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At least 793 records · Page 44
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NASA NTRS1978

Future prospects of infrared observations of active nuclei

Future developments in infrared observations are briefly discussed. The prospects for large advances in ground-based broad-band observations probably lie in increased availability of dedicated telescopes and observing time. Several specific areas of potential advancement are described. The plans for the utilization of space for infrared astronomy call for an unbiased all-sky survey in the wavelengths 8-120 microns followed by complementary missions using a cooled moderate sized telescope and a large uncooled telescope.

Neugebauer, G.↗
NASA NTRS1978

Hydrogen emission-line spectra in quasars and active galactic nuclei

An attempt is made to account for the observational fact that the intensity ratios of the lower Balmer lines seen in emission from QSOs, type 1 Seyfert galaxies, and other related active galaxies are not what would be expected from the conventional picture of lines generated in a recombining hydrogen plasma. Previous observations of emission-line intensities in QSOs and Seyfert galaxies are reviewed. The various processes included in the rate coefficients of the calculations are then described, viz., free-free, bound-free, and bound-bound transitions, radiative transfer, and angular-momentum mixing. The strengths of the various emission lines are calculated by using more accurate values for the transition rates, including excitation and deexcitation simultaneously, and providing for finite rates in optically forbidden transitions. Numerical results are presented for the cases of ground-state collisional excitation, reabsorption of the upper Balmer lines, and collisional deexcitation. The results are applied to construct a model for the emission-line gas in a QSO and to discuss limits on models of Seyfert galaxies.

Krolik, J. H.↗
NASA NTRS1978

The mean mass of the abundant cosmic-ray nuclei from boron to silicon at 1.2 GeV per atomic mass unit

Results are presented for an investigation of cosmic-ray isotopic composition in which a balloon-borne instrument was used to perform an isotopic analysis of the abundant elements from B through Si in the energy range from 1.0 to 1.6 GeV/amu. The technique employed, which depends on the effect of earth's magnetic field on incoming cosmic rays, and reduction of the observational data are described in detail. Mean masses of the elements B, C, N, O, Ne, Mg, and Si are extracted from normalized Cerenkov spectra and evaluated in terms of the 'leaky box' model of galactic propagation and confinement. The implications of the results for cosmic-ray sources are shown to be consistent with the nucleosynthesis of certain isotopes in the cosmic rays in explosive carbon burning under the conditions of peak temperature, density, and neutronization expected in an explosion of a massive highly evolved star. The possibility of alternate interpretations is noted.

Dwyer, R.↗
NASA NTRS1978

An interpretation of special and general isotopic anomalies in r-process nuclei

My analysis of new discoveries by McCulloch and Wasserburg of Ba and Nd isotopic anomalies in inclusions of the Allende meteorite argues that (1) these anomalies contain special extinct radioactivities resulting from radioactive decay within grains formed in and ejected from the supernova interior, (2) the inclusions studied are fused assemblies of interstellar grains that were never totally vaporized, and (3) theoretical separation into r and s abundances suggests that fluctuations between r and s components has occurred during the accumulation processes. These points lend support to a new chemical picture of the early solar system that I have developed, although many interpretations remain possible. Measurements of the neutron-capture cross sections of Nd isotopes are urgently needed to experimentally validate these conclusions.

Clayton, D. D.↗
NASA NTRS1979

Isophotometry of the nuclei of M31 and M51

Isophotic contour maps derived from blue plates confirm the change in position angle as a function of distance from the center of M31 in the red region reported by Peterson, Ford, and Rubin (1977). The techniques used for M31 were then applied to a study of M51, for which similar results were obtained for the nuclear region. The spiral arms of M51 were traced for an additional half turn near the center of the galaxy.

Feibelman, W. A.↗
NASA NTRS1979

An instrument to measure the spectrum of cosmic ray iron and other nuclei to above 100 GeV/nucleon

A balloon-borne instrument system for the study of cosmic ray composition in the energy region beyond 100 GeV/nucleon is described. The large area detector incorporates an ionization spectrometer or calorimeter to determine the energy of the incident particles which is calibrated in flight by a gas Cerenkov counter filled with freon 12 at 20 psi. Calibration of calorimeter response indicates that the effects of ionization energy loss and nucleon fragmentation are correctly accounted for in a Monte Carlo simulation. A charge measuring module consisting of five scintillators and a lucite Cerenkov counter is used to determine the charge and position of the incoming particles. A spatial resolution of 5 to 10 cm has been obtained by using the ratios of pulse heights in different photomultiplier tubes aimed at the same scintillators within the module. The detector represents the heaviest cosmic ray experiment that can be flown over 30 km using the dynamic launching technique commonly employed.

Arens, J. F.↗
NASA NTRS1978

Super enrichments of Fe-group nuclei in solar flares and their association with large He-3 enrichments

Data on solar flares and perodic particle intensity enhancements in the energy range from 1 to 20 MeV/n are examined. It is found that: (1) Fe/He-4 ratios range from about 1 to 1000 times the solar ratio of 0.0004; (2) these high ratios mitigate against extended storage and large amounts of nuclear processing; (3) the CNO/He-4 ratio has a much smaller range of variability and a mean value of 0.02; (4) large He-3 and Fe enrichments are strongly associated, but not on a one-to-one basis; (5) large Fe enhancements sometimes occur without correspondingly large He-3 enrichments; and (6) none of the models so far advanced adequately explains the observed He-3 and heavy-nucleus enrichments.

Anglin, J. D.↗
NASA NTRS1979

The isotopic and elemental abundances of neon nuclei accelerated in solar flares

The relative isotopic abundances of Ne-20 and Ne-22 in seven solar flares were determined from measurements of the satellite IMP 8, yielding the ratio Ne-20/Ne-22 = 7.7 (+2.3, -1.5) for solar chromospheric matter. This value is in agreement with the ratio for the component neon-A (the 'primordial' component) found in carbonaceous chondrites. An elemental abundance ratio Ne/O = 0.14 + or - 0.01 also has been obtained which agrees closely with earlier reported measurements. It is shown that the effects of preferential acceleration relative to solar-system abundances with increasing charge number observed for some solar flares - though biasing the elemental ratio - does not appear to influence the neon isotopic abundances.

Dietrich, W. F.↗
NASA NTRS1979

The Penrose photoproduction scenario for NGC 4151: A black hole gamma-ray emission mechanism for active galactic nuclei and Seyfert galaxies

A consistent theoretical interpretation is given for the suggestion that a steepening of the spectrum between X-ray and gamma ray energies may be a general, gamma-ray characteristic of Seyfert galaxies, if the diffuse gamma ray spectrum is considered to be a superposition of unresolved contributions, from one or more classes of extragalactic objects. In the case of NGC 4151, the dominant process is shown to be Penrose Compton scattering in the ergosphere of a Kerr black hole, assumed to exist in the Seyfert's active galactic nucleus.

Leiter, D.↗
NASA NTRS1980

Laboratory investigations of cloud nuclei from combustion of Space Shuttle propellant

Small quantities of solid rocket motor propellant, of the type to launch the Space Shuttle, were burned at ambient pressure in the laboratory to provide aerosol samples for characterization. A portion of each sample was injected into an isothermal cloud chamber and the remainder into a 770-liter holding tank. Portable ice nucleus (IN) counters, filter devices for IN determinations and a cloud condensation nucleus (CCN) counter sampled from the tank. The measurements show that particles resulting from the combustion of the propellant are active IN (3.3 times 10 to the 8th to 1.5 times 10 to the 11th/g active at 20 C). The portable counters and filters detected significantly fewer IN than the isothermal cloud chamber. The propellant aerosol is a prolific source of CCN that swamped the instrument.

Hindman Ii, E. E.↗
NASA NTRS1980

The isotopic composition of galactic cosmic-ray iron nuclei

High-resolution observations made in interplanetary space of 83-284 MeV per nucleon galactic cosmic-ray iron isotopes are reported and it is directly established that Fe-56 is the dominant cosmic-ray Fe isotope. The following percentage abundances for Fe at the cosmic-ray source are found: Fe-54 = 9(+8, -5)%, Fe-55 is less than or equal to 7%, Fe-56 = 91(+5, -11)%, Fe-57 is less than or equal to 8%, and Fe-58 is less than or equal to 6%. When compared to calculated nucleosynthesis yields and other observations, these results place significant constraints on the neutron excess of the environment where cosmic-ray Fe originates.

Mewaldt, R. A.↗
NASA NTRS1980

The ultraviolet continua of the nuclei of M31 and M81

Ultraviolet measurements of the central 2.5 arcmin of M31 and M81 made with the ANS satellite are reported. A library of standard-star observations is also included for comparison. In agreement with OAO 2 results, it is found that both galaxies exhibit a significant UV excess as compared to expectations for a pure, old metal-rich stellar population including just the main sequence and giant branch. Model calculations show that the excess UV light could be produced either by young stars or by commonly occurring hot stars in an old stellar population. At the present time, several lines of evidence slightly favor old stars. An ultraviolet excess of similar magnitude seems to be common to all early-type galaxies yet observed in the UV and thus may significantly affect the appearance of redshifted galaxies.

Wu, C.-C.↗
NASA NTRS1980

Elemental composition of solar energetic nuclei

It is found for four major solar-flare events that the average solar energetic particle (SEP) abundances are similar to abundances in the solar wind and to recent coronal measurements. The average SEP and galactic cosmic-ray source abundances are also similar for elements with nuclear charge in the range 8-28, but are different for helium, carbon, and nitrogen.

Cook, W. R.↗
NASA NTRS1980

Comment on 'magnetosheath burst of predominantly medium nuclei observed with Imp 8 on February 16, 1974' by T. P. Armstrong, S. M. Krimigis, and R. P. Lepping

Armstrong et al. (1978) have observed a unique series of bursts with an unusually high content of Z greater than 2 particles and a high degree of collimation from the direction of the sun. The bursts were observed only with the CPME (Charged Particle Measurement Instrument) on Imp 8, but not with the CPME on Imp 7 nor with the ULET (Ultralow Energy Telescope) on Imp 8, whose response characteristics matched those of the CPME on Imp 7. The present comparison of the measurements made with the CPME and ULET cast doubt on the correctness of the reported observations.

Hovestadt, D.↗