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  • ASTROPHYSICS  (119)
  • STRUCTURAL MECHANICS
  • 1980-1984  (136)
  • 1970-1974
  • 1960-1964
  • 1981  (136)
  • 1
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    Rapid variability of 10-140 keV X-rays from Cygnus X-1 (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: On five occasions in 1977 and 1978, Cygnus X-1 was observed using the low-energy detectors of the UCSD/MIT Hard X-ray and Low-Energy Gamma Ray experiment on the HEAO 1 satellite. Rapid (times between 0.08 and 1000 sec) variability was found in the 10-140 keV band. The power spectrum was white for frequencies between 0.001 and 0.05 Hz and was proportional to the inverse of the frequency for frequencies between 0.05 and 3 Hz, indicating correlations on all time scales less than approximately 20 s. The shape of the energy spectrum was correlated with intensity; it was harder at higher intensity. If the emission is produced by Comptonization of a soft photon flux in a hot cloud, the heating of the cloud cannot be constant; it must vary on time scales up to approximately 20 s. A variable accretion rate could cause the observed effects.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 246
    Format: text
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  • 2
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    A heating mechanism for the chromospheres of M dwarf stars (1981)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The atmospheric structure of the dwarf M-stars which is especially important to the general field of stellar chromospheres and coronae was investigated. The M-dwarf stars constitute a class of objects for which the discrepancy between the predictions of the acoustic wave chromospheric/coronal heating hypothesis and the observations is most vivid. It is assumed that they represent a class of stars where alternative atmospheric heating mechanisms, presumably magnetically related, are most clearly manifested. Ascertainment of the validity of a hypothesis to account for the origin of the chromospheric and transition region line emission in M-dwarf stars is proposed.
    Keywords: ASTROPHYSICS
    Type: Smithsonian Astrophysics Observatory 2nd Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Vol. 1; p 73-79
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  • 3
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    A variable mixing-length ratio for convection theory (1981)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: It is argued that a natural choice for the local mixing length in the mixing-length theory of convection has a value proportional to the local density scale height of the convective bubbles. The resultant variable mixing-length ratio (the ratio between the mixing length and the pressure scale height) of this theory is enhanced in the superadiabatic region and approaches a constant in deeper layers. Numerical tests comparing the new mixing length successfully eliminate most of the density inversion that typically plagues conventional results. The new approach also seems to indicate the existence of granular motion at the top of the convection zone.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 244
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  • 4
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    Rotation of dwarf star chromospheres in the ultraviolet (1981)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Periodic variations in the ultraviolet fluxes of chromospheric emission line multiplets are investigated for F, G and K stars as evidence of rotational modulation. Vacuum ultraviolet spectra were obtained with the IUE spacecraft for six stars as many as 11 times over the period April 23 to December 3, 1980. Variations in the emission fluxes of the hydrogen Lyman-alpha, Si II and Mg II lines are observed with periods up to 47 days. The periodicity, which is identified with rotational modulation, is found to persist over many rotational cycles, although the periods and time dependences of the fluxes from the different ionic species are not identical, probably due to differential rotation and global distributions. The spread of the UV periods is observed to be within 10%, with one or two peaks per cycle and a ratio of modulated to umodulated flux ranging from 1.1 to 3.0, analogous to solar behavior.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 248
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  • 5
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    Abundances of argon, sulfur, and neon in six galactic H II regions from infrared forbidden lines (1981)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Airborne measurements of the Ar II (6.99 micron) and S III (18.71 micron) forbidden lines for six compact H II regions are presented, as well as ground-based 2-4 micron and 8-13 micron spectroscopy if not already published. From these data and radio data, lower limits to the elemental abundances of Ar, Ne, and S are deduced. G29.9-0.0, at 5 kpc from the galactic center, is overabundant in all these elements. The other five regions (at distances 6-13 kpc from the center) mainly appear to be consistent with standard abundances, with the exception of G75.84 + 0.4 at 10 kpc from the galactic center, which is overabundant in S. However, preliminary results on G12.8-0.2 at 6 kpc from the galactic center suggest a possible underabundance. A large statistical sample of H II regions is required in order to determine if there is a radial gradient in the heavy element abundances of the Galaxy.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 250
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  • 6
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    The Gemini-Monoceros X-ray enhancement - A giant X-ray ring (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A 1.5 deg spatial resolution map of the 1/4 KeV diffuse X-ray background enhancement in the Gemini and Monoceros constellations is found to show a circular ring-shaped emission feature with a diameter of 20 deg. The feature and possible X-ray contributions from the Mon OB1 association and neighboring supernova remnants are discussed. From 300 pc, the region has a radius of 50 pc, with an emitting electron density of 0.01 per cu cm. A shell of expanding neutral hydrogen and nonthermal radio spur is observed outside the ring with the X-ray emitting pulsar PSR 0656 + 14 lying close to the center of the ring. Origins of the ring are discussed, ruling out formation by the association Mon OB1. The ring is considered to be a field supernova remnant formed by the progenitor of the central pulsar, and providing constraints on theories of remnant evolution. This conclusion is found to agree with estimated supernova rates, and the absence of additional examples of this stage of evolution is an observational selection effect.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 248
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  • 7
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    Supernova remnant revolution in an inhomogeneous medium. I - Numerical models (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The first numerical simulations of supernova remnant evolution in an inhomogeneous gas are presented. Evolution in the lowest density substrate (the intercloud) is assumed to be spherically symmetric with a large intercloud filling factor and many dense regions (clouds) within the remnant; however, mass momentum and energy transfer between cloud and intercloud are included and the position and morphology of individual clouds tracked. Evolution is considered in several different models of the interstellar medium, both those in which the intercloud gas is diffuse (0.001 to 0.01/cu cm) and those in which it is relatively dense (n approximately 0.3/cu cm) under a variety of assumptions about the efficiency of thermal evaporation from the clouds into the intercloud medium.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 247
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  • 8
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    Short time-scale variability of chromospheric Ca II in late-type stars (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The short time-scale variability of singly ionized calcium chromospheric emission has been investigated in a few late-type stars. Emission-line variations with time scales of a few minutes to hours are seen in Alpha Tau (K5 III), Lambda And (G8 III-IV), and Epsilon Eri (K2 V). The existence of substantial chromospheric flux changes (10 to the 30th to 10 to the 32nd ergs) over short periods of time suggests that the calcium emission arises from a few small, coherent regions. Frequencies present in the data are discussed in the context of acoustic wave predictions and estimated acoustic cutoff frequencies for giants and dwarfs.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 246
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  • 9
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    Cloud fluid compression and softening in spiral arms and the formation of giant molecular cloud complexes (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: With regard to the galactodynamics of the cloudy interstellar medium, the paper considers the response of such a gas to a forcing potential in the tight-winding density wave theory. The cloud fluid is treated in the hydrodynamic limit with an equation of state which softens at high densities. It is shown that in the inner regions of the galaxy, cooling of the cloud fluid in the arms can result in gravitational instability and the formation of large bound complexes of clouds which are identified with the giant molecular clouds (GMCs). Masses, dimensions, distributions, and scale heights of the GMCs are predicted by the theory. It is suggested that the interstellar gas density in the disk is regulated by the gravitational instability mechanism in the arms which siphons material into star formation. Implications for the evolution of individual GMCs and for galactic morphology are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 245
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  • 10
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    Galaxy formation in an intergalactic medium dominated by explosions (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The evolution of galaxies in an intergalactic medium dominated by explosions of star systems is considered analogously to star formation by nonlinearly interacting processes in the interstellar medium. Conditions for the existence of a hydrodynamic instability by which galaxy formation leads to more galaxy formation due to the propagation of the energy released at the death of massive stars are examined, and it is shown that such an explosive amplification is possible at redshifts less than about 5 and stellar system masses between 10 to the 8th and 10 to the 12th solar masses. Explosions before a redshift of about 5 are found to lead primarily to the formation of massive stars rather than galaxies, while those at a redshift close to 5 will result in objects of normal galactic scale. The model also predicts a dusty interstellar medium preventing the detection of objects of redshift greater than 3, numbers and luminosities of protogalaxies comparable to present observations, unvirialized groups of galaxies lying on two-dimensional surfaces, and a significant number of black holes in the mass range 1000-10,000 solar masses.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 243
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