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  • ASTROPHYSICS  (119)
  • SPACE SCIENCES
  • 1995-1999
  • 1980-1984  (119)
  • 1970-1974
  • 1981  (119)
  • 11
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    Hard X-ray spectrum of Cygnus X-1 (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Long-term measurements of the hard X-ray spectrum from 3 keV to 8 MeV of the black-hole candidate Cygnus X-1 in its low state are reported. Observations were made from October 26 to November 18, 1977 with the A2 (Cosmic X-ray) and A4 (Hard X-ray and Low-Energy Gamma-Ray) experiments on board HEAO 1 in the spacecraft's scanning mode. The measured spectrum below 200 keV is found to agree well with previous spectra which have been fit by a model of the Compton scattering of optical or UV photons in a very hot plasma of electron temperature 32.4 keV and optical depth 3.9 or 1.6 for spherical or disk geometry, respectively. At energies above 300 keV, however, flux excess is observed which may be accounted for by a distribution of electron temperatures from 15 to about 100 keV.
    Keywords: ASTROPHYSICS
    Type: Nature; 293; Sept. 24
    Format: text
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  • 12
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    X-ray emission from M87 - A pressure confined cooling atmosphere surrounding a low mass galaxy (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A conventional model for the mass of the galaxy M87 (500-billion solar masses) is used to show that a radiative cooling flow agrees well with spectroscopically derived mass flow rates (10 solar masses per year) and surface brightness profiles of X-ray emission around the galaxy. A temperature of 100-million K, and a density of a few x 10 to the -4th/cu cm are obtained without the use of a massive halo. The atmosphere contains 1-trillion solar masses of gas, with a temperature profile increasing outward and gas acting as a reservoir for the flow. The present models indicate that thermal conductivity in M87 is less than 0.002 of its classical value, and does not significantly affect properties of the surrounding gas.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 247
    Format: text
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  • 13
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    The Mg 2 h and k lines in a sample of dMe and dM stars (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Both Mg II h and k line fluxes are presented for a sample of 4 dMe and 3 dM stars obtained with the IUE satellite in the long wavelength, low dispersion mode. The observed fluxes are converted to stellar surface flux units and the importance of chromospheric non radiative heating in this sample of M dwarf stars is intercompared. In addition, the net chromospheric radiative losses due to the Ca II H and K lines in those stars in the sample for which calibrated Ca II H and K line data exist are compared. Active region filling factors which likely give rise to the observed optical and ultraviolet chromospheric emission are estimated. The implications of the results for homogeneous, single component stellar model chromospheres analyses are discussed.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center The Universe at Ultraviolet Wavelengths: The First Two Yrs. of Intern. Ultraviolet Explorer; p 279-286
    Format: application/pdf
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  • 14
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    A search for apsidal motion in 4U0115+63 (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The measurement of apsidal motion provides one of the few experimental tests of models of stellar interiors. Binary X-ray pulsars are suited for a potentially important application of the apsidal motion test because of their generally close orbits and the precision with which their orbits can often be measured. The orbit of the X-ray pulsar 4U0115+63 was determined by Rappaport et al. (1978). The orbital parameters were determined with sufficient precision to make possible a measurement of apsidal motion if a second observation of the source could be made. However, 4U0115+63 has not been observed to be active since its 1978 outburst. An analysis has, therefore, been conducted of the archival Uhuru data of the first recorded outburst of this source in early 1971. The results of this analysis are combined with the 1978 observations. It is concluded that apsidal motion would have been detectable if the companion were a rapidly rotating star with a mass not less than 30 solar masses.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 251
    Format: text
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  • 15
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    A search for expanding supershells of gas around OB associations (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The results of a search for giant, rapidly expanding shells of gas surrounding OB associations are recorded. Two out of thirteen nearby associations (Orion and Carina) show supershells of gas with radii of about 100 pc, expansion velocities of about 100 km/sec, and ages of about 400,000 years. Observations of the newly discovered Carina supershell are presented. It is suggested that these features are formed by supernova explosions in the associations. The birthrate of association supernovae than lies between 1.5 x 10 to the -14th/cu pc per/year and 10 to the -13th/cu pc per/year.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 250
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  • 16
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    The cool half of the H-R diagram in soft X-rays (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The results of an Einstein Observatory program to map the occurrence of hot coronae (T greater than 1 million K) in the cool half of the Hertzsprung-Russell diagram are reported. F-M dwarfs, and late F through early K star giants characterized by 10,000 K chromospheric and 20,000-200,000 K FUV emission lines were studied in one region, while a second region study included red giants later than K2 III and supergiants later than G5 Ib with weaker chromospheric emission and no high temperature species. Program goals comprised determination of the C IV division as seen in soft X-rays, and identification of stellar parameters which distinguish strong from weak coronal X-ray sources. A summary of target stars, X-ray fluxes, and UV emission profiles is provided, and coronal emissions, comparisons of C IV and wind boundaries, hybrid-spectrum supergiants, the energy balance of stellar outer atmospheres, stellar rotation and coronae, and evolutionary considerations are discussed, along with lines of future research.
    Keywords: ASTROPHYSICS
    Type: PB82-144183 , Astrophysical Journal; vol. 250
    Format: text
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  • 17
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    On the origin and distribution of C IV and Si IV ions in the neighboring interstellar medium (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: IUE spectrometer observational data are reported on the C IV, Si IV, and N V absorption lines in 46 distant stars at FWHM resolution of 30 km/sec. A simple analytic theory is given for these contributions, including the effects of ionization by a soft X-ray background. While for some of the most distant field stars a photoionized contribution may be ruled out, C IV absorption lines are detected which are shallow and broad. It is suggested that what are being detected in these cases are contributions from the hot interstellar gas that is responsible for the O VI absorption lines. Most of the Si IV and C IV lines probably arise in the H II regions surrounding the observed star or its neighbors.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 248
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  • 18
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    Relations among stellar X-ray emission observed from Einstein, stellar rotation and bolometric luminosity (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The correlation between observed stellar X-ray luminosities, bolometric luminosities, and projected rotational velocities for stars of various spectral types and luminosity classes are determined. Early type stars (O3 to A5) have X-ray luminosities independent of rotational velocities, and correlating with bolometric luminosities. Late type stars of spectral type G to M have luminosities well correlated to equatorial rotational velocities, and are independent of luminosity class. The dependence of late type stars is found to be equivalent to a relation between the X-ray surface flux and the stellar angular velocity. F stars are intermediate with X-ray luminosities higher than would be predicted on the basis of the early type star relation, although lower than expected from the late type velocity dependence. The location of RS CVn stars as a class is also discussed, and it is found that the heating of late type stellar coronas does not result from direct conversion of ratational energy.
    Keywords: ASTROPHYSICS
    Type: PB82-133869 , Astrophysical Journal; vol. 248
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  • 19
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    The distribution and morphology of X-ray-emitting gas in the core of the Perseus cluster (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: An unresolved source has been found coincident with the nucleus of NGC 1275 in a high-resolution X-ray image of the core of the Perseus cluster. Absorption in the optical features at high velocity with respect to NGC 1275, which are thought to be associated with a foreground galaxy, does not produce any detectable X-ray absorption. The emission tends to become asymmetric in the presence of the lower-velocity filaments, but no obvious, detailed correlation is found between X-ray enhancements and individual filaments. Deprojection of surface brightness to yield temperature and density profiles of the intracluster gas shows results consistent with a quasi-hydrostatic radiative accretion flow onto NGC 1275, and the pressure-driven mass inflow onto the central galaxy is then 200-400 solar masses/yr. The possibility of a problem in the relation of line-of-sight velocity dispersion to cluster gravitational mass is confirmed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 248
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  • 20
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    A search for interstellar molecules in the spectra of highly reddened stars (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A total of ten stars were observed with cameras of the International Ultraviolet Explorer (IUE) in both high and low dispersion. One star, X Persei (HD 24534, 6.0 BE), was analyzed in detail. Ultraviolet observations of the column densities of CO match those derived from the radio to within a factor of 4, with the difference probably due to the larger beam size of the radio measurement and the assumption of a thermal population in the rotational levels of CO. Upper limits are given to the log column densities for OH, HCl, and CH2 of 14.0, 12.3 and 12.8. The carbon abundance was found to be about solar with a possible depletion of about a factor of 2. With precautions concerning both noise and correct background, the IUE can be used for studies of interstellar molecules.
    Keywords: ASTROPHYSICS
    Type: The Universe at Ultraviolet Wavelengths: The First Two Yrs. of Intern. Ultraviolet Explorer; p 581-588
    Format: application/pdf
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