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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Visibility of comet nuclei

Photography of the nucleus of comet Halley is the goal of several planned space missions. The nucleus of a comet is surrounded by a cloud of dust particles. If this cloud is optically thick, it will prevent observation of the nuclear surface. Broadband photometry of nine comets has been analyzed to determine the visibility of their nuclei. Only in the case of comet West near perihelion was the dust dense enough to interfere with imaging. Comparison of the visual brightness of the well-observed comets with that of Halley in 1910 leads to the conclusion that the nucleus of Halley can be imaged without significant obscuration by the dust.

Ney, E. P.↗
NASA NTRS1981

Could life have evolved in cometary nuclei

The suggestion by Hoyle and Wickramasinghe (1978) that life might have originated in cometary nuclei rather than directly on the earth is discussed. Factors in the cometary environment including the conditions at perihelion passage leading to the ablation of cometary ices, ice temperatures, the absence of an atmosphere and discrete liquid and solid surfaces, weak cometary structure incapable of supporting a liquid core, and radiation are presented as arguments against biopoesis in comets. It is concluded that although the contribution of cometary and meteoritic matter was significant in shaping the earth environment, the view that life on earth originally arose in comets is untenable, and the proposition that the process of interplanetary infection still occurs is unlikely in view of the high specificity of host-parasite relationships.

Bar-Nun, A.↗
NASA NTRS1982

The charge and isotopic composition of Z = 6-14 cosmic ray nuclei at their source

Using data from a cosmic ray charge-isotope telescope flown on balloons, both the charge and isotopic composition are determined of Z = 6-14 cosmic ray nuclei. A low abundance for the elements N and Ne in the cosmic ray source relative to solar cosmic rays is observed. For the isotopes, a cosmic ray source ratio Ne-22/Ne-20 is found that is 3.52 plus or minus 0.67 times the solar ratio, this enhancement being due in part to an enhancement of Ne-22 and in part to an underabundance of Ne-20. Also observed are possible enhancements of the Mg-26/Mg-24 ratio, which is 1.40 plus or minus 0.24 times the solar t ratio, and the C-13/C-12 ratio, which is 2.90 plus or minus 0.93 times the solar ratio. For Ne, the underabundance of this element, coupled with the overabundnace of the isotope Ne-22, is seen as providing an important new clue to the nucleosynthesis processes producing these differences.

Webber, W. R.↗
NASA NTRS1981

The charge and isotopic composition of Z equals 7-16 cosmic ray nuclei at their source

Using the combined data from our 1976 and 1977 balloon flights, the charge and isotopic composition of Z equals 7-16 cosmic ray nuclei have been determined. A low abundance of both N and Ne in the cosmic ray source relative to solar cosmic rays is observed. However, the cosmic ray source ratio, Ne-22/Ne is 2.75 plus or minus 0.42 times the solar ratio. A possible enhancement of the Mg-26/Mg ratio, which is 1.29 plus or minus 0.14 times the solar ratio is also observed.

Webber, W. R.↗
NASA NTRS1981

Cosmic ray detector for high energy iron nuclei

An experiment to directly measure the differential energy spectra of nuclei with charge between 15-28 inclusive with a balloon borne instrument is presented. A High Energy Gas Cerenkov Spectrometer (HEGCS) is described, and consists of a 3 m diam drum 4.5 m tall containing three light diffusion chambers. A hodoscopic array of scintillators emit light into the top and bottom light chambers which serve for track recordings of the trajectory and charge of incident cosmic rays. A center chamber pressure vessel, the HEGCS, has a 4.0 m sr and a threshold of 50 GeV/amu. The hexagonal array scintillators have 24 elements/array feeding photomultiplier tubes at the vertices between the scintillators, with other PMTs located around the top and bottom chambers to capture escaped scintillator light. The HEGCS is filled with freon-12, with reflective paint on the inner surface to convert UV Cerenkov photons into visible photons near 425 nm.

Streitmatter, R. E.↗
NASA NTRS1981

X-ray imaging observations of active galactic nuclei and quasars

After discussing the X-ray properties of active galactic nuclei and quasars which have been ascertained by means of Einstein Observatory imaging observations, the observation of X-ray jets coincident with the optical and radio jets in 3C 273 and M87 is considered. The sensitivity of the Einstein Observatory makes possible the search for variability in Seyfert galaxies on time scales shorter than one day. It is noted that if such short time scale variability occurs in all Seyferts, X-ray emission lines due to Compton smearing are unlikely, as is borne out by observations. It appears that integrated emission from quasars makes up a large fraction of the X-ray background in the 1-3 keV band, with the observational consequence that the average value of many quasar spectra should be the same as the background spectrum.

Henry, J. P.↗
NASA NTRS1981

The heavy nuclei experiment on HEAO-3

The HEAO-3 experiment to study the heavy and ultraheavy nuclei in cosmic radiation is described. The array is double-ended and consists of three main elements. Two pressure chambers, filled with an argon-methane mixture at about 850 torr, each contain two x-y hodoscopes made of wires spaced 1 cm apart and three parallel plate ionization chambers. Between the pressure chambers is mounted a Cerenkov counter composed of two layers of Pilot 425 radiator looked at by eight photomultipliers. The instrument is designed to achieve a charge resolution of 0.3 charge units over the charge range 17-120, which requires a measurement accurate to 0.6 percent at Z = 100. Examination of selected data shows that the instrument has the intrinsic resolution needed.

Binns, W. R.↗
NASA NTRS1982

Fragmentation of Fe-56 nuclei on CH2, carbon, and hydrogen targets - Individual charge changing and total cross sections

Elemental and isotopic differences between cosmic rays coming from different sources may have important implications for the nucleosynthesis process occurring in the stars that ultimately produce the galactic cosmic rays. The cosmic-ray composition observed at earth is modified from that accelerated in the source regions by passage through approximately 6 g/sq cm of interstellar matter consisting of approximately 90% H and 10% He by number. The extrapolation of the observed composition at earth to the sources requires an accurate understanding of the fragmentation cross sections in H. With the recent acceleration of heavy nuclei up to Fe to relativistic energies, the direct measurement of the astrophysically interesting cross section is now possible. The present investigation is concerned with a determination of the individual elemental cross sections, Fe-56 incident on H, taking into account the application of the elemental cross section data to the cosmic-ray propagation problem.

Webber, W. R.↗
NASA NTRS1982

Chemical composition of cometary nuclei

Observational evidence pertaining to the origin and composition of cometary material is reviewed. Arguments favoring the undifferentiated character of the icy conglomerate are summarized. Theoretical descriptions of the sublimation of a cometary nucleus and the velocity field of the expanding gas are presented and compared with observations. The nature of cometary dust and the atomic abundances of H, C, N, O, and S in the volatile fraction are examined, and data on the dust and volatile fractions are combined to derive elemental abundances. It is shown that O, N, and S in cometary nuclei appear to have essentially cosmic abundances but that both H and C are drastically depleted with respect to the cosmic abundances. The apparent depletion of C by a factor of more than three is discussed. It is suggested that the missing carbon might be hidden in the dust fraction in the form of heavy organic molecules or might have remained in either the primeval solar nebula or interstellar space.

Delsemme, A. H.↗
NASA NTRS1982

The rotation of comet nuclei

Spin-vector research on cometary nuclei is reviewed with emphasis on the actual determination of rotation period and spin-axis orientation. The rotation periods of 47 comets are compared with those of 41 asteroids with diameters of not more than 40 km. It is shown that the median periods for the comets is 15.0 hr as compared with 6.8 hr for the asteroids and that the preliminary distribution curve for the logarithms of the comet periods is not Gaussian and is flatter than the corresponding curve for the asteroids. Slow accumulation at low relative velocities is suggested as the cause of the longer comet rotation periods.

Whipple, F. L.↗
NASA NTRS1983

The propagation of ultraheavy cosmic ray nuclei

The propagation of ultraheavy cosmic ray nuclei (Z greater than 30) has been studied using a model based on the leaky-box formalism. This model has been used to examine the possible composition of the source, the path length distribution, and the mean escape length of the confinement region. Various abundance ratios have been found that have a sensitive dependence on one or more of these factors. Comparison of the theoretical values of these ratios with published data suggests that the source composition is similar to that of solar system matter, but that in the Pt-Pb region, enrichment in material produced by the r-process cannot be ruled out. Data for the secondary-to-primary ratio (Ru-44 - Cd-48)/(Sn-50 - Ba-56) show the presence of significantly more secondaries than are predicted, even when a truncated path length distribution is used in the model.

Brewster, N. R.↗
NASA NTRS1983

The dynamics and fueling of active nuclei

It is generally believed that quasars and active galactic nuclei produce their prodigious luminosities in connection with the release of gravitational energy associated with accretion and infall of matter onto a compact central object. In the present analysis, it is assumed that the central object is a massive black hole. The fact that a black hole provides the deepest possible central potential well does imply that it is the most natural candidate for the central engine. It is also assumed that the quasar is associated with the nucleus of a conventional galaxy. A number of difficulties arise in connection with finding a suitable stellar fueling model. A simple scheme is discussed for resolving these difficulties. Attention is given to fueling in a nonaxisymmetric potential, the effects of a massive accretion disk, and the variability in the disk luminosity caused by star-disk collisions assuming that the energy deposited in the disk is radiated.

Norman, C.↗
NASA NTRS1983

The cosmological evolution and luminosity function of X-ray selected active galactic nuclei

The cosmological evolution and the X-ray luminosity function of X-ray selected active galactic nuclei (AGNs) are derived and discussed. The sample used consists of 31 AGNs extracted from a fully identified sample of X-ray sources from the Einstein Observatory Medium Sensitivity Survey and is therefore exclusively defined by its X-ray properties. The distribution in space is found to be strongly nonuniform. The amount of cosmological evolution required by the X-ray data is derived in the framework of pure luminosity evolution and is found to be smaller than the amount determined from optically selected samples. The X-ray luminosity function is derived. It can be satisfactorily represented by a single power law only over a limited range of absolute luminosities. Evidence that the luminosity function flattens at low luminosity or steepens at high luminosity, or both, is presented and discussed.

Maccacaro, T.↗
NASA NTRS1983

Condensation nuclei events at 30 km and possible influences of solar cosmic rays

Two recent observations have provided the basis for study of a relationship between solar activity and the formation of small particles in the earth's atmosphere; the discovery of annual increases of condensation nuclei (CN) at 30 km and the detection of sulfuric acid molecules in large negative ion clusters in the 25-35-km altitude region. These observations have led to formulation and testing of a model wherein CN are formed in a 'polar cloud chamber' supersaturated with sulfuric acid vapor and triggered by ionization associated with solar flare cosmic radiation. It is concluded that such a model provides a potential explanation of the observations.

Hofmann, D. J.↗
NASA NTRS1983

Unusual behavior in the condensation nuclei concentration at 30 km

The result obtained with an improved balloon-borne condensation nuclei (CN) counter that is capable of operating to altitudes of at least 30 km are presented. Of major interest is the appearance of a quasiannual variation near 30 km which could be described by a sudden concentration increase of unusually small particles occurring in the winter or spring followed by a 1- to 3-month decay period to background levels. The magnitude of the variation has increased dramatically following the recent generally high levels of volcanic activity affecting the lower stratosphere. Several potential explanations for the event are considered, but none appear entirely satisfactory or complete at the present time. The explanation with the fewest drawbacks would attribute the production of growth of the new cn to a highly supersaturated H2SO4 vapor layer generated by any one of several proposed processes in the upper high latitude stratosphere.

Rosen, J. M.↗
NASA NTRS1982

Einstein Observatory results on active galactic nuclei

The results of Einstein Observatory surveys of active galactic nuclei (AGN) are reviewed. The ubiquity of X-ray emission from AGNs was confirmed. The relations between X-ray and optical luminosities, between X-ray and radio properties, and between X-ray and optical-UV line emission found by the surveys are summarized and briefly discussed. The possible causes of observed X-ray emission from jets in Cen-A, 3C273, and M87 are considered. The active nucleus discovered in the optically 'dull' galaxy NGC 4156 is covered, and a model for NGC 4151 based on detailed spectral studies is briefly discussed. This model establishes the global symmetry of the AGN clouds, their approximate sizes, and their ionization state. Difficulties encountered in attempting to explain the cosmic X-ray background in terms of AGN contributions are addressed.

Mushotzky, R. F.↗
NASA NTRS1983

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. II - 207 sources

Time delay and fringe frequency measurements at 2.29 GHz have been obtained with VLBI on baselines of about 10,000 km, in order to determine the positions of milliarcsec nuclei in 207 extragalactic radio sources. Accuracy ranges are estimated to lie between 0.1 and 1.0 arcsec in both right ascension and declination, with all sources having uncertainties of less than 4 arcsec in both coordinates. These sources constitute part of an all-sky VLBI catalog of milliarcsec radio sources, for 752 of which arcsec positions have thus far been determined.

Morabito, D. D.↗
NASA NTRS1983

Relativistic particles and gamma-rays in quasars and active galactic nuclei

A model for a class of quasars and active galactic nuclei is described in which a shock around a massive black hole randomizes the infall kinetic energy of spherically accreting matter producing a nonthermal spectrum of high energy protons. These protons may be responsible for the secondary production (via tau + or - decay) of the radio emitting high energy electrons and also of high energy gamma rays (via Pi decay and inverse Compton interactions of the electrons). The correlation between radio and gamma ray emission implied by the model is in good agreement with observations of 3C273. Observation of the flux of high energy neutrinos from quasars may provide a test for the model.

Protheroe, R. J.↗