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At least 1,711 records · Page 95

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DOE OSTI2025

Long Range Plan: Dense matter theory for heavy-ion collisions and neutron stars

Since the release of the 2015 Long Range Plan in Nuclear Physics, major events have occurred that reshaped our understanding of quantum chromodynamics (QCD) and nuclear matter at large densities, in and out of equilibrium. The US nuclear community has an opportunity to capitalize on advances in astrophysical observations and nuclear experiments and engage in an interdisciplinary effort in the theory of dense baryonic matter that connects low- and high-energy nuclear physics, astrophysics, gravitational waves physics, and data science. This is a white paper prepared by a group of nuclear physicists during the 2023 LRP process.

Lovato, Alessandro↗

NASA NTRS1974

A comment on gamma ray lines from He-4

It is pointed out that excited states of He-4 cannot produce gamma ray line emission of astrophysical significance, contrary to a recent suggestion of Reina et al. (1974). The only possible gamma-ray lines in He-4 are above 25 MeV, but their excitation cross sections appear to be so small that they are probably of little astrophysical significance.

Kozlovsky, B.↗

NASA NTRS1977

Magnetic fields greater than 10 to the 20th power gauss

Zaumen (1976) found that spontaneous pair production in a uniform magnetic field should be a feasible process for field strengths at least of the order of 10 to the 20th power gauss. This note points out that a magnetic field of this order of magnitude is most unlikely to occur in realistic astrophysical situations because of the large dynamical and quantum-mechanical effects such a field would produce. It is suggested that Zaumen's calculation would probably have little bearing on the suspected evolution of astrophysical systems since other processes (either dynamical or quantum-mechanical) apparently limit the field strength before such high magnetic fields would be reached. An upper limit of about 10 to the 16th power gauss is obtained by considering the isotropy of the 3-K blackbody radiation, the formation of collapsed objects in very high magnetic fields, and magnetic bremsstrahlung processes in quantum electrodynamics.

Lerche, I.↗

NASA NTRS1978

On dielectronic recombination and resonances in excitation cross sections

It is found that the contribution to the dielectronic recombination rate of stabilizing transitions of the outer electron is within the 30% accuracy of the Burgess rates calculated by neglecting this process even for Delta n = 0 transitions for astrophysically important ions. Autoionization to excited levels, ignored in the Burgess procedure and shown by Jacobs et al. (1977) to be important for some ions in the calculation of the dielectronic recombination rate, is discussed with reference to the resonance excitation of some astrophysically important X-ray lines. In particular, the contribution of resonances to the excitation of several lines in the helium and neon isosequences is estimated.

Raymond, J. C.↗

NASA NTRS1980

Cosmic gamma-ray lines - Theory

The various processes that lead to gamma-ray line emission and the possible astrophysical sources of such emission are reviewed. The processes of nuclear excitation, radiative capture, positron annihilation, and cyclotron radiation, which may produce gamma-ray line emission from such diverse sources as the interstellar medium, novas, supernovas, pulsars, accreting compact objects, the galactic nucleus and the nuclei of active galaxies are considered. The significance of the relative intensities, widths, and frequency shifts of the lines are also discussed. Particular emphasis is placed on understanding those gamma-ray lines that have already been observed from astrophysical sources.

Lingenfelter, R. E.↗

NASA NTRS1980

Future prospects for gamma-ray

Astrophysical phenomena discussed are: the very energetic and nuclear processes associated with compact objects; astrophysical nucleo-synthesis; solar particle acceleration; the chemical composition of the planets and other bodies of the solar system; the structure of our galaxy; the origin and dynamic pressure effects of the cosmic rays; the high energy particles and energetic processes in other galaxies, especially active ones; and the degree of matter antimater symmetry of the universe. The gamma ray results of GAMMA-I, the gamma ray observatory, the gamma ray burst network, solar polar, and very high energy gamma ray telescopes on the ground provide justification for more sophisticated telescopes.

Fichtel, C.↗

NASA NTRS1981

The Sun as a star

The ways in which solar astrophysics serves to improve the methodology for the interpretation of stellar observations and the construction of stellar atmospheric models are summarized. The astrophysical processes highlighted are: stellar mass; stellar rotation; stellar magnetism; stellar composition; stellar companions; and evolutionary history.

Rutten, R. J.↗

NASA NTRS1981

Interpretations and implications of gamma-ray lines from solar flares, the galactic centre and gamma-ray transients

Gamma-ray line emission from the Sun results from the nuclear interactions of energetic protons and nuclei with the solar atmosphere. These interactions produce gamma-ray lines from neutron capture, positron annihilation, and nuclear deexcitation. Observation of such gamma-rays can provide unique information on high energy processes at the Sun. Details of solar gamma-ray spectroscopy are discussed along with the galactic center 0.511 MeV line. The richness of astronomy at 0.511 MeV is indicated by the great variety of astrophysical positron production mechanisms and by the many astrophysical sites where such mechanisms could operate. Attention is also given to lines from gamma-ray transients, and the prospects for gamma-ray line detections, taking into account gamma-ray lines from processes of nucleosynthesis and lines from low-energy cosmic ray interactions.

Ramaty, R.↗

NASA NTRS1981

Potentials of surfaces in space

The subject of surface potentials on natural and artificial bodies in space is reviewed with particular emphasis on recent developments. Following a brief survey of work done up to the early 1970s on the charging of astrophysical objects in interstellar and interplanetary space and spacecraft charging, the various charging mechanisms in space are examined, including the collection of plasma particles, photoemission, and secondary electron emission by electron impact and ion impact, and the effects on charging of nonisotropic plasmas, wakes, and environmental magnetic and electric fields are considered. The concept of equilibrium potential is discussed, along with differential charging, potential barriers and discharge processes. The measurement of spacecraft potentials is then considered, and recent work on spacecraft potential modification and control by active and passive means is presented. Finally, astrophysical applications where charging effects may be important are discussed, and areas for further work are indicated.

Whipple, E. C.↗

NASA NTRS1982

A comparative study of computational methods in cosmic gas dynamics

Many theoretical investigations of fluid flows in astrophysics require extensive numerical calculations. The selection of an appropriate computational method is, therefore, important for the astronomer who has to solve an astrophysical flow problem. The present investigation has the objective to provide an informational basis for such a selection by comparing a variety of numerical methods with the aid of a test problem. The test problem involves a simple, one-dimensional model of the gas flow in a spiral galaxy. The numerical methods considered include the beam scheme, Godunov's method (G), the second-order flux-splitting method (FS2), MacCormack's method, and the flux corrected transport methods of Boris and Book (1973). It is found that the best second-order method (FS2) outperforms the best first-order method (G) by a huge margin.

Van Albada, G. D.↗

NASA NTRS1982

Simplified expressions for the gyrosynchrotron radiation from mildly relativistic, nonthermal and thermal electrons

Simplified expressions are presented for the gyrosynchrotron radiation from mildly relativistic electrons, both for nonthermal (power-law) and for thermal (Maxwellian) distributions. Expressions are given for the emissivity, absorptivity, effective temperature, frequency of peak brightness, and degree of circular polarization. The expressions are designed for the range from about 10 to about 100 of harmonic numbers, 20 to 80 deg of viewing angle, 3 to 7 electron energy spectral index (nonthermal electrons), and 10 to the 8th to 10 to the 9th K in temperature (thermal electrons). This range generally covers the cases of interest for the 10 keV to 1 MeV electrons that are probably important in solar and stellar flares and possibly in other astrophysical situations. The accuracy of these expressions, within the stated range of validity, is generally much better than a factor of 2, while the values of some of the quantities cover up to 15 orders of magnitude. The simplicity of the expressions should make them useful for semiquantitative investigations of models of astrophysical sources

Dulk, G. A.↗

NASA NTRS1982

Extra-solar astronomy with a 2.4 m normal incidence X-ray telescope at 0.1 arcsec resolution

The advantages and disadvantages for astrophysical observations of a normal incidence X-ray telescope are considered. The baseline configuration is for a 2.4 meter diameter mirror operating at 0.23-0.28 keV with a CCD detector having a pixel size equivalent to 0.1 arcsec. This resolution enables X-ray images to be made comparable to those of Space Telescope in the optical and to those of the Very Large Array in the radio for a large number of sources. Specific examples cover essentially the entire range of astrophysics.

Elvis, M.↗

NASA NTRS1983

The gas-grain interaction in the interstellar medium - Thermal accommodation and trapping

The paper develops a numerical model for calculating thermal accommodation coefficients alpha sub T and trapping functions f sub t for gases incident on solid surfaces. The method is especially designed for astrophysical applications in that it treats economically and with moderate accuracy (+ or - 20%) the dependences of alpha sub T and f sub t on finite and different surface and gas temperatures for a large number of gas-surface combinations. In particular, the method is applied to the astrophysical combinations of hydrogen and helium gases incident on graphite, silicon, and ice surfaces. Graphs are presented of the dependence of alpha sub T and f sub t on interstellar gas temperatures in the range of 10 to 10,000 K and grain temperatures in the range 10 to 1000 K, assuming the current estimates of the gas-surface physical parameters such as the composition and the Debye temperature of the grain material, the repulsive range of the surface potential, and the gas-grain adsorption energy.

Burke, J. R.↗

NASA NTRS1983

Energy dynamics in stressed magnetic fields - The filamentation and flare instabilities

The thermal and tearing instabilities are believed to be the two primary temperature modification mechanisms in sheared astrophysical magnetic fields. The former gives rise to the formation of cool filaments and the latter to the release of magnetic energy. It has long been known that these processes are interrelated, most conspicuously in the case of the solar corona where prominences often precede flares within the same magnetic structure. It is also clear, from first principles, that the energy transport underlying the thermal instability should have a strong effect on the resistivity which facilitates magnetic tearing, and that the energy release of the latter should affect the temperature drop of the former. This paper describes some results of the first calculations which attempt to unify the dynamic treatment of these two coexisting instabilities. Growth rates as a function of resistivity, and examples of the primary mode structures are provided, along with a discussion of some critical aspects of the interaction of these two astrophysical energy flux mechanisms.

Van Hoven, G.↗

NASA NTRS1983

Acceleration and transport processes - Verification and observations

The general problem of diffusive transport and acceleration of energetic charged particles is considered. The transport of solar-flare particles, solar modulation of galactic cosmic rays and shock acceleration processes on the solar wind are examined and observational tests are summarized. It is concluded that the basic diffusive transport equation is a useful approximation in situations like the solar wind, where turbulent scattering by magnetic irregularities is sufficient to maintain near isotropy. The application of this equation to the interstellar medium andd other, more distant astrophysical regimes is then discussed and implications for gamma-ray astrophysics are outlined. Finally the evidence for interstellar turbulence is reviewed and its consequences briefly discussed.

Jokipii, J. R.↗

NASA NTRS1985

Research in particles and fields

Research activities in Cosmic Rays Gamma Rays, and Astrophysical Plasmas are covered The investigation of the astrophysical aspects of cosmic rays and gamma rays and of the radiation and electromagnetic field environment of the Earth and other planets are studied. These investigations are carried ut by means of energetic particle and photon detector systems flown on spacecraft and balloons. The emphasis is on precision measurements with high resolution in charge mass and energy. An extensive bibliography is given.

Stone, E. C.↗

NASA NTRS1985

Relativistic electron in curved magnetic fields

Making use of the perturbation method based on the nonlinear differential equation theory, the author investigates the classical motion of a relativistic electron in a class of curved magnetic fields which may be written as B=B(O,B sub phi, O) in cylindrical coordinates (R. phi, Z). Under general astrophysical conditions the author derives the analytical expressions of the motion orbit, pitch angle, etc., of the electron in their dependence upon parameters characterizing the magnetic field and electron. The effects of non-zero curvature of magnetic field lines on the motion of electrons and applicabilities of these results to astrophysics are also discussed.

An, S.↗

NASA NTRS1985

Neutron-rich nuclei in cosmic rays and Wolf-Rayet stars

Wolf-Rayet stars figure prominently in astrophysical research. As a bonus, they seem to offer, in the recent past, an interesting connection between classical astronomy and high energy astrophysics due to their unusual composition and their huge mechanical power. The material flowing from WC stars (carbon-rich WR stars) contains gas which has been processed through core-helium burning, i.e., considerably enriched into 12C,16O, 22Ne, and 25,26Mg. This composition is reminiscent of the cosmic ray source anomalies. Encouraging agreement is obtained with observation in the mass range 12 A 26 assuming acceleration of wind particles at the shock that delineates the WR cavity, and adequate dilution with normal cosmic rays, but silicon poses.

Prantzos, N.↗