the amplification of errors associated with Abel deconvolution, without the bias that results from assuming an ad hoc fitting function. We also show how to estimate confidence bands for both functions. We apply our algorithms to the determination of the density profiles near the center of the Coma galaxy cluster, and the globular cluster M15. Both systems are found to exhibit an approximate power-law dependence of space density on radius near the center: roughly r-1 in the case of Coma, and approximately r-2 in the case of M15. We find no evidence for a core in either system. Application of our techniques to other problems, including the construction of smooth potential solvers for the collisionless N-body problem, is discussed. Publication: The Astronomical Journal Pub Date: August 1994 DOI: 10.1086/117088 Bibcode: 1994AJ....108..514M Keywords: Algorithms; Density Distribution; Galactic Clusters; Star Clusters; Star Distribution; Computational Astrophysics; Kernel Functions; Stellar Models; Astrophysics; GALAXIES: STARS CLUSTERS full text sources ADS | data products SIMBAD (3)

Gravitational radiation from pairs of realistic, nonaccreting compact stars

view Abstract Citations (2) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gravitational Radiation from Pairs of Realistic, Nonaccreting Compact Stars Spyrou, N. K. ; Kokkotas, K. D. Abstract The dependence is described of the gravitational-radiation luminosity on the changing internal characteristics of the members of a binary system which are assumed to be realistic compact stars. The new, general relativistic post-Newtonian contribution to the gravitational-radiation luminosity is due to the evolution of the nonaccreting members of the stellar pair, and it is nonvanishing even in the case of equal inertial masses of the members. For nonaccreting members of equal inertial masses in circular relative motion, the rate of emission of gravitational radiation due to the change of the members' internal characteristics is the same for both kinds of change, namely deceleration or acceleration of the axial rotations, and/or increase or decrease of the radii. In this case the value of the emission rate although generally small, can be comparable to the corresponding purely orbital one of the same post-Newtonian order. Also the dependence of the gravitational-wave phase change on the changing internal characteristics is examined and is compared with that due to simply orbital, Newtonian, or post-Newtonian characteristics. Finally, the underlying theoretical framework permits an analogous investigation for different inertial masses and, in principle, also for mass-accreting binaries. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174483 Bibcode: 1994ApJ...431..254S Keywords: Binary Stars; Gravitational Waves; Stellar Evolution; Stellar Interiors; Stellar Luminosity; Stellar Mass; Computational Astrophysics; Magnetic Fields; Relativity; Stellar Mass Accretion; Astrophysics; RADIATION MECHANISMS: NONTHERMAL; RELATIVITY; STARS: BINARIES: CLOSE; STARS: INTERIORS; STARS: EVOLUTION full text sources ADS | data products SIMBAD (1)

The effect of constant horizontial magnetic fields on model atmospheres of AP stars

view Abstract Citations (21) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Effect of Constant Horizontal Magnetic Fields on Model Atmospheres of AP Stars LeBlanc, F. ; Michaud, G. ; Babel, J. Abstract Model atmospheres have been constructed in LTE for A-type magnetic stars. The Lorentz force caused by the interaction between the ambipolar diffusion of hydrogen and the magnetic field, has been taken into account in the hydrostatic equation. Calculations were carried out using a modified version of the ATLAS line blanketed program with metal solar abundances, but no helium. Models with Teff = 8500, 10,000 and 12,000 K, log g = 4.0 and constant horizontal magnetic fields up to 10 kG in the 12,000 K model and up to 8 kG in the cooler ones are presented. The Lorentz force increases effective gravity in the hydrogen ionization region and thus modifies the structureof magnetic atmospheres. In a Teff = 10,000 K star, effective gravity is up to 7 times greater than gravity in the presence of a 5 kG horizontal field in the hydrogen ionization zone. The pressure gradient is proportionately increased in the ionization region of these magnetic Ap stars. The effect is present in stars with Teff up to at least 12,000 K but the effect decreases as Teff further increased and hydrogen is completely ionized in the atmosphere. Balmer lines are broadened in these magnetic atmospheres leading to a variation of 0.01 mag in the beta index in the Teff - 10,000 and 12,000 K models. This is smaller than the variation of 0.02 mag with rotational period typically observed in Ap stars. This still might play a role in explaining the beta variations of stars like 56 Ari and CU Vir. Observed metallicity gradients and magnetic field configurations could also play a role. The effect on Balmer lines in the Teff = 8500 K model is considerably reduced, since the ionization zone occurs at too large an optical depth for the pressure gradient increase to affect observations. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174492 Bibcode: 1994ApJ...431..388L Keywords: Abundance; Ambipolar Diffusion; Magnetic Stars; Stellar Atmospheres; Stellar Magnetic Fields; Stellar Models; Atmospheric Ionization; Balmer Series; Computational Astrophysics; Lorentz Force; Pressure Gradients; Astrophysics; DIFFUSION; STARS: ATMOSPHERES; STARS: MAGNETIC FIELDS; STARS: PECULIAR full text sources ADS | data products SIMBAD (2)

Misdirected quasars and evolved stars in distant radio galaxies

view Abstract Citations (77) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Misdirected Quasars and Evolved Stars in Distant Radio Galaxies di Serego Alighieri, Sperello ; Cimatti, Andrea ; Fosbury, Robert A. E. Abstract We present the results of spectropolarimetry of radio galaxies with redshift close to 1, complemented by imaging polarimetry. These show: (1) a flat (in flambda) polarized UV continuum, (2) broad polarized Mg II emission line, (3) narrow unpolarized forbidden emission lines, (4) a drop in the polarization of the continuum to the red of 4000 A, (5) a strong absorption feature at 2598 A, and (6) perpendicularity between the E vector of polarization, as measured with imaging polarimetry, and the optical/radio axis. These data provide evidence that distant radio galaxies harbor a quasar which is hidden from direct view but seen by scattering from the interstellar medium in the galaxy. Hot electrons cannot be the dominant scattering agent because of the presence of polarized Mg II lines with a width similar to that observed in quasars. The drop in the continuum polarization to the red of the 4000 A break suggests dilution by a red stellar population. The absorption line at 2598 A is probably due to interstellar Fe II. We discuss a two-component model consisting of a dust scattered quasar and an evolved stellar population, which reproduces simultaneously the polarization measurements and the UV/optical spectral energy distribution. Our results provide strong observational support to the unified model for the most luminous active galactic nuclei and to the idea that the alignment effect in distant radio galaxies is due to scattering, and they add an important tool for the study of the early evolution of galaxies. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174472 Bibcode: 1994ApJ...431..123D Keywords: Galactic Evolution; Interstellar Matter; Optical Polarization; Quasars; Red Shift; Absorption Spectra; Active Galactic Nuclei; Astronomical Models; Astronomical Polarimetry; Computational Astrophysics; Hot Electrons; Line Spectra; Astrophysics; GALAXIES: EVOLUTION; LINE: IDENTIFICATION; POLARIZATION; GALAXIES: QUASARS: GENERAL; ULTRAVIOLET: GALAXIES full text sources ADS | data products SIMBAD (15) NED (3)

Statistically efficient parallel testing of the Friedmann and chronometric flux-redshift predictions in the radio band

view Abstract Citations (6) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Statistically Efficient Parallel Testing of the Friedmann and Chronometric Flux-Redshift Predictions in the Radio Band Segal, I. E. ; Nicoll, J. F. ; Blackman, E. Abstract The flux-redshift predictions of Friedmann and chronometric cosmology are subjected to statistically efficient parallel tests on the basis of the 3C catalog of radio sources, as updated by Spinrad et al. (1985). Sub-samples limited in flux, redshift, and spectral index are treated to this end. Luminosity functions are estimated by an optimal nonparametric procedure that does not postulate spatial homogeneity or require completeness in redshift. Directly observed quantities are compared with the predictions of the respective cosmologies, and objective statistical significance levels are determined. Within the frame of the Friedmann cosmology, the disperson in log flux invariably exceeds the observed value, and the same is true of the correlation of log luminosity with redshift. Over the full redshift range of the sample, extending to z ~ 2.5, the deviations are significant at the ~4 σ, level. On the parallel basis the chronometric predictions are highly consistent with the observations, without evolution or any adjustable cosmological parameter such as q_0_. Moreover, the chronometric cosmology predicts quite accurately the deviations of the Friedmann predictions from direct observation, while the Friedmann cosmology is unable to explain the excellent fit of the chronometric predictions. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174382 Bibcode: 1994ApJ...430...63S Keywords: Computational Astrophysics; Cosmology; Galactic Evolution; Prediction Analysis Techniques; Radio Sources (Astronomy); Red Shift; Statistical Tests; Astronomical Catalogs; Chronology; Luminosity; Astrophysics; COSMOLOGY: THEORY; GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: EVOLUTION; RADIO CONTINUUM: GALAXIES full text sources ADS |

Luminosity-line-width relations and the extragalactic distance scale. 2: A comparison with Types IA and II supernovae

view Abstract Citations (79) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Luminosity--Line-Width Relations and the Extragalactic Distance Scale. II. A Comparison with Types IA and II Supernovae Pierce, Michael J. Abstract We present a comparison of galaxy distances estimated via the luminosity-line-width or Tully-Fisher relation and those estimated using supernovae. In the first case, we compare to distances estimated by assuming Type Ia supernovae (SNe Ia) are standard candles. The sample is restricted to those late-type galaxies which have had both reasonably well observed supernova events and which are also suitable for an application of the luminosity-line-width relation. Given this selection criteria, the resulting sample of 16 galaxies should be essentially free of "Malmquist-like" effects. Consequently, the observed dispersion between the two methods should only reflect that intrinsic in the two techniques and enable a fair assessment of the precision of the two methods. The Hubble diagram for the sample has an rms dispersion of only 0.4 mag, implying that both techniques are reasonably accurate indicators of extragalactic distances and have small intrinsic dispersion (~0.3 mag, or 15% in distance). The calibration of the luminosity-line-width relations obtained in Paper I using Cepheid and RR Lyrae variables provides an absolute calibration for SNe Ia of M_B_ = - 18.74 +/- 0.14. Existing data on more distant SNe Ia are used to derive a global estimate of the Hubble Constant, the result being H_0_ = 86 +/- 7 km s^-1^ Mpc^-1^. Alternatively, if we force H_0_ ~ 45 km s^-1^ Mpc^-1^, the luminosity- line-width relations would still have a small rms dispersion but must then have a discontinuous absolute calibration of {DELTA}m = 1.4 mag over a small range in distance, namely, 4.0 < D_TF_ < 6.5 Mpc. We also present a similar comparison using distances estimated via the method of modeling the expanding photospheres of Type II supernovae (SNe II). In this case, the mean distance ratio for 11 SNe II and their parent galaxies is D_TF_/D_SN II_ = 0.89 +/- 0.05, with an rms dispersion of only 11% in distance, or equivalently 0.22 mag. This implies that the SN II method is at least as reliable as assuming SNe Ia are standard candles, and that the luminosity-line-width relations produce accurate distance estimates as well (0.16 mag). That is, the distances estimated via these two methods are completely consistent within the stated internal errors. Consequently, discrepancies in the value of the Hubble Constant estimated using the SN II method and the TF relation are not significant. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174381 Bibcode: 1994ApJ...430...53P Keywords: Astronomical Models; Calibrating; Computational Astrophysics; Distance; Hubble Constant; Luminosity; Supernovae; Galactic Radiation; Hubble Diagram; Line Spectra; Mathematical Models; Astrophysics; COSMOLOGY: DISTANCE SCALE; STARS: SUPERNOVAE: GENERAL full text sources ADS | data products NED (65) SIMBAD (63) Related Materials (2) Part 1: 1992ApJ...387...47P Part 3: 2000ApJ...533..744T

How unusual is the locally quiet Hubble flow?

view Abstract Citations (34) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS How Unusual is the Locally Quiet Hubble Flow? Schlegel, David ; Davis, Marc ; Summers, Frank ; Holtzman, Jon A. Abstract The local galaxy distribution offers an interesting constraint to cosmological models of structure formation. The galaxies are distributed in a long, filamentary structure, presumably the result of large amplitude gravitational instability, yet the local velocity field is cold. In particular, there are no blueshifted galaxies within 5h^-1^ Mpc except those within the Local Group radius of 1h^-1^ Mpc. Using numerical simulations we demonstrate that such a situation is extremely rare for an observer in {OMEGA} = 1 CDM models, but is not uncommon in mixed dark matter models that include massive neutrinos. Publication: The Astrophysical Journal Pub Date: June 1994 DOI: 10.1086/174164 arXiv: arXiv:astro-ph/9310040 Bibcode: 1994ApJ...427..527S Keywords: Astronomical Models; Computational Astrophysics; Computerized Simulation; Cosmology; Density Distribution; Hubble Constant; Local Group (Astronomy); Mass Distribution; Universe; Velocity Distribution; Dark Matter; Filaments; Infrared Astronomy Satellite; Many Body Problem; Mass To Light Ratios; Neutrinos; Red Shift; Virgo Galactic Cluster; Astrophysics; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; COSMOLOGY: THEORY; COSMOLOGY: DARK MATTER; GALAXIES: CLUSTERING; GALAXIES: DISTANCES AND REDSHIFTS; Astrophysics E-Print: 9 pages including 6 figures, uuencoded compressed PostScript, CfPA93-TH-33 full text sources arXiv | ADS |

Nonlinear approximations to gravitational instability: A comparison in second-order perturbation theory

view Abstract Citations (28) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonlinear Approximations to Gravitational Instability: A Comparison in Second-Order Perturbation Theory Munshi, Dipak ; Starobinsky, Alexei A. Abstract Nonlinear approximation methods such as the Zeldovich approximation, and more recently the frozen flow and linear potential approximations, are sometimes used to simulate nonlinear gravitational instability in the expanding Universe. We investigate the relative accuracy of these approximations by comparing them with the exact solution using second order perturbation theory. We evaluate the density and velocity fields in these approximations to second order, and also determine the skewness parameter $S_3 = \langle \delta^3 \rangle / \langle (\delta^{(1)})^2\rangle ^2$ for each of the approximations again in second order. We find that $S_3 = 4, ~3, ~3.4$ for the Zeldovich approximation, the frozen flow and the linear potential approximations respectively as compared to $S_3 = {34/7}$ for the exact solution. Our results show that, of all the approximations considered, the Zeldovich approximation is the most accurate in describing the weakly nonlinear effects of gravity. Moreover, the Zeldovich approximation is much closer to the exact results for matter and velocity distributions than the other approximations if the slope of the power spectrum of density perturbations is $-3< n \le -1$. Publication: The Astrophysical Journal Pub Date: June 1994 DOI: 10.1086/174255 arXiv: arXiv:astro-ph/9311056 Bibcode: 1994ApJ...428..433M Keywords: Approximation; Computational Astrophysics; Dynamic Stability; Galactic Structure; Gravitational Fields; Nonlinear Equations; Accuracy; Density Distribution; Perturbation Theory; Velocity Distribution; Astronomy; GRAVITATION; INSTABILITIES; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; Astrophysics E-Print: Latex file, pages 20 , Two figures (available at 144.16.31.6 through anonymous ftp), IUCAA 31/93 full text sources arXiv | ADS |

Mergers, cooling flows, and evaporation revisited

view Abstract Citations (22) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Mergers, Cooling Flows, and Evaporation Revisited Fabian, A. C. ; Canizares, C. R. ; Boehringer, H. Abstract Sparks has recently proposed that the evaporation of cold gas clouds embedded in a hot intracluster medium can mimic the spectral appearance of a cooling flow without any cooling actually occurring. If correct, this would solve a major puzzle: the lack of clear evidence for large quantities of previously cooled material in cooling flow clusters. We show here that his evaporation model is untenable since it requires planar geometry to explain the observed X-ray spectra of cooling flows but is then in conflict with the observed X-ray images; the model investigated also has implausible boundary conditions. Moreover, the supposed evaporating region was itself embedded in a cooling flow, and the solution, which is assumed static, is highly unstable. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173959 Bibcode: 1994ApJ...425...40F Keywords: Cold Gas; Computational Astrophysics; Cooling Flows (Astrophysics); Elliptical Galaxies; Galactic Clusters; Intergalactic Media; X Ray Astronomy; Boundary Conditions; Geometry; Mathematical Models; Stellar Models; X Ray Spectra; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: COOLING FLOWS; GALAXIES: ELLIPTICAL AND LENTICULAR; CD; GALAXIES: INTERGALACTIC MEDIUM full text sources ADS | data products SIMBAD (1)

The K-band surface brightness of late-type stars and the distance to cataclysmic variables

view Abstract Citations (43) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The K-Band Surface Brightness of Late-Type Stars and the Distance to Cataclysmic Variables Ramseyer, Tod F. Abstract The method for determining the distance to cataclysmic variables using the K-band surface brightness is reexamined with a new, larger dataset. The K-band surface brightness of dwarf stars is constant over the range 3.0 less than or equal to V-K less than or equal to 5.0, and declines at larger values of V-K. The behavior of the K-band surface brightness depends on luminosity class. Theoretical models of M dwarf stars are roughly consistent with the observations. This new surface brightness relationship reconciles the different distance determinations for HT Cas. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173980 Bibcode: 1994ApJ...425..243R Keywords: Cataclysmic Variables; Computational Astrophysics; Dwarf Stars; Extremely High Frequencies; Infrared Astronomy; Infrared Stars; Late Stars; Brightness; Radii; Stellar Luminosity; Stellar Models; Astrophysics; INFRARED: STARS; STARS: NOVAE; CATACLYSMIC VARIABLES; STARS: INDIVIDUAL CONSTELLATION NAME: HT CASSIOPEIAE; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (1)

Planet detectability by an adaptive optics stellar coronagraph

view Abstract Citations (22) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Planet Detectability by an Adaptive Optics Stellar Coronagraph Nakajima, T. Abstract We show the possibilities for imaging Jupiter-like planets around nearby bright stars, assuming the availability of stellar coronagraphs coupled with modest adaptive optics mounted on large ground-based telescopes. The adaptive optics sharpens the point-spread function (PSF) of the planet, permits the use of an occulting disk smaller than the seeing disk, reduces the PSF envelope of the bright star, and therefore enhances the contrast between the planet and background. We have generated the PSF of the planet and the PSF envelope of the main star, using Monte Carlo simulations based on the Kolmogorov theory of turbulence. We calculate the signal-to-noise ratio of a model planet as a function of the angular separation based on photon statistics and realistic assumptions on the system performance. We have derived a criterion for optimizing the combination of the degree of adaptive compensation and the telescope diameter. It is found that a stellar coronagraph with modest adaptive optics mounted on a large ground-based telescope will be capable of detecting Jupiter-like planets around nearby bright stars such as alpha Cen, Sirius, and Procyon at wavelengths between 0.7 and 2.2 micrometers. Near-infrared observations are preferred because usable telescopes and isoplanatic angles are larger at infrared wavelengths than optical wavelengths for a given adaptive optics system. We have also found seven other target stars around which planets will be above the detection limit. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173990 Bibcode: 1994ApJ...425..348N Keywords: Adaptive Optics; Astronomical Photography; Computational Astrophysics; Coronagraphs; Extrasolar Planets; Gas Giant Planets; Monte Carlo Method; Planetary Systems; Signal To Noise Ratios; Stars; Stellar Envelopes; Imaging Techniques; Near Infrared Radiation; Points (Mathematics); Real Time Operation; Stellar Models; PLANETS; EXTRASOLAR PLANETS; ASTRONOMY; EQUIPMENT; PLANETARY SYSTEMS; TECHNIQUES; EARTH-BASED OBSERVATIONS; SIMULATION; NEAR INFRARED; WAVELENGTH; CALCULATIONS; OPTICAL PROPERTIES; TECHNICAL ASPECTS; Astrophysics; Planets; STARS: PLANETARY SYSTEMS; INSTRUMENTATION: MISCELLANEOUS full text sources ADS | data products SIMBAD (9)

Discovery of Cepheids in IC 4182: Absolute peak brightness of SN IA 1937C and the value of H[SUB]0[/SUB

view Abstract Citations (156) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Discovery of Cepheids in IC 4182: Absolute Peak Brightness of SN IA 1937C and the Value of H 0 Saha, A. ; Labhardt, Lukas ; Schwengeler, Hans ; Macchetto, F. D. ; Panagia, N. ; Sandage, Allan ; Tammann, G. A. Abstract Repeated imaging of a field in the Sdm galaxy IC 4182 with the Wide Field Camera of the Hubble Space Telescope has led to the discovery of 28 Cepheid variables. The distance modulus to this galaxy, derived from the Cepheids, is (m - M) = 28.36 +/- 0.09, and the absolute magnitude at peak brightness of the Type Ia supernova SN 1937C which appeared in IC 4182 is M_V_(max) = - 19.72 +/- 0.13. The Hubble diagram using peak V magnitudes of distant (Virgo cluster and beyond) Type Ia supernovae (selected a priori on the basis of light curve and spectral conformity with the prototype) shows an observational random scatter of 0.36 mag, with evidence that the intrinsic scatter is even smaller. Using the observed scatter to estimate the uncertainty of calibrating the Hubble diagram using only the single calibrating supernova SN 1937C, it is shown that H_0_ = 52 +/- 9km s^-1^ Mpc^-1^. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173957 Bibcode: 1994ApJ...425...14S Keywords: Brightness; Cepheid Variables; Computational Astrophysics; Cosmology; Galaxies; Interstellar Extinction; Spaceborne Astronomy; Supernovae; Astronomical Photometry; Color-Magnitude Diagram; Hubble Diagram; Hubble Space Telescope; Light Curve; Astrophysics; STARS: VARIABLES: CEPHEIDS; COSMOLOGY: OBSERVATIONS; COSMOLOGY: DISTANCE SCALE; GALAXIES: INDIVIDUAL ALPHANUMERIC: IC 4182; STARS: INDIVIDUAL ALPHANUMERIC: SN 1937C full text sources ADS | data products SIMBAD (13) NED (2) MAST (1) ESA (1)

Standard and nonstandard plasma neutrino emission revisited

view Abstract Citations (185) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Standard and Nonstandard Plasma Neutrino Emission Revisited Haft, Martin ; Raffelt, Georg ; Weiss, Achim Abstract On the basis of Braaten and Segel's representation of the electromagnetic dispersion relations in a QED plasma we check the numerical accuracy of several published analytic approximations to the plasma neutrino emission rates. As we find none of them satisfactory we derive a new analytic approximation which is accurate to within 4\%\ where the plasma process dominates. The correct emission rates in the parameter regime relevant for the red giant branch in globular clusters are larger by about $10-20$\% than those of previous stellar evolution calculations. Therefore, the core mass of red giants at the He flash is larger by about $0.005\M_\odot$ or 1\% than previously thought. Our bounds on neutrino magnetic dipole moments remain virtually unchanged. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173978 arXiv: arXiv:astro-ph/9309014 Bibcode: 1994ApJ...425..222H Keywords: Computational Astrophysics; Emission Spectra; Magnetic Dispersion; Neutrinos; Numerical Analysis; Plasmas (Physics); Quantum Electrodynamics; Approximation; Magnetic Dipoles; Stellar Cores; Stellar Evolution; Astrophysics; ELEMENTARY PARTICLES; PLASMAS; RADIATION MECHANISMS: MISCELLANEOUS; STARS: GIANTS; STARS: INTERIORS; Astrophysics E-Print: LaTeX, 16 pages, 12 figures on request from authors, MPI-Ph/93-65 full text sources arXiv | ADS | Related Materials (1) Erratum: 1995ApJ...438.1017H

Numerical study of two-fluid C-type shock waves

view Abstract Citations (31) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Numerical Study of Two-Fluid C-Type Shock Waves Toth, Gabor Abstract C-type magnetohydrodynamic shocks in the partially ionized interstellar medium are studied in the two-fluid approximation. The ionized fluid can move along the magnetic field lines, while it interacts with the neutral fluid via ion-neutral elastic scattering. I use an explicitly flux-conserving two-dimensional Eulerian flux-corrected transport code to study the dynamics of two-fluid shocks. A numerical instability intrinsic to two-fluid problems was discovered, and a solution to the problem is proposed. The code can successfully simulate C-type shocks. The results of the linear stability analysis by Wardle are confirmed, and the nonlinear behavior of the instability is explored. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173973 Bibcode: 1994ApJ...425..171T Keywords: Computational Astrophysics; Elastic Scattering; Euler Equations Of Motion; Interstellar Matter; Magnetohydrodynamics; Numerical Analysis; Shock Waves; Two Dimensional Models; Two Fluid Models; Algorithms; Boundary Conditions; Computerized Simulation; Differential Equations; Grid Generation (Mathematics); Linear Equations; Nonlinearity; Astrophysics; METHODS: NUMERICAL; MAGNETOHYDRODYNAMICS: MHD; SHOCK WAVES full text sources ADS |

The role of the equation of state in the 'prompt' phase of type II supernovae

view Abstract Citations (86) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Role of the Equation of State in the ``Prompt'' Phase of Type II Supernovae Swesty, F. Douglas ; Lattimer, James M. ; Myra, Eric S. Abstract We examine the role of the equation of state (EOS) of hot, dense matter in the prompt phase of stellar collapse. In order to achieve this goal, we have carried out radiation hydrodynamic simulations using an adjustable EOS that is consistent with constraints placed on the EOS by neutron star observations, nuclear systematics, and laboratory experiments. Our simulations of stellar collapse show that these constraints restrict the role that the EOS can play in determining the dynamics of shock propagation. We find that certain nuclear force parameters do not substantially affect the dynamics of collapse as strongly as previously believed. In particular, we find that the shock stall radius is practically independent of the compression modulus and symmetry energy when other constraints on the EOS are satisfied. In contrast, the nuclear symmetry energies have more profound effects on the collapse via electron capture, and these effects may be detectable by means of the neutrino signature of a nearby supernova. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173974 Bibcode: 1994ApJ...425..195S Keywords: Collapse; Computational Astrophysics; Computerized Simulation; Equations Of State; Hydrodynamics; Shock Wave Propagation; Supernovae; Electron Capture; Free Energy; Helmholtz Equations; Interstellar Matter; Shock Waves; Stellar Models; Astrophysics; DENSE MATTER; EQUATION OF STATE; SHOCK WAVES; STARS: NEUTRON; STARS: SUPERNOVAE: GENERAL full text sources ADS | data products SIMBAD (1)

The formation of compact groups of galaxies. I: Optical properties

view Abstract Citations (115) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Formation of Compact Groups of Galaxies. I. Optical Properties Diaferio, Antonaldo ; Geller, Margaret J. ; Ramella, Massimo Abstract The small crossing time of compact groups of galaxies (t_cr_ H_0_ <~0.02) makes it hard to understand why they are observable at all. Our dissipationless N-body simulations show that within a single rich collapsing group compact groups of galaxies continually form. The mean lifetime of a particular compact configuration is ~1 Gyr. On this time scale, members may merge and/or other galaxies in the loose group may join the compact configuration. In other words, compact configurations are continually replaced by new systems. The frequency of this process explains the observability of compact groups. Our model produces compact configurations (CG's) with optical properties remarkably similar to Hickson's [ApJ, 255,382(1982)] compact groups (HCG's): (1) CG's have a frequency distribution of members similar to that of HCG's; (2) CG's are 10 times as dense as loose groups; (3) CG's have dynamical properties remarkably similar to those of HCG's; (4) most of the galaxy members of CG's are not merger remnants. The crucial aspect of the model is the relationship between CG's and the surrounding rich loose group. Our model predicts the frequency of occurrence of CG's. A preliminary analysis of 18 rich loose groups is consistent with the model prediction. We suggest further observational tests of the model. Publication: The Astronomical Journal Pub Date: March 1994 DOI: 10.1086/116900 Bibcode: 1994AJ....107..868D Keywords: Astrodynamics; Compact Galaxies; Computational Astrophysics; Frequency Distribution; Gravitational Collapse; Optical Properties; Stellar Models; Computerized Simulation; Many Body Problem; Numerical Analysis; Astrophysics; GALAXIES: CLUSTERING full text sources ADS | Related Materials (1) Part 2: 1995AJ....109.2293D

Nonlinear evolution of tidally distorted accretion disks: Two-dimensional simulations

view Abstract Citations (43) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonlinear Evolution of Tidally Distorted Accretion Disks: Two-dimensional Simulations Ryu, Dongsu ; Goodman, Jeremy Abstract According to a previously published linear analysis, the tidal distortion of accretion disks in binary star systems produces a local hydrodynamic instability to m = 1 internal waves, which may have arbitrarily small wavelengths in the absence of viscosity. The instability is three-dimensional and approximately incompressible. To explore the nonlinear outcome of this instability, we develop a shearing-sheet approximation on scales comparable to the disk thickness. The large-scale azimuthal variation of the disk is represented by varying the local metric with time (local orbital phase). The hydrodynamic equations can then be posed two-dimensionally on local meridional planes. We solve these equations with a second-order gasdynamical code based on the Total-Variation-Diminishing scheme. Our simulations confirm the predicted linear growth rate. The modes saturate chaotically at velocities scaling as the product of the linear growth rate and the wavelength. If the wavelength is small compared with the disk thickness, the modes remain nearly incompressible even when nonlinear. The two-dimensional power spectrum of velocities after saturation is roughly isotropic and extends over a broad range of scales in an approximately power-law fashion. We measure the heating rate associated with the nonlinear dissipation of the modes. The dissipation implies a secular torque on the disk and a return of angular momentum to the secondary star via the tidal potential. The estimated torque is somewhat larger than the tidal torque produced by maximal disk viscosity (alpha approximately 1). At least in these two-dimensional simulations, however, there is no significant angular momentum flux within the disk. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173725 Bibcode: 1994ApJ...422..269R Keywords: Accretion Disks; Binary Stars; Computational Astrophysics; Computerized Simulation; Evolution (Development); Flow Distortion; Nonlinearity; Tides; Two Dimensional Models; Angular Momentum; Current Sheets; Hydrodynamic Equations; Numerical Analysis; Stellar Mass; Tvd Schemes; Velocity; Astrophysics; ACCRETION; ACCRETION DISKS; HYDRODYNAMICS; INSTABILITIES full text sources ADS |

The size distribution of interstellar dust particles as determined from extinction

view Abstract Citations (438) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Size Distribution of Interstellar Dust Particles as Determined from Extinction Kim, Sang-Hee ; Martin, P. G. ; Hendry, Paul D. Abstract We have investigate the character of the changes in the size distribution, and illustrate this with two contrasting cases: the diffuse interstellar medium (RV = 3.1), and a dense cloud region (RV = 5.3). For this exploratory investigation we adopted spherical bare silicate and bare graphite particles as in the Mathis, Rumpl, & Nordsieck (MRN) modeling. To extract the size distributions of the two components as objectively as possible we used the Maximum Entropy Method which gives the smoothest solution compatible with the chi squared confidence level on the goodness of fit to the extinction data. Abundance constraints were implemented directly in the method in order that the elements incorporated in the grains did not exceed their cosmically available abundances or contradict depletion data. We found that the silicate and graphite size distributions depart significantly (and robustly) from a simple power law. We also show how the size distribution falls off smoothly beyond a+, perhaps compatible with an exponential cutoff. The implications for the origin and evolution of the grain size distribution are discussed. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173714 Bibcode: 1994ApJ...422..164K Keywords: Abundance; Chemical Composition; Computational Astrophysics; Cosmic Dust; Infrared Radiation; Interstellar Extinction; Light Scattering; Mass Distribution; Size Distribution; Brightness; Curve Fitting; Maximum Entropy Method; Numerical Analysis; Parameterization; Astrophysics; ISM: DUST; EXTINCTION full text sources ADS |

An iterative technique for computing rotating white dwarf models

view Abstract Citations (10) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Iterative Technique for Computing Rotating White Dwarf Models Geroyannis, V. S. ; Papasotiriou, P. J. Abstract This paper has a twofold purpose. First, it aims at extending and applying the so-called complex-plane iterative technique (CIT) to the computation of classical white dwarf models distorted by rotation (either rigid or differential), and thus at calculating various significant physical characteristics of such models. Second, it aims at comparing the results of CIT with corresponding results of other elaborate methods and thus at establishing the reliability of this technique. Publication: The Astrophysical Journal Supplement Series Pub Date: January 1994 DOI: 10.1086/191868 Bibcode: 1994ApJS...90..501G Keywords: Computation; Computational Astrophysics; Iterative Solution; Stellar Models; Stellar Rotation; White Dwarf Stars; Differential Equations; Numerical Analysis; Numerical Integration; Rigid Rotors; Spherical Coordinates; Astrophysics; STARS: INTERIORS; STARS: ROTATION; STARS: WHITE DWARFS full text sources ADS |

Second-order perturbation theory in an expanding universe - Spherical harmonics expansion

view Abstract Citations (12) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Second-Order Perturbation Theory in an Expanding Universe: Spherical Harmonics Expansion Zaroubi, Saleem ; Hoffman, Yehuda Abstract The formalism of gravitational instability in an expanding universe is developed here to second order in terms of the spherical harmonics representation and is applied to study the evolution of single localized perturbations to an otherwise flat Einstein-de Sitter universe. A simple analytical form of the radial dependence of the various harmonics is used to model the structure expected for a scale-free Gaussian perturbation field. The general formalism is applied to study the dynamics of the monopole terms, i.e., the spherically averaged variables, of a proto- object made of monopole and quadrupole terms only. Our main result is that at the inner part, i.e., the core, the dynamics is dominated by the monopole term, and it reproduces the exact spherical top-hat model. However, the outer region, i.e., the halo, is dominated by the quadrupole terms, and its dynamics differs significantly from the top-hat model. The l = 2 terms lead to the formation of a secondary maximum in the run of the density (averaged over spherical shells) versus radius. This is a pure second-order effect whose very clear signature is that it occurs at a radius of the transition to the nonlinear regime. Indeed, such a secondary maximum has been observed in the outer parts of real clusters of galaxies and in N-body numerical simulations. Another pure second- order effect discovered here is the case of a pure quadrupole primordial perturbation. For such a perturbation the vanishing monopole does not grow in the linear regime, but is unstable to second order. We find a considerable growth for typical values of parameters, and conjecture that this is the main dynamical factor in the formation of filaments in the large-scale structure of the universe. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173052 Bibcode: 1993ApJ...414...20Z Keywords: Cosmology; Galactic Clusters; Hubble Constant; Perturbation Theory; Spherical Harmonics; Computational Astrophysics; Digital Simulation; Universe; Astrophysics; COSMOLOGY: THEORY; GALAXIES: CLUSTERING; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE full text sources ADS |