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  • ASTROPHYSICS  (27)
  • ASTRONOMY  (13)
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  • 1
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    The CI pressure shift and gravitational redshift of the cool DBQA5 white dwarf LDS678A (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A high resolution ultraviolet spectrum of the helium rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite is presented. LDS 678A is the coolest metallic line generate (DQ or DZ) yet observed with the IUE scale. These observations provide a detailed line profile of the strong C I 2479 absorption line with equivalent width (W sub 2479 = 2.35 plus or minus 0.06 angstroms) from which theoretical line profile fits yield a C abundance (log C/He = 6.4). The presence of carbon in a helium rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift for the C I line, a gravitational redshift is deduced from which a most probable mass of 0.55 solar mass is derived.
    Keywords: ASTROPHYSICS
    Type: ESA, Evolution in Astrophysics: IUE Astronomy in the Era of New Space Missions; p 467-470
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  • 2
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    IUE observations of a hot DAO white dwarf: Implications for diffusion theory and photospheric stratification (1988)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Observations of the DAO white dwarf PG1210+533, including the first high dispersion spectrum of a hybrid H-He object of this nature were obtained by IUE. In contrast with hot DAs in the 50,000 K temperature range, PG1210+533 shows no narrow interstellar-like metal lines, in spite of an optically observed He/H abundance of 0.1. This lack of metal makes accretion from the ISM an unlikely source for the He in the PG1210+533 photosphere. A significant discovery in the high dispersion spectrum is the existence of a sharp, non-LTE like, core seen in the He II 1640 line. Such features are detected in DO white dwarfs. A small aperture SWP low dispersion observation reveals the Lyman alpha profile of PG1210+533 to be surprisingly weak and narrow. Fits of this profile using pure H models yielded a T(eff) = 56,000 K. Fits of the Balmer H gamma profile however, yield T(eff) = 42,300 K and log g = 8.5 + or - 0.5 for the same models. It is unlikely that homogeneously mixed H-He atmospheres can resolve the inconsistency between the Lyman alpha and H gamma features in this star. Stratified models involving thin H photospheres may be necessary to explain these results.
    Keywords: ASTRONOMY
    Type: ESA, A Decade of UV Astronomy with the IUE Satellite, Volume 1; p 263-265
    Format: application/pdf
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  • 3
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    Spectrophotometric and model-atmosphere analyses of the hot DO and DAO white dwarfs from the Palomar-Green survey (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Recent optical and spectrophotometric observations of all currently known DAO and DO hot white dwarf candidates from the Palomar-Green Survey are presented. The optical observations were carried out using the Kitt Peak National Observatory Intensified Image Dissector Scanner (IIDS) system, the Multiple Mirror Telescope (MMT), and two photon-counting Reticon systems. Ultraviolet spectrophotometry was obtained using the International Ultraviolet Explorer (IUE) short-wavelength and long-wavelength cameras. The following spectroscopic subgroups were identified in the observations: the cool DO stars with spectra showing H I features together with He II + H blends; the hot DO stars, with spectra exhibiting weak He II (+H) lines; and hydrogen-rich DAO stars with broad H lines and sharp H II lines at 4686 A. A fourth subgroup of stars resembling PG 1159 with He at He II lines at 4686, and a C, N ion blend in the optical spectrum were also found. The spectroscopic data were analyzed in terms of a grid of hot, high-gravity LTE model atmospheres composed of a mixture of hydrogen and helium, and atmospheric parameters are deduced for each class of objects. On the basis of the model atmosphere analysis, the local space density of the DO stars is estimated to be 1.9-3.0 x 10 to the -7th per cubic pc, depending on the scale height.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 58; 379-411
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  • 4
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    The cosmological evolution and luminosity function of X-ray selected active galactic nuclei (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 266
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  • 5
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    The ultraviolet spectrum of the magnetic degenerate star GD229 (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The ultraviolet spectrum obtained with the IUE satellite of the magnetic degenerate white dwarf star GD 229 is presented. The ultraviolet energy distribution measured by the short- and long-wavelength cameras covering the range 1170-3200 A, when combined with optical data, indicates a best fit blackbody temperature of 16,000 K, which is significantly lower than previous estimates. A broad absorption trough in the 2000-3000 A region is observed which may be produced by opacity in the fundamental cyclotron frequency for surface magnetic fields from 3.2 to 5 x 10 to the 8th gauss. The absence of a strong Lyman-alpha sigma + component suggests that the primary atmospheric constituent is not hydrogen, with various absorption features probably attributable to Zeeman components of neutral helium, or possibly Mg II, C II and other metal lines. The superposition of many strong Zeeman features is also an alternative interpretation of the absorption in the 2000-3000 A interval.
    Keywords: ASTRONOMY
    Type: Astronomical Society of the Pacific; vol. 93
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  • 6
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    Observations of six flat spectrum sources from the 5 GHz survey (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Observations at radio, optical, and X-ray wavelengths of six sources from the Bonn-NRAO 5 GHz survey are reported. The sources were selected on the basis of their flux densities and spectral indices at 5 GHz. All have been shown to contain compact radio cores and to emit strongly at X-ray wavelengths; five are strongly polarized at optical wavelengths. The measured flux densities suggest that the sources are of comparable luminosity (per fractional bandwidth) in the X-ray and optical regions. The interpretation of these results in terms of a synchrotron-self-Compton mechanism is briefly discussed.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; vol. 247
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  • 7
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    BE Ursae Majoris /PG 1155 + 492/ - A unique cataclysmic-variable-like object (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The variable spectrum of BE Ursae Majoris, which is similar to that of the cataclysmic variables (CVs), shows (1) an emission-line spectrum with the high-excitation CN 4650 A blend as the strongest optical feature, weaker H and He lines, and a strong Balmer jump, and (2) an absorption spectrum dominated by He. Although the object shows no evidence of photometric variability on either short or long time-scales, it has been shown by Kurochkin (1964, 1971) to have a 2.29-day, sinusoidal, 1.5-mag variation. The luminosity is dominated by a UV power-law component, though the 4000-10000 A continuum is flat. The hypothesis that this object is an active mass transfer binary is not supported by its photometric behavior, energy distribution and long period, despite the spectroscopic variability. Nevertheless, three components whose nature is not understood remain: (1) an ultraviolet source whose energy distribution is explainable neither by a uniform temperature primary star nor by an accretion disk; (2) a reprocessed component powered by the UV star; and (3) a component similar to a CV disk, producing the optically thick Balmer lines.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; vol. 251
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  • 8
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    Discovery of carbon stars in the Draco dwarf spheroidal galaxy (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A grating prism survey of the Draco dwarf spheroidal galaxy that is 97% areally complete has led to the discovery of three carbon stars. Membership of these stars in Draco is firmly established from luminosity, proper motion, and radial velocity considerations. Optical and preliminary infrared photometry suggest that the Draco carbon stars are more closely related to the CH stars in omega Cen than to the luminous carbon stars found in the Fornax and Carina dwarfs. One of the carbon stars possesses an unusual emission line spectrum which might be indicative of either a very hot degenerate companion or a preplanetary evolutionary stage. Carbon stars have now been located in all four of the dwarf spheroidals that have been examined using transmission grating prism techniques. The rarity of these stars in galactic globulars, systems with which the dwarf spheroidals are often compared, indicates a fundamental population difference whose cause is not yet fully understood.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 254
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  • 9
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    A medium sensitivity X-ray survey using the Einstein Observatory - The log N-log S relation for extragalactic X-ray sources (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The paper presents results of an X-ray survey of about 50 sq deg of the high galactic latitude sky at sensitivities in the range of 7 x 10 to the -14 to 5 x 10 to the -12 ergs/sq cm/s. The number-flux relation is derived for the extragalactic population to yield a best-fit power-law slope of 1.53 + or - 0.16, and the content of the sample is analyzed in terms of types of sources, appearing to be significantly different from the content of similar samples selected at higher fluxes. The medium sensitivity sample of extragalactic sources is dominated by active galactic nuclei, while samples selected at higher fluxes and higher energies are dominated by clusters of galaxies. Thus, the number-flux relation for extragalactic sources may be interpreted to a first approximation as the sum of the two different distributions with flatter and steeper slopes describing clusters and AGNs, respectively.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; vol. 253
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  • 10
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    Discovery of a narrow line quasar (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A stellar object is reported which, while having X-ray and optical luminosities typical of quasars, has narrow permitted and forbidden emission lines over the observed spectral range. The narrow-line spectrum is high-excitation, the Balmer lines seem to be recombinational, and a redder optical spectrum than that of most quasars is exhibited, despite detection as a weak radio source. The object does not conform to the relationships between H-beta parameters and X-ray flux previously claimed for a large sample of the active galactic nuclei. Because reddish quasars with narrow lines, such as the object identified, may not be found by the standard techniques for the discovery of quasars, the object may be a prototype of a new class of quasars analogous to high-luminosity Seyfert type 2 galaxies. It is suggested that these objects cannot comprise more than 10% of all quasars.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; vol. 252
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