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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 505 records · Page 28
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NASA NTRS2006

Theory of Radiation Transfer in Neutron Star Atmospheres

The possibility for direct investigation of thermal emission from isolated neutron stars opened about a quarter of century ago with the launch of the first X-ray observatories Einstein and EXOSAT stimulated developing models of the neutron star surface radiation which began at the end of 80's. Confronting observational data with theoretical models of thermal emission allows one to infer the surface temperatures, magnetic fields, chemical composition, and neutron star masses and radii. This information, supplemented with the model equations of state and neutron star cooling models, provides an opportunity to understand the fundamental properties of the superdense matter in the stars' interiors. Almost all available models are based on the assumption that thermal radiation emitted by a neutron star is formed in the superficial star's layers--atmosphere. The neutron star atmospheres are very different from those of usual stars due to the immense gravity and huge magnetic fields. In this presentation we review the current status of the neutron star atmosphere modeling, present most important results, discuss problems and possible future developments.

Zavlin, Vyacheslav↗
NASA NTRS1980

Hydromagnetics and period changes in RR Lyrae stars

Balazs-Detre and Detre have drawn an interesting parallel between the observed time scales of variability in the sun and in RR Lyrae stars. Additional information is presented here to support their conjecture that an analog of the solar magnetic cycle is operating in RR Lyrae stars. Rough considerations of the expected changes of photospheric radius and of magnetic energy content during a magnetic cycle suggest that the pulsation periods of these stars should also change in time. Within the large observational and theoretical uncertainties, the predicted period changes are compatible with those observed.

Stothers, R.↗
NASA NTRS1985

Modeling the effects of latitudinal gradients in stellar winds, with application to the solar wind

A steady, axisymmetric, quasi-radial, global model previously developed for stellar winds with embedded magnetic fields has been extended to include latitudinal gradient effects on the azimuthal velocity and magnetic field. The linear results at large radii are presented for large-amplitude latitudinal variations in the radial magnetic field, mass loss rate, and radial velocity of the wind. The magnetohydrodynamic (MHD) equations predict meridional flows that develop naturally from internal magnetic stresses. The flows open flux tubes in the star's equatorial plane, redistributing mass and magnetic flux as a function of stellar latitude. The plasma spins up to conserve angular momentum in fields and plasma. The results are generally applicable to stellar winds (including radiatively driven winds), provided that the internal structure is not dominated by rotation. The asymptotic solutions do not explicitly depend on the form of the energy equation, although the assumed O(1) state which drives these solutions depends on the deposition of energy and momentum throughout the wind.

Nerney, S.↗
NASA NTRS1982

Special types of B stars

The properties of the Beta Cephei stars are examined with emphasis on spectral types, luminosities, periods, and light and velocity variations. Typical members of the group and their position in the observational and theoretical HR diagram are discussed. Those B stars with anomalous spectra (chemically peculiar) are considered including hot subdwarfs, extreme helium, weak helium, strong helium, helium variable, magnetic and mercury-manganese stars.

Lesh, J. R.↗
NASA NTRS1973

On the theory of group generation of stars

The hypothesis proposed is that topology of a rotating gaseous cloud can be variable in the contraction process. Due to rotation an originally spherical cloud is transformed into a toroidal body. The contraction of a thin torus is considered with different suppositions on cooling the gas. In the determined time the torus will become gravitationally unstable. The excitation of Jeans' waves is shown to result in the disintegration of the torus into fragments. The number of the fragments and their mass distributions are calculated. The proposed hypothesis on toroidal stages in stellar evolution can remove some difficulties in the theory of structure and evolution of stars, such as absence of limitary stars, distribution of rotation velocities of early-type stars, origin of poloidal magnetic fields and decline rotators with the magnetic axis orthogonal to the axis of rotation.

Zhilyayev, B. Y.↗
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 NTRS1981

Synchrotron cooling and annihilation of an e/+/-e/-/ plasma - The radiation mechanism for the 5 March, 1979 transient

Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the 5 March, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation-dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. In this layer, about 10 to the 12th generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the 0.15 s duration of the impulsive phase.

Ramaty, R.↗
NASA NTRS1983

Observation of winds in cool stars

Sufficient observational material - ultraviolet spectroscopic measures, quantitative optical spectroscopy, and X-ray photometry exists to enable discernment of the presence and character of mass loss in cool stars and to establish meaningful constraints on theoretical models. Two determinants of atmospheric wind structure - temperature and gravity - may suffice in a most superficial way to define the wind and atmospheric structure in a star; however more extensive observations demonstrate the importance of magnetic surface activity and its particular geometrical configuration. Successive observations of an active binary system and a supergiant star reveal that magnetic activity and perhaps mass loss occur on restricted regions of a stellar surface and that long lived structures are present in a wind.

Dupree, A. K.↗
NASA NTRS1989

Solar and stellar magnetic fields and structures - Observations

A review of stellar magnetic field measurements is presented which includes both a critique of recent spectral diagnostic techniques and a summary of important trends now appearing in the data. Both the Zeeman broadening techniques using unpolarized light and techniques based on circular and linear polarization data are discussed. An agenda for the development of a self-consistent model for the magnetic atmospheres of active stars is presented.

Linsky, Jeffrey L.↗
NASA NTRS1990

Line asymmetries in G and K dwarfs - Dependence on spectral type and activity

Observations of photospheric line asymmetries in 12 G and 11 K dwarfs, particularly the Fe I 6151.62 A line, are examined to determine the amplitudes and shapes of the bisectors. Bisector shapes are compared for magnetically inactive and active stars, and correlations between the slope and the stellar angular velocity are investigated. G stars have increasingly negative slopes with larger stellar angular velocity while K-star slopes become more positive, an effect which is shown to be consistent with recent hydrodynamical models.

Saar, Steven H.↗
NASA NTRS1991

Some aspects of the scientific significance of high energy gamma ray astrophysics

The attraction of high energy gamma-ray astronomy lies in this radiation relating directly to those processes in astrophysical situations which deviate most from thermo-dynamic equilibrium. Some examples of these phenomena which are known to or expected to emit gamma rays are cosmic rays as they interact in intergalactic space, the high energy particles in the magnetic fields of neutron stars, the death of a black hole, the explosion and residual of a supernova, lumps of Weakly Interacting Massive Particles, energetic solar particles interacting near the sun, and very high energy particles in the extreme conditions associated with active galaxies. Although the intensities are known to be low as seen near the earth, a partially compensating characteristic is that the very penetrating nature of high energy gamma rays increases the probability that they can escape from their origin and reach the solar system.

Fichtel, Carl E.↗
NASA NTRS1993

Massive low surface brightness galaxies

A multi-wavelength study of an extreme type of galaxy which will assist us in our attempts to understand the formation and evolution of galaxies was completed. In particular, a subset of low surface brightness (bar-mu(sub B) is approximately greater than 25 mag arcsec(sup -2)), giant galaxies (LSBG's) which contain large amounts of atomic gas (M(HI) is approximately greater than 10(exp 10) solar mass), have blue optical diameters similar to those of giant spiral galaxies, but which do not seem to have prodigious amounts of ongoing star formation were observed. Our sample was drawn from the first and second Palomar Sky Surveys. This population of galaxies has been largely ignored because of selection effects which make it difficult to detect optically. The question of how these massive systems differ from the higher surface brightness 'normal' spiral galaxies is addressed. Using B and R surface photometry, in conjunction with H-alpha, HI, CO-12, and far-infrared data, an attempt is made to determine if these galaxies had an early epoch of star formation that has since faded, have ongoing star formation with an unusual interplanetary magnetic field (IMF), or are perhaps galaxies which have never efficiently formed stars due to a lack of molecular clouds.

Knezek, Patricia M.↗