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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Can cluster environment modify the dynamical evolution of spiral galaxies?

Over the past decade many effects of the cluster environment on member galaxies have been established. These effects are manifest in the amount and distribution of gas in cluster spirals, the luminosity and light distributions within galaxies, and the segregation of morphological types. All these effects could indicate a specific dynamical evolution for galaxies in clusters. Nevertheless, a more direct evidence, such as a different mass distribution for spiral galaxies in clusters and in the field, is not yet clearly established. Indeed, Rubin, Whitmore, and Ford (1988) and Whitmore, Forbes, and Rubin (1988) (referred to as RWF) presented evidence that inner cluster spirals have falling rotation curves, unlike those of outer cluster spirals or the great majority of field spirals. If falling rotation curves exist in centers of clusters, as argued by RWF, it would suggest that dark matter halos were absent from cluster spirals, either because the halos had become stripped by interactions with other galaxies or with an intracluster medium, or because the halos had never formed in the first place. Even if they didn't disagree with RWF, other researchers pointed out that the behaviour of the slope of the rotation curves of spiral galaxies (in Virgo) is not so clear. Amram, using a different sample of spiral galaxies in clusters, found only 10% of declining rotation curves (2 declining vs 17 flat or rising) in opposition to RWF who find about 40% of declining rotation curves in their sample (6 declining vs 10 flat or rising), we will hereafter briefly discuss the Amram data paper and compare it to the results of RWF. We have measured the rotation curves for a sample of 21 spiral galaxies in 5 nearby clusters. These rotation curves have been constructed from detailed two-dimensional maps of each galaxy's velocity field as traced by emission from the Ha line. This complete mapping, combined with the sensitivity of our CFHT 3.60 m. + Perot-Fabry + CCD observations, allows the construction of high-quality rotation curves. Details concerning the acquisition and reduction procedures of the data are given in Amram. We present and discuss our preliminary analysis and compare them with RWF's results.

Amram, P.↗
NASA NTRS1997

Damped Ly alpha absorbers at high redshift: Large disks or galactic building blocks?

The nature of the physical structures giving rise to damped Lyman alpha absorption systems (DLAS) at high redshifts is investigated. The proposal that rapidly rotating large disks are the only viable explanation for the observed asymmetric profiles of low ionization absorption lines is examined. Using hydrodynamic simulations of galaxy formation, it is demonstated that irregular protogalactic clumps can reproduce the observed velocity width distribution and asymmetries of the absorption profiles equally well. The velocity broadening in the simulated clumps is due to a mixture of rotation, random motions, infall and merging. The observed velocity width correlates with the virial velocity for the dark matter halo of the forming protogalactic clump. The typical virial velocity of the halos required to lead to the DLAS population is approximately 100 km/s. It is concluded that the evidence that DLAS at high redshift are related to large, rapidly rotating disks, is not compelling.

Haehnelt, Martin G.↗
NASA NTRS2021

Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. III. Model Description and Verification of Parameter Estimation Codes

We describe the X-ray pulse profile models we use and how we use them to analyze Neutron Star Interior Composition Explorer(NICER)observations of rotation-powered millisecond pulsars to obtain information about the mass–radius relation of neutron stars and the equation of state of the dense matter in their cores. Here we detail our modeling of the observed profile of PSR J0030+0451 that we analyzed in Miller et al. and Riley et al. and describe a cross-verification of computations of the pulse profiles of a star with R/M 3, in case stars this compact need to be considered in future analyses. We also present our early cross-verification efforts of the parameter estimation procedures used by Miller et al. and Riley et al. by analyzing two distinct synthetic data sets. Both codes yielded credible regions in the mass–radius plane that are statistically consistent with one another, and both gave posterior distributions for model parameter values consistent with the values that were used to generate the data. We also summarize the additional tests of the parameter estimation procedure of Miller et al. that used synthetic pulse profiles and the NICER pulse profile of PSR J0030+0451. We then illustrate how the precision of mass and radius estimates depends on the pulsar’s spin rate and the size of its hot spot by analyzing four different synthetic pulse profiles. Finally, we assess possible sources of systematic error in the estimates made using this technique, some of which may warrant further investigation.

Slavko Bogdanov↗
NASA NTRS1980

Models for galactic X-ray sources

Attention is given to those compact galactic X-ray sources whose X-ray luminosities are considerably in excess of the solar luminosity. It is pointed out that the key breakthrough in the development of an understanding of compact galactic X-ray sources was the discovery of X-ray pulsars with the UHURU satellite. There is now overwhelming evidence that these objects are neutron stars in close binary stellar systems. The X-ray pulsations are thought to be thermal emission from the magnetic polar caps of a neutron star that is accreting matter from a companion star and whose magnetic field is misaligned with its rotation axis. Among the compact galactic X-ray sources that are not X-ray pulsars, some still show direct evidence of binary membership, such as X-ray eclipses. There is evidence that the galactic-bulge sources are, in fact, close binary stellar systems. It is concluded, that the great majority of bright galactic X-ray sources, with only a tiny handful of exceptions (such as the Crab and Vela pulsars), are likely to be binaries.

Joss, P. C.↗
NASA NTRS1989

Interstellar matter in early-type galaxies - Optical observations

Results of optical observations of 26 bright elliptical galaxies selected on the basis of IRAS data are discussed. Optical broadband imaging (using B and R filters) and narrow-band imaging (using H-alpha interference filters) have been performed to study dust patches and ionized gas. Long-split spectroscopy has also been made to determine gas kinematics and relative line ratios. The spectroscopic data confirm the presence and distribution of interstellar matter (dust lanes and ionized gas) seen in the direct imaging. Decoupled kinematics of interstellar gas and stars favors an external origin of the interstellar matter. However, for one isolated galaxy, an internal origin is not excluded. The rotation curves determined by optical emission lines are symmetric around the center in most galaxies observed. Galaxy masses and mass-to-light ratios are estimated using the rotation curves of the ionized gas.

Kim, Dong-Woo↗
NASA NTRS1993

Evolution of molecular clouds

The evolution of interstellar molecular hydrogen was studied, with a special interest for the formation and evolution of molecular clouds and star formation within them, by a two-dimensional hydrodynamical simulation performed on a rectangular grid of physical sizes on the order of 100 pc. It is filled with an initial density of approx. 1 cm(exp -3), except for one cell (approx. 1 pc(exp 2)) at the center of the grid where an accretion core of 1-10(exp 3) solar masses is placed. The grid is co-moving with the gridcenter that is on a circular orbit around the Galactic center and that also is the guiding center of epicyclic approximation of orbits of the matter surrounding it. The initial radial velocity is zero; to account for differential rotation the initial tangential velocity (i.e. the movement around the galactic center) is proportional to the radial distance to the grid center. The rate is comparable to the rotation rate at the Local Standard of Rest. The influence of galactic rotation is noticed by spiral or elliptical forms, but on much longer time scales than self gravitation and cooling processes. Density and temperature are kept constant at the boundaries and no inflow is allowed along the tangential boundaries.

Sevenster, M.↗
NASA NTRS2019

Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set

We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437−4715, J0030+0451, J1231−1411, and J2124−3358, selected as targets for constraining the mass–radius relation of neutron stars and the dense matter equation of state (EoS) via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass–radius relation and the dense matter EoS. We find that the long-term timing and flux behavior and the Fourier domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437−4715, for which multiple temperatures are required.

Neutron stars↗
NASA NTRS1974

The internal structure of the convective envelope of the sun

An axisymmetric hydrodynamic model of the sun consisting of a core at radiant equilibrium and a convective envelope is studied with consideration of gravitational forces, rotation, turbulence effects, and electromagnetic forces. The dimensionless parameters that control the equations of plasma motion within the sun and the boundary conditions are investigated; estimates are given for these parameters on the basis of experimental data and the existing theoretical models. Stability conditions are derived for convective equilibrium, developed cellular convection, and developed turbulent convection with consideration of rotation and turbulent Reynolds stresses. These conditions characterize the possible physical state of the matter in the convective envelope. Using estimates of the dimensionless parameters and models of some of them, a method of successive approximations is proposed for solution of the problem as a whole.

Yavorskaya, I. M.↗
NASA NTRS1979

Numerical modelling of the formation process of planets from protoplanetary cloud

Evolution of the plane protoplanetary cloud, consisting of a great number of gravitationally interacting and uniting under collision bodies (protoplanets) moving in the central field of a large mass (the Sun or a planet), is considered. It is shown that in the course of protoplanetary cloud evolution the ring zones of matter expansion and compression occur with the subsequent development leading to formation of planets, rotating about their axes mainly directly. The principal numerical results were obtained through digital simulation of planetary accumulation.

Kozlov, N. N.↗
NASA NTRS1988

The growth of magnetic fields prior to galaxy formation

Hydromagnetic dynamo theory is applied to the problem of magnetic field amplification in the epoch after recombination but prior to galaxy formation. Large-scale density fluctuations acquire angular momentum from tidal torques exerted by neighboring fluctuations, and small-scale subfluctuations possess helicity due to the Coriolis forces associated with the rotation of the large-scale host fluctuations. Cosmic expansion and the interaction of matter with the ambient radiation field are taken into account. Despite many uncertainties, it is concluded that significant dynamo amplification may have occurred.

Zweibel, Ellen G.↗
NASA NTRS1990

The polar-ring galaxies NGC 2685 and NGC 3808B (VV 300)

Polar-ring galaxies (PRG) are among the most interesting examples of interaction between galaxies. A PRG is a galaxy with an elongated main body surrounded by a ring (or a disk) of stars, gas, and dust rotating in a near-polar plane (Schweizer, Whitmore, and Rubin, 1983). Accretion of matter by a massive lenticular galaxy from either intergalactic medium or a companion galaxy is usually considered as an explanation of the observed structure of PRG. In the latter case there are two possibilities: capture and merging of a neighbor galaxy, and accretion of mass from a companion galaxy during a close encounter. Two PRG formation scenarios just mentioned are illustrated here by the results of our observations of the peculiar galaxies NGC 2685 and NGC 3808B.

Reshetnikov, V. P.↗
NASA NTRS1994

A study of a high resolution IUE spectrum of AM Canum Venaticorum

We have obtained the first high resolution IUE spectrum of the helium-rich, cataclysmic variable star AM CVn. The spectrum is greatly underexposed, but we can still detect both wide and narrow line profiles. We report broad, shortward-shifted, P-Cygni-like absorption and in some cases emission lines in the far UV high ionization resonance lines of C, N, O, and Si, but the profiles are considerably disk/boundary layer outflows, absorption disk continuum light in H-rich CVs. The highest ionizations show evidence of a narrow jet or conical flow. For other, lower ionized lines, we find some evidence of a stellar origin. The broad He II (lambda 1640 A) absorption profile with blue shifted emission core has a remarkably similar overall structure to the He II (lambda 1640 A) broad absorption trough in the IUE spectrum of the prototypical cool DO white dwarf HZ 21. The sharp absorption lines seem most convincingly in the resonance doublets of N V (lambda 1238 A, lambda 1242 A) and C IV (lambda 1548 A, lambda 1550 A) and in He II (lambda 1640 A) exhibit a precise velocity coincidence. These sharp features are almost certainly due to circumbinary matter because they are obviously unaffected by the rapid orbital motion (or rapid stellar rotation) in this short period system during the long (9.3 hour) IUE echelle exposure. Our observations support an evolution through shell episodes of a close binary system which ends up with an expanding envelope as seen for HZ 21, and suggests one possible evolutionary channel for production of DOs (DBs).

Solheim, J.-E.↗
NASA NTRS2021

The Radius of PSR J0740+6620 from NICER and XMM-Newton Data

PSR J0740+6620 has a gravitational mass of 2.08±0.07Me, which is the highest reliably determined mass of any neutron star. As a result, a measurement of its radius will provide unique insight into the properties of neutron star core matter at high densities. Here we report a radius measurement based on fits of rotating hot spot patterns to Neutron Star Interior Composition Explorer(NICER)and X-ray Multi-Mirror(XMM-Newton)X-ray observations. We find that the equatorial circumferential radius of PSR J0740+6620 is-+13.71.52.6km(68%).We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030+0451,the masses of two other∼2Mepulsars, and the tidal deformability constraints from two gravitational wave events, to three different frameworks for equation-of-state modeling, and find consistent results at∼1.5–5 times nuclear saturation density. For a given framework, when all measurements are included, the radius of a 1.4Meneutron star is known to±4%(68% credibility)and the radius of a 2.08Meneutron star is known to±5%. The full radius range that spans the±1σcredible intervals of all the radius estimates in the three frameworks is 12.45±0.65 km for a 1.4Meneutron star and 12.35±0.75 km for a 2.08Me neutron star.

M C Miller↗
NASA NTRS2006

General Relativistic Magnetohydrodynamic Simulations of Jet Formation with a Thin Keplerian Disk

We have performed several simulations of black hole systems (non-rotating, black hole spin parameter a = 0.0 and rapidly rotating, a = 0.95) with a geometrically thin Keplerian disk using the newly developed RAISHIN code. The simulation results show the formation of jets driven by the Lorentz force and the gas pressure gradient. The jets have mildly relativistic speed (greater than or equal to 0.4 c). The matter is continuously supplied from the accretion disk and the jet propagates outward until each applicable terminal simulation time (non-rotating: t/tau S = 275 and rotating: t/tau S = 200, tau s equivalent to r(sub s/c). It appears that a rotating black hole creates an additional, faster, and more collimated inner outflow (greater than or equal to 0.5 c) formed and accelerated by the twisted magnetic field resulting from frame-dragging in the black hole ergosphere. This new result indicates that jet kinematic structure depends on black hole rotation.

Mizuno, Yosuke↗
NASA NTRS1979

Synchronous rotation in magnetic X-ray binaries

AM Herculis is thought to be a binary stellar system that contains an accreting magnetic degenerate dwarf whose rotation is synchronous with the orbital period. This synchronism is remarkable, particularly because of the small moment of inertia of a degenerate dwarf and the large specific angular momentum of the accreted matter. This paper demonstrates that ohmic dissipation from the magnetic interaction of the stars is capable of bringing about exact synchronism, provided that some other process has brought the rotation period of the degenerate dwarf to the same order of magnitude as the orbital period. It is also shown that magnetostatic interaction in the synchronous state leads to oscillatory drifts in phase about exact synchronism with periods of approximately 1-10 yr. These phase drifts could manifest themselves in long-term periodic variability in the X-ray or optical properties of the source. Accretion torques could excite such oscillatory motions but need not disrupt synchronism once it has been established.

Joss, P. C.↗
NASA NTRS2019

MeV Emission from Pulsar Wind Nebulae: Understanding Extreme Particle Acceleration in Highly Relativistic Outflows

The Earth is constantly bombarded from outer space by energetic particles. Where and how these "cosmic rays" are produced is poorly understood, with various particle types and energies likely originating from different sources. Particularly mysterious is the source of high-energy e+/- produced in our Galaxy, especially those responsible for both the high fraction of e+ in the GeV cosmic ray lepton spectrum and the e+/- and observed excess of microwaves and gamma-rays detected towards the Galactic center and bulge. While these particles could be evidence for exotic forms of dark matter, they might also be produced by "normal" astrophysical sources such as pulsars the strongly magnetized, rapidly rotating neutron stars whose rotational energy powers an ultra-relativistic outflow (commonly referred to as a "pulsar wind") whose interaction with the surrounding medium creates a pulsar wind nebula .While the detection of TeV emission from numerous PWNe strongly suggest they contain e+/- with PeV or higher energies, how and to what energies these particles are produced is unknown, let alone their dependence on the properties of the pulsar, pulsar wind, and surrounding medium. A major reason for this uncertainty is the lack of information concerning their MeV properties, since the synchrotron emission from the highest energy e+/- peaks in this waveband. Only by combining the MeV spectrum of PWNe measured by proposed missions with that obtained at lower (primarily radio and X-ray) and higher (TeV) photon energies by current and hopefully future facilities is it possible to measure the full spectrum of e+/- in these sources. The resultant insights into the underlying acceleration mechanism would significantly impact many areas of astrophysics from indirect searches for dark matter to the origin of cosmic rays to the physics of relativistic outflows observed from active galactic nuclei, gamma-ray bursts, and some gravitational wave events.

Gelfand, Joseph D.↗
NASA NTRS1995

Magnetohydrodynamic simulations of outflows from accretion disks

Magnetohydrodynamic simulations have been made of the formation of outflows from a Keplerian disk threaded by a magnetic field. The disk is treated as a boundary condition, where matter is ejected with Keplerian azimuthal speed and poloidal speed less than the slow magnetosonic velocity, and where boundary conditions on the magnetic field correspond to a highly conducting disk. Initially, the space above the disk, the corona, is filled with high specific entropy plasma in thermal equilibrium in the gravitational potential of the central object. The initial magnetic field is poloidal and is represented by a superposition of monopoles located below the plane of the disk. The rotation of the disk twists the initial poloidal magnetic field, and this twist propagates into the corona pushing and collimating matter into jetlike outflow in a cylindrical region. Matter outflowing from the disk flows and accelerates in the z-direction owing to both the magnetic and pressure gradient forces. The flow accelerates through the slow magnetosonic and Alfven surfaces and at larger distances through the fast magnetosonic surface. The flow velocity of the jet is approximately parallel to the z-axis, and the collimation results from the pinching force of the toroidal magnetic field. For a nonrotating disk no collimation is observed.

Ustyugova, G. V.↗
NASA NTRS1983

Accreting degenerate dwarfs in close binary systems

Advances in the study of cataclysmic variables made during the past few years are reviewed. The classification of cataclysmic binaries and their dynamic properties are summarized. The hard and soft X-ray emission from these objects is discussed, and two alternative accretion geometries for producing this radiation from deep in the potential well of the degenerate dwarf are considered. The ultraviolet and optical spectrum is addressed, including disk emission and contributions from the companion star. Magnetic fields in cataclysmic variables are discussed, and the temporal behavior of these stars is addressed, including periodic modulations associated with orbital motion and rotation as well as flickering and pulsation reflecting the mass transfer process and the dynamics of matter near the surface of the accreting star. The outburst process is considered, including classical novae, recurrent novae, and dwarf novae.

Cordova, F. A.↗