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  • General Chemistry  (64)
  • ASTROPHYSICS  (47)
  • ASTRONOMY  (15)
  • 550 - Earth sciences
  • 1990-1994  (127)
  • 1
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    Correction to the luni-solar precession from very long baseline interferometry (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The redetermination of the luni-solar precession is performed by using Very Long Baseline Interferometry (VLBI) catalogues of extragalactic radio sources containing positions at observation epochs which cover 9 years. The positions of the sources have been determined by the VLBI network of the Crustal Dynamics Project of NASA from measurements covering consecutive half year and one year intervals between 1981 and 1989. In the course of their reduction the International Astronomical Union (IAU) recommended precession terms were applied. On the assumption of an imperfection of the luni-solar precession it is expected that the positions of each source obtained at different epochs but uniformly reduced to the reference system J2000.0 exhibit an apparent proper motion. We attributed this motion tentatively to precession and solved for a correction of the luni-solar precession by making a least squares fit to the position differences between homonymous sources in the set of catalogues. The 18.6 yr terms of nutation entered the process as parameters the values of which were set by choice from the reservoir of recent determinations. Twenty-five sources contributed to the analysis with the effect that the annual and semi-annual catalogues contain 16 sources on the average. The analysis confirmed the slightly overestimated IAU value of the luni-solar precession yielding a mean correction of -3.59 +/- 1.14 mas/yr from the annual and -3.84 +/- 1.16 mas/yr from the semi-annual catalogues. We discuss these secular terms and their dependence on the adopted nutation terms.
    Keywords: ASTRONOMY
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 284; 3; p. 1000-1006
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  • 2
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    Near-infrared and ultraviolet spectrophotometry of the young planetary nebula Hubble 12 (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The young planetary nebula Hubble 12 is observed using near-IR and UV spectrophotometry. The brightness of the O I lines, which is greater than in any other planetary nebula yet measured, indicates that fluorescent excitation by stellar continuum is the principal mechanism generating these lines. Extinction, electron density, and electron temperature are determined using infrared measurements combined with UV data and published optical observations. The range in extinction, density, and temperature implies that, within the ionized region, pockets of emission with distinctly different conditions exist. Logarithmic abundances for helium, oxygen, and sulfur are presented.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 105; 3; p. 1002-1009.
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  • 3
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    The ultraviolet spectrum of the planetary nebula Vy 2-2 (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: New ultraviolet short-wavelength (SWP) IUE observations for the proto-planetary nebula Vy 2-2 were obtained in the low-dispersion mode. An exposure of 400-min duration reveals a weak stellar continuum and several definite emission lines as well as a number of absorption features. Emission features of semiforbidden C III, semiforbidden Si III, and forbidden Ne III are clearly observed, while C IV, Si II, and possibly C II appear in absorption. Emission features of He II, semiforbidden O III, and Al III are also indicated, though less certain. The UV spectrum tends to confirm that Vy 2-2 is a young, dense planetary nebula excited by a central star with T(eff) greater than 60,000 K.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 105; 688; p. 595-598.
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  • 4
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    Quasi-periodic oscillations in low-mass x ray binaries (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Variability on short time scales in the x-ray flux is a very general property of binary x-ray sources. Not until after the discovery of intensity-dependent quasi-periodic oscillations (QPO) and associated red noise form luminous low-mass x-ray binaries were systematic studies of the shape of the power-spectral components made. A brief account is given of the main developments since this discovery which have led to a new picture of the properties of low-mass x-ray binaries.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 251-260
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  • 5
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    X-ray planetary nebulae (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A study of IUE archival low-dispersion spectra was conducted in an attempt to correlate the C IV/He II and C III)/C IV emission line ratios as indicators of X-ray emission in planetary nebulae. The presence of P Cygni lines revealed a strong correlation for planetaries known to exhibit X-ray emission. Of the twelve planetaries definitely known to emit X-rays, eleven have been observed with IUE and all but one show ultraviolet lines with very strong P Cygni profiles for at least one, and sometimes all, of the N V, O V, or C IV lines which are indicative of strong stellar winds. Twenty planetary nebulae are identified as likely candidates for X-ray sources, based on their high IUE emission line flux ratios for C IV/He II as well as prevalence of P Cygni lines. Some misconceptions based on outdated IUE data are corrected.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 106; 701; p. 756-764
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  • 6
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    On planetary nebula nuclei of the spectral type 'continuous' (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A detailed reexamination of existing and new International Ultraviolet Explorer (IUE) low-and high-resolution spectra, plus a search of recent literature, shows that for most of the central stars of planetary nebulae the spectral classification previously called 'continuous-type' does not really exist. The stellar continua show primarily photospheric absorption features of N V, O V, C IV, H, He II, Fe VII, Fe VI, and Fe V, and/or P Cygni profiles.
    Keywords: ASTROPHYSICS
    Type: Publications of the Astronomical Society of the Pacific (ISSN 0004-6280); 106; 695; p. 56-58
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  • 7
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    Ultraviolet high-excitation Fe II fluorescence lines excited by O VI, C IV, and H I resonance emission as seen in IUE spectra (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Archival high-dispersion spectra from the IUE are used in a search for Bowen emission lines of Fe II excited by the stronger transition of the O VI resonance doublet. The possibility of using these Fe II emission lines as a diagnostic of the strength of the far-ultraviolet emission of O VI at 1032 A is explored. It is found that the Fe II emission lines are quite common and strong in symbiotic stars, particularly those of the type known as 'symbiotic novae', as well as in normal novae. The lines are observed in central stars of some planetary nebulae of the O VI sequence besides a few central stars of type WR. High density, high excitation, and high temperature are suggested to be requirements for the excitation of the Fe II fluorescence lines. It is pointed out that while these lines were observed in PG 1159-035 and K1-16, they were not observed in AGNs.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 373; 649-656
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  • 8
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    Far-infrared properties of NGC 6822 (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: IRAS FIR maps of NGC 6822, one of the few noninteracting Magellanic irregular galaxies resolved in the FIR by IRAS, are examined and compared to optical and H I data. The diffuse FIR emission at large radii from the center of the galaxy is found to be due to Galactic cirrus. There is a general correspondence between H-alpha and FIR emission in that FIR peaks are associated with major H II complexes, but the ratio of the FIR to the H-alpha emission varies between these complexes by about a factor of five. H I mass-to-FIR luminosity ratios cover a similar range for the three major H I cloud complexes in NGC 6822. More than about half of the FIR flux is produced in regions near H II region complexes. The remaining flux comes from diffuse FIR emission that extends over the entire optically visible galaxy. A model for the galaxy is discussed in which the galaxy is fairly transparent and the FIR is strongest where there is a strong UV source and higher optical depth due to dust in gas clouds.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 371; 142-147
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  • 9
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    Ultraviolet observations of the enigmatic bipolar nebula M1-92 (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: IUE observations of the bipolar nebula M1-92 indicate that this object is an evolved object and possibly a preplanetary nebula. Data acquired in the SWP camera clearly show a flat UV continuum indicative of a relatively unobscured O subdwarf. Data acquired in the long wavelength region of the SWP and in the LWP camera reveal an object which mimics the characteristics of an F2 supergiant. These results, combined with other data at visual and infrared wavelengths, strongly favor that the central object of M1-92 is an evolved binary system. The He I 5876 and 6678 A absorption seen in optical spectra is extremely problematic. This, plus the presence of Fe II, Mg II, and Mg I absorption arising from resonance transitions and transitions from low-lying metastable levels, leads to the suggestion that the object resembling an F2 star is actually an early-type subdwarf cloaked in a thick circumstellar nebula. This interpretation and the similarities of the IR flux distribution to Vy 2-2, a preplanetary prototype, is consistent with M1-92 being in the process of forming a planetary nebula. The bipolar flow, the optical Fe II and ultraviolet Mg II emission of M1-92 are also characteristics of T Tauri stars. Although constraints definitely rule out that M1-92 is a pre-main-sequence object, these similarities imply that in many cases it may be difficult to distinguish between T Tauri stars and protoplanetary nebulae.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 354; 262-266
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  • 10
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    FG Sagittae - No hot companion? (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The nucleus of the planetary nebula He 1-5 (= PK 60 -7 deg 1), the variable star FG Sge, was observed with the SWP camera of the International Ultraviolet Explorer satellite to detect a hot companion of the star, if such a companion exists. The observation found no evidence for the existance of a hot companion in the 1200-2000 A range of the SWP camera and supported the contention that FG Sge underwent a helium shell flash during the past century, and that the surrounding nebula, He 1-5, is a nebula of fossil ionization. Despite the currently accepted fossil ionization model, constraints posed by the satellite detection limit, the observed H-beta flux, and the adopted radii for white dwarfs still allow the possibility of a putative hot companion photoionizing this nebula.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 100; 1248-125
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