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  • ASTROPHYSICS  (26)
  • 1
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    Voyager EUV and FUV Observations (1984)
    In:  CASI
    Details
    Publication Date: 2014-09-16
    Description: The Voyager 1 and 2 ultraviolet spectrometers are sensitive over the wavelength range 500 to 1700 A. In the extreme ultraviolet (EUV), at wavelengths shortward of the Lyman limit (912 A), Voyager observations detected emission from three out of a sample of 11 nearly hot DA white dwarfs. These observations imply very low HI column densities in the directions of the three stars detected. In the far ultraviolet (FUV), at wavelengths between 912 and 1200 A, Voyager observations of O and B stars can be used to study interstellar reddening at the shortest wavelengths and to provide useful estimates of interstellar H2 column densities.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center Local Interstellar Medium, No. 81; p 91-95
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  • 2
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    An analysis of the bright white dwarf CD - 38 deg 10980 (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The bright DA white dwarf CD - 38 deg 10980 is comprehensively analyzed using all available data. Detailed comparisons of the overall energy distribution and Lyman-alpha and Balmer profiles are made with theoretical predictions based on a homogeneous grid of model atmospheres. These comparisons yield a consistent, well-defined effective temperature and surface gravity for this star of 24,700 + or - 250 K and log g = 7.95 + or - 0.15, respectively. A new gravitational redshift of 28.4 + or - 4.8 km/s is derived from an IUE high-dispersion spectrum. Conclusions regarding the mass and radius of CD - 38 deg 10980 are also presented.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 293; 294-302
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  • 3
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    Far-ultraviolet background observations at high galactic latitude. I - The Coma Cluster (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A series of high-galactic-latitude observations made with the Voyager 2 ultraviolet spectrometer are used to place stringent upper limits on the far-ultraviolet emissions from the central regions of the Coma Cluster of galaxies. In particular, consideration of the 912-1150 A region yields lower limits upon the radiative decay lifetimes of any neutrino species with masses in the range 22.1-27.8 eV, assuming massless decay products. Limits on the radiative decay lifetimes of such neutrinos are found to vary from 2.4 x 10 to the 25th to 7.1 x 10 to the 24th s, respectively, as a function of neutrino rest mass. This represents a substantial improvement over previous measurements.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 292; 16-21
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  • 4
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    The far-ultraviolet energy distribution of Sirius B from Voyager 2 (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Observations of Sirius obtained with the Voyager 2 ultraviolet spectrometer clearly reveal the presence of flux from the white dwarf Sirius B at wavelengths between 950 and 1100 A. These observations are in good agreement with all previous ultraviolet observations of Sirius B, and in particular with the IUE observations of Boehm-Vitense, Dettmann, and Kapranidis. A joint analysis of the Voyager 2 and IUE observations yields a temperature range of 26,000-28,000 K. A reexamination of current ultraviolet, visible, and X-ray observations produces good general agreement, but no single, mutually consistent, temperature for Sirius B. The Voyager 2 observations can be used to place a firm upper limit of 28,000 K on the temperature of Sirius B.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 280; 679-687
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  • 5
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    Far-ultraviolet spectrophotometry of two very hot O type subdwarfs (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: As a result of a spectroscopic survey of stars classified as nonemission OB+, Drilling (1983) has detected 12 new subluminous O stars. It was found that these stars are the hottest known O type subdwarfs. The effective temperatures of the stars are 60,000 K or higher. It has been possible to observe two of these stars with Voyager 1, taking into account LSE 21 and LS IV +10.9 deg. LSE 21 is one of the hottest of the new subdwarfs, with an effective temperature of at least 100,000 K. The optical spectrum indicates a hydrogen-rich atmosphere of high surface gravity. LX IV +10.9 deg is one of the cooler objects with an effective temperature of 65,000 K. The optical spectrum indicates an extremely helium-rich atmosphere and a somewhat lower surface gravity than LSE 21. The Voyager 1 observations confirm the temperature scale set up by Schoenberger and Drilling (1984) for the hottest O type subdwarfs.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 283; L67-L70
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  • 6
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    The circumstellar nature of the metallic features in a hot DA white dwarf (1995)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A new co-added IUE echelle spectrum of the bright DA white dwarf CD -38 deg 10980, together with a newly determined radial velocity for this star, indicate that the sharp lined Si and C absorption features seen in the UV are clearly circumstellar in origin. Absorption in both excited and ground state transitions occurs at a velocity displaced by -12.1 +/- 2.0 km.s with respect to the photospheric velocity. Weak features due to the Si IV doublet are seen at a velocity intermediate between that of the circumstellar features and the photosphere. First time estimates of column densities for excited and ground states of C II, Si II, and Si III are derived. These quantities are used with electron density estimates derived from these species to determine the location and physical conditions of the circumstellar gas in the vicinity of CD -38 deg 10980. If collisional excitation alone is responsible for the excited levels of Si III observed in CD -38 deg 10980, then electron densities in the circumstellar gas must exceed 10(exp 9)/cu cm. Substantially lower electron densities are possible if the circumstellar gas is located near enough to the star so that photoexcitation is the dominant process responsible for the excited lines seen in the UV. Strong limits are placed on the photospheric abundance of Si and C in the star itself. These limits are in sharp contrast to the theoretical predictions of radiative levitation in which Si, but not C, is expected in the photosphere of a white dwarf such as CD -38 deg 10980. The interstellar line of sight to CD -38 deg 10980 is also investigated.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 443; 2; p. 753-763
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  • 7
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    The Lyman-alpha observations of comet Kohoutek from Mariner 10 (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The paper presents the Lyman-alpha data from postperihelion observations of comet Kohoutek made with Mariner 10 UV spectrometer during the 1974 January 11-24 period when the comet distance from the sun varied from 0.521 AU to 0.860 AU. These data are analyzed to determine the hydrogen production rates. It is noted that observations against the low interplanetary background allowed a measurement of the hydrogen corona over 30 million km (or 15 minutes) as seen from the Mariner 10 spacecraft. The bulk of the observations can be fitted with a non-steady-state model assuming two velocity components (20 km/sec and 8 km/sec) of equal proportions. A 70% increase in the hydrogen production rate is found within 2 days from January 17 to 19 while the comet receded from the sun. Finally, it is noted that the postperihelion production rates from the Mariner 10 observations are comparable to the preperihelion production rates reported previously.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 232
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  • 8
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    Far-ultraviolet observations of BW Vulpeculae with Voyager 2 (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: BW Vul has been the subject of numerous ground-based spectroscopic and photometric studies. However, observations in the ultraviolet have been limited. The Voyager 2 observations presented in the current study include the first observations of BW Vul shortward of 1200 A. Voyager 2 observations in the 912-1700 A region are employed to investigate the variations in the temperature, radius, and luminosity of BW Vul during a pulsation cycle. Attention is given to details regarding the observations, the light curves representing the variation of BW Vul at effective wavelengths of 1050 A and 1395 A, the UV spectra, comparisons with other stars, comparisons with models, and the amplitude-wavelength relation.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 281; 766-773
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  • 9
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    Constraints on the optical properties of interstellar dust in the far-ultraviolet - Voyager observations of the diffuse sky background (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Four regions of the sky have been observed at various Galactic latitudes and H I column densities, with the ultraviolet spectrometer aboard Voyager 2 finding no evidence for the presence of a diffuse radiation field between 912 and 1100 A at a level of 100-200 photons per sq cm s sr A. As starlight scattered by dust grains may be expected to be a major contributor to the diffuse background, especially at low Galactic latitudes, it is possible to place a very stringent upper limit of less than 0.1 on the reflectivity of the dust in this spectral range. This albedo is much lower than the canonical value of about 0.5 for standard graphite-silicate dust models.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 383; 198-204
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  • 10
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    Voyager 2 observations of NGC 7023 - Dust scattering shortward of 1600 A (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: We report the first observational determination of the nebular-to-stellar flux ratio out to wavelengths as short as 1000 A for the reflection nebula NGC 7023, using a combination of spectroscopic data from Voyager 2 and from the Hopkins Ultraviolet Telescope (HUT) on the Astro 1 mission. After accounting for contributions by H2 fluorescence to the nebular flux, the wavelength variation of the ratio of the residual dust scattered flux to that observable from the embedded star is consistent with a reduction of the dust albedo by 25 percent over the wavelength interval 1300-1000 A. This is most easily interpreted, if the particles responsible for the FUV rise in the extinction curve are low-albedo, not necessarily zero-albedo, grains. The direct proportionality between the nebular surface brightness, measured at 22 arcsec offset by HUT, and the nebular flux suggests that the phase function of scattering does not vary in the 2000-1000 A range.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 410; 2; p. 714-718.
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