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  • Analytical Chemistry and Spectroscopy  (1,518)
  • ASTROPHYSICS  (1,388)
  • Chemical Engineering  (638)
  • 1990-1994  (3,544)
  • 1985-1989
  • 1993  (3,544)
Collection
Keywords
Publisher
Years
  • 1990-1994  (3,544)
  • 1985-1989
Year
  • 11
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    The OB associations of 30 Doradus in the Large Magellanic Cloud. II - Stellar content and initial mass function (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: From photometric and spectroscopic observations of the stars in 30 Doradus we determine the effective stellar temperatures and bolometric magnitudes. We construct the theoretical H-R diagram, and using stellar evolution models we then bin the stars by mass to obtain the initial mass function (IMF). The IMF shows marked curvature; it flattens out at lower masses, even for those masses above which the photometry is complete. Best estimates of the IMF slope yield values of Gamma -1.5 +/- 0.2 for mass above 12 solar masses, where the Salpeter slope is Gamma = -1.35. However, there are indications that different regions of 30 Doradus have different IMF slopes, perhaps as the result of sequential star formation effects.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 106; 4; p. 1471-1483.
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  • 12
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    Star formation in the disks of H I-rich S0 galaxies (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: We present the results of a H-alpha emission-line imaging survey of a sample of neutral-gas-rich S galaxies. We find evidence of disk H II regions in 14 of our sample of 32 galaxies, detect nuclear or faint diffuse circumnuclear H-alpha + forbidden N II emission in another 11 galaxies without disk H II regions, and obtain upper limits for 8 galaxies. We find a striking dichotomy between Ss with and without H II regions; either a galaxy has a number of H II regions, most often distributed into distinct rings or ringlike structures, or there are none down to detection limits equivalent to a single unreddened H II region ionized by single O stars. We find that the S0s without disk H II regions have a lower median M sub H I/L(B) than those with disk H II regions, but the distributions have a large range of dispersion. Our data suggest that S0s may lie in a regime where local threshold effects, perhaps primarily kinematic in origin, are more important in determining the star formation in these galaxies than the global stability mechanisms that recent empirical models for large-scale star formation have suggested prevail in later-type spirals.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 106; 4; p. 1405-1419.
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  • 13
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    CCD imaging of Comet Wilson (1987VII) - A quantitative coma analysis (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: Distinctive cometary components (dust, ions, and radicals) are studied on the basis of 2D, narrow-band CCD images of Comet Wilson (1987VII). The fact that Comet Wilson showed no significant structures in the neutral coma during its first perihelion passage is additional evidence for the hypothesis that dynamically new comets do not show a heterogeneous nucleus, but still have a relatively uniform surface. The deviations from the 1/rho law for the decrease of surface brightness as a function of nuclear distance are explained by a combination of short-term variations in the dust production and the effects of solar radiation pressure. The C2 production rate remains basically constant during the whole observational period, while the CN production rate decreases with increasing heliocentric distance. It is inferred that the formation of C2 might be due both to photolytic destruction of some parent molecules as well as to chemical reactions between other species.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 104; 2; p. 206-225.
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  • 14
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    Estimate of the distance to the solar wind termination shock from gradients of anomalous cosmic ray oxygen (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: The radial gradient of anomalous cosmic ray oxygen measured instantaneously between Voyager 2 and Pioneer 10 during 1985-1988 is correlated with the inferred tilt of the heliospheric neutral sheet. This is consistent with a simple model in which the radial gradient is related to the length of the neutral sheet between the two spacecraft. With this model we show that the radial gradient and the tilt of the neutral sheet near the solar wind termination shock can be inferred from the Voyager and Pioneer observations. By comparing the time history of the inferred tilt with that derived from solar observations, we estimate that the termination shock was at 62 +5/-7 AU at solar minimum in 1987. At solar maximum the shock should be located at about 90 AU due to the increased pressure of the solar wind.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A9; p. 15,165-15,168.
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  • 15
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    Activation of extended red emission photoluminescence in carbon solids by exposure to atomic hydrogen and UV radiation (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: We report on new laboratory results which relate directly to the observation of strongly enhanced extended red emission (ERE) by interstellar dust in H2 photodissociation zones. The ERE has been attributed to photoluminescence by hydrogenated amorphous carbon (HAC). We are demonstrating that exposure to thermally dissociated atomic hydrogen will restore the photoluminescence efficiency of previously annealed HAC. Also, pure amorphous carbon (AC), not previously photoluminescent, can be induced to photoluminesce by exposure to atomic hydrogen. This conversion of AC into HAC is greatly enhanced by the presence of UV irradiation. The presence of dense, warm atomic hydrogen and a strong UV radiation field are characteristic environmental properties of H2 dissociation zones. Our results lend strong support to the HAC photoluminescence explanation for ERE.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 415; 1; p. L51-L54.
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  • 16
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    Genesis of a planet in Messier 4 (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The anomalous spin period second derivative of the binary millisecond pulsar PSR 1620-26 in the globular cluster M4 is best explained by a sub-Jovian mass planet in a moderately eccentric about 7 AU orbit about the pulsar binary. We consider formation scenarios for PSR 1620-26. A planet scavenged from a single main-sequence star during an exchange encounter naturally produces systems such as PSR 1620-26. The position of the pulsar just outside the core of M4 is shown to fit naturally with the preferred formation scenario and permit a planet to have survived in the inferred orbit about the binary. It is possible that the orbital eccentricity of the binary was induced by the planet. A confirmation of a planet in eccentric orbit about PSR 1620-26 would strongly suggest that planets form ubiquitously around low-mass main-sequence stars, even stars of low metallicity.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 415; 1; p. L43-L46.
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  • 17
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    Detection of accreting gas toward HD 45677 - A newly recognized, Herbig Be proto-planetary system (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: We report the detection of high-velocity, accreting gas toward the BE(e) star with IR excess and bipolar nebula, HD 45677. High-velocity (+200 to +400 km/s), variable column density gas is visible in all IUE spectra from 1979 to 1992 in transitions of Si II, C II, Al III, Fe III, Si IV, and C IV. Low-velocity absorption profiles from low oscillator strength transitions of Si II, Fe II, and Zn II exhibit double-peaked absorption profiles similar to those previously reported in optical spectra of FU Orionis objects. The UV absorption data, together with previously reported analyses of the IR excess and polarization of this object, suggest that HD 45677 is a massive, Herbig Be star with an actively accreting circumstellar, protoplanetary disk.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 415; 1; p. L39-L42.
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  • 18
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    Population II Li-6 as a probe of nucleosynthesis and stellar structure and evolution (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We discuss the importance of Population II Li-6 as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars. The observation of Li-6 in the subdwarf HD 84937 is shown to be consistent with the existing Population II LiBeB data within the context of a simple three-component model: (1) standard big bang nucleosynthesis, (2) Population II cosmic-ray nucleosynthesis, (3) standard (nonrotating) stellar LiBeB depletion. If this interpretation is correct, we predict a potentially detectable boron abundance for this star: about 2 x 10 exp -12. Subsequent Population II LiBeB observations, and in particular further observations of Population II Li-6, are shown to be crucial to our understanding of the primordial and early galactic creation and destruction mechanisms for light elements.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 415; 1; p. L35-L38.
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  • 19
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    Gamma-ray bursts as a probe of large-scale structure in the universe (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: If gamma-ray bursts are cosmological in origin, the sources of the bursts are expected to trace the large-scale structure of luminous matter in the universe. We show that, if this is so and if the Burst and Transient Source Experiment yields the locations of approximately greater than 3000 gamma-ray bursts, it may be possible to use them to probe the structure of luminous matter on the largest scales known, consistent with recent determinations from pencil beam surveys and studies of superclusters. A positive result would provide compelling evidence that most gamma-ray bursts are cosmological in origin and would allow comparison between the distributions of luminous matter and dark matter on large scales. Conversely, a negative result might cast doubt on the cosmological origin of the bursts, provide evidence that the clustering of burst sources on large scales is less than that expected from pencil beam surveys and studies of superclusters, or indicate that gamma-ray bursts have some more exotic origin.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 415; 1; p. L1-L4.
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  • 20
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    Cepheid velocity curves from lines of different excitation and ionization. I - Observations (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: Radial velocity curves were obtained from several hundred optical absorption lines for FF Aql, Delta Cep, Eta Aql, and Chi Cyg. An iodine absorption cell has been used to reduce relative radial velocity errors to a few hundred meters per second. The shape and amplitude of the resulting radial velocity curves are in good agreement with previous results, particularly Delta Cep, which is shown to have been extremely constant over most of the last century. Velocities from atoms in low excitation potential (EP) levels (Fe I, Fe II, Sc II, Y II, and Ti II) show positive velocity residuals of as much as 5 km/s compared to high EP Fe I lines during the phase of rapidly decreasing velocity. The 8.12 eV line of Si II at 6348 A has a negative velocity residual during the same phase. The asymmetry of the high EP Fe I, Ti II, and the Si II lines are similar and greater than the asymmetry of the low EP Fe I, Fe II, Sc II, and Y II lines.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 415; 1; p. 323-334.
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