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  • 1
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    Infrared spectra of crystalline phase ices condensed on silicate smokes at T less than 20 K (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Infrared spectra of H2O, CH3OH, and NH3 condensed at T less than 20 K on amorphous silicate smokes reveal that predominantly crystalline phase ice forms directly on deposit. Spectra of these molecules condensed on aluminum substrates at T less than 20 K indicate that amorphous phase ice forms. On aluminum, crystalline phase H2O and CH3OH are formed by annealing amorphous deposits to 155 K and 130 K, respectively (or by direct deposit at these temperatures); crystalline NH3 is formed by direct deposit at 88 K. Silicate smokes are deposited onto aluminum substrates by evaporation of SiO solid or by combustion of SiH4 with O2 in flowing H2 followed by vapor phase nucleation and growth. Silicate smokes which are oxygen-deficient may contain active surface sites which facilitate the amorphous-to-crystalline phase transition during condensation. Detailed experiments to understand the mechanism are currently in progress. The assumption that amorphous phase ice forms routinely on grains at T less than 80 K is often used in models describing the volatile content of comets or in interpretations of interstellar cloud temperatures. This assumption needs to be reexamined in view of these results.
    Keywords: ASTROPHYSICS
    Type: The Astrophysical Journal (ISSN 0004-637X); 428; 2, pt; p. L81-L84
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  • 2
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    The accumulation and structure of comets (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The paper reviews evidence for the accumulation of the terrestrial planets and comets from solid grains, with emphasis on the various proposals for the formation of cometary nuclei. With three exceptions, all hypotheses conclude or imply that a single compact object forms. Several hypotheses start with Goldreich-Ward-type gravitational instabilities. The collapse for this case also occurs at low velocities in the cm/s to m/s range. Experiment and theory show that under these conditions, low-density, filamentary clusters form that are fractal aggregates with a fractal dimension approximately equal to 2. In order to form cometary nuclei, the initial temperature must be about 50 K and not undergo a significant temperature rise during the accumulation process. The calculations show that accumulation will occur at low temperatures. Models of cometary nuclei are reviewed, and a simple model of the structure that results fom the accumulation of fluffy aggregates is described.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 335-359.
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  • 3
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    Refractory grain processing in circumstellar shells Diagnostic infrared signatures (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Recent advances in infrared speckle interferometry reported by Ridgway, et al. (1986) have made possible the determination of the temperatures at the inner radius of certain dusty outflows. When combined with recent data on the thermal annealing and hydrous alteration rates of amorphous magnesium silicate grains, this information allows one to predict that grains heated to high temperatures around stars such as NML Cygnus will be more crystalline than will cooler grains around stars like IRC +10420. In 1985, Jura and Morris (1985) showed that water vapor can condense on previously nucleated refractory grains in some stellar outflows. Stochastic heating events might provide sufficient energy to produce hydrated silicates from orginally amorphous grains provided that the loss of water from such materials does not occur too rapidly. Observable consequences of both types of grain processing are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 310; L83-L86
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  • 4
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    A fractal model of a cometary nucleus formed by random accretion (1986)
    In:  Other Sources
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    Publication Date: 2013-08-29
    Description: Numerical simulations of the structure of aggregates of grains in the solar system show they are fractals with low density and irregular shapes. Comet nuclei forming from a mixture of ice and dust grains are likely to have nonuniform composition as well as structure. Such a nucleus appears to account for observed characteristic of comets. The structure has significant implications for the evolution of cometary nuclei.
    Keywords: ASTROPHYSICS
    Type: ESA, Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 3: Posters; p 523-524
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  • 5
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    Atlas of Comet Halley 1910 II (1986)
    In:  CASI
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    Publication Date: 2019-06-28
    Description: With the impending return of Halley's Comet in 1986, a major effort began to collect the material obtained at its last appearance in 1910. This material displays the evolving coma and tail phenomena, and is useful for comparison with the present quantitative studies of spectroscopic and structural phenomena. Images in the atlas are arranged in chronological order by day. Days that have multiple images with varying scale are arranged in two sequences. Photographs showing tail phenomena are first, followed by photographs obtained with longer focus instruments showing the head or near-nuclear region. Drawings of Comet Halley, made from visual observations in 1835 and 1910, also are included.
    Keywords: ASTROPHYSICS
    Type: NASA-SP-488 , NAS 1.21:488 , LC-86-16341
    Format: application/pdf
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  • 6
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    Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. 6: Variability of NGC 3783 from ground-based data (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The Seyfert 1 galaxy NGC 3783 was intensely monitored in several bands between 1991 December and 1992 August. This paper presents the results from the ground-based observations in the optical and near-IR bands, which complement the data set formed by the International Ultraviolet Explorer (IUE) spectra, discussed elsewhere. Spectroscopic and photometric data from several observatories were combined in order to obtain well-sampled light curves of the continuum and of H(beta). During the campaign the source underwent significant variability. The light curves of the optical continuum and of H(beta) display strong similarities to those obtained with the IUE. The near-IR flux did not vary significantly except for a slight increase at the end of the campaign. The cross-correlation analysis shows that the variations of the optical continuum have a lag of 1 day or less with respect to those of the UV continuum, with an uncertainty of is less than or equal to 4 days. The integrated flux of H(beta) varies with a delay of about 8 days. These results confirm that (1) the continuum variations occur simultaneously or with a very small lag across the entire UV-optical range, as in the Seyfert galaxy NGC 5548; and (2) the emission lines of NGC 3783 respond to ionizing continuum variations with less delay than those of NGC 5548. As observed in NGC 5548, the lag of H(beta) with respect to the continuum is greater than those of the high-ionization lines.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 425; 2; p. 609-621
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  • 7
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    Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. III - Further observations of NGC 5548 at optical wavelengths (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The results of the second year of an intensive ground-based spectroscopic and photometric study of variability in the bright Seyfert 1 galaxy NGC 5548 are reported in order to study the relationship between continuum and emission-line variability. Relative to the first year of the monitoring program, the nucleus of NGC 5548 was considerably fainter and the continuum variations slower during the second year, but the continuum H-beta cross-correlation results for the two years are nearly identical. The variations in the broad H-beta emission-line lag behind those in the continuum by somewhat less than 20 days, as concluded from the first year's data.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 392; 2 Ju; 470-484
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  • 8
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    Laboratory experiments on carbonaceous material as a source for the red rectangle visual emissions (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The authors subjected mixtures of CO, Ar, N2, H2O, and CH4 with 42 percent and 8 percent argon to an electrical discharge, froze out the reaction products at T about 20 K, and subsequently warmed the sample to room temperature. This resulted in a stable residue having broad-band fluorescence. It is suggested tht interstellar grains in the Red Rectangle/HD 44179 are coated with a residue of generally similar composition that is responsible for the broad emission feature in the 5400-7500 A spectral region.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 336; 838-842
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  • 9
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    The structure of the broad-line region in the Seyfert galaxy Markarian 590 (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We have undertaken a nine-month study of continuum and emission-line variability in the Seyfert galaxy Mrk 590 in order to determine the structure of the broad-line region. The H-beta variations are found to lag behind those of the optical continuum by about 19 days. We apply a maximum entropy method to solve for the transfer function which relates the line and continuum variability. This analysis suggests that there is a deficit of emission-line response due to gas along the line of sight to the continuum source, as in the case of NGC 5548, although these data do not allow us to reject with confidence models with significant line-of-sight response. We also show that the H-beta line variability is apparently confined to the core of the emission line, as suggested previously by Ferland, Korista, and Peterson (1990).
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 402; 2; p. 469-478.
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  • 10
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    Steps toward determination of the size and structure of the broad-line region in active nuclei. 7: Variability of the optical spectrum of NGC 5548 over years (1994)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: We report on the results of a continuation of a large monitoring program of optical spectroscopy of the Seyfert 1 galaxy NGC 5548. The new observations presented here were obtained between 1990 December and 1992 October, and extend the existing database to nearly 1400 days, dating back to 1988 December. The continuum variations are generally smooth and well-resolved, except during the third year of this 4 year project, when the variations were apparently more rapid and of lower amplitude than observed at other times. The broad H(beta) emission line is found to vary in response to the continuum variations with a lag of about 18 days, but with some changes from year to year. The H(beta) transfer functions for each of the 4 yr and for the entire 4 yr database are derived by using a maximum entropy method.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 425; 2; p. 622-634
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