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  • ASTROPHYSICS  (31)
  • 1
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    The stellar composition of X-ray surveys from the Einstein Observatory (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A new class of X-ray-luminous 'yellow' stellar objects which contributes significantly to the stellar log N-log S distribution, but which cannot be reconciled with normal G and K main-sequence stars. This identification results from a new analysis of the stellar content of three samples of X-ray-selected X-ray sources observed with the Einstein Observatory, namely the 'Medium Sensitivity Survey', the 'High Sensitivity Survey', and the 'Hyades Region Survey'. In this paper, both X-ray and optical properties of the stellar samples in these surveys are reported. The actual stellar content of the surveys is compared with predictions based on current knowledge of stellar X-ray luminosity functions and the stellar composition and spatial distribution in the Galaxy. It is shown that a plausible identification for the excess population of 'yellow' stars is with the active, RS CVn-like binaries.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 324; 1010-101
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  • 2
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    On stellar X-ray emission (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Stellar X-ray astronomy represents an entirely new astronomical discipline which has emerged during the past five years. It lies at the crossroads of solar physics, stellar physics, and general astrophysics. The present review is concerned with the main physical problems which arise in connection with a study of the stellar X-ray data. A central issue is the extent to which the extrapolation from solar physics is justified and the definition (if possible) of the limits to such extrapolation. The observational properties of X-ray emission from stars are considered along with the solar analogy and the modeling of X-ray emission from late-type stars, the modeling of X-ray emission from early-type stars, the physics of stellar X-ray emission, stellar X-ray emission in the more general astrophysical context, and future prospects.
    Keywords: ASTROPHYSICS
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  • 3
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    Coordinated Einstein and IUE observations of a disparitions brusques type flare event and quiescent emission from Proxima Centauri (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The Einstein Imaging Particle Counter observed a major X-ray flare in its entirety during a 5-hr period of simultaneous observations, with the IUE, of the dM5e flare star Proxima Centauri in August, 1980. The detailed X-ray light curve, temperature determinations during various intervals, and UV line fluxes obtained before, during, and after the flare indirectly indicate a 'two-ribbon flare' prominence eruption. The calculated ratio of coronal to bolometric luminosity for the event is about 100 times the solar ratio. The Proxima Cen corona is analyzed in the context of static loop models, in light of which it is concluded that less than 6% of the stellar surface seems to be covered by X-ray emitting active regions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 267
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  • 4
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    The cool half of the H-R diagram in soft X-rays (1981)
    In:  Other Sources
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    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
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  • 5
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    Relations among stellar X-ray emission observed from Einstein, stellar rotation and bolometric luminosity (1981)
    In:  Other Sources
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    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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  • 6
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    Einstein Observations of X-ray emission from A stars (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Results are reported from the combined CfA Stellar Survey of selected bright A stars and an Einstein Guest Observer program for Ap and Am stars. In an initial report of results from the CfA Stellar Surveys by Vaiana et al. (1981) it was noted that the spread in observed X-ray luminosities among the few A stars observed was quite large. The reasons for this large spread was studied by Pallavicini et al. (1981). It was found that the X-ray emission from normal stars is related very strongly to bolometric luminosity for early-type stars and to rotation rate for late-type stars. However, an exception to this rule has been the apparently anomalous behavior of A star X-ray emission, for which the large spread in luminosity showed no apparent correlation with either bolometric luminosity or stellar rotation rate. In the present study, it is shown that the level of emission from normal A stars agrees with the correlation observed for O and B stars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 271; Aug. 1
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  • 7
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    Einstein X-ray survey of the Pleiades - The dependence of X-ray emission on stellar age (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The data obtained with two pointed observations of 1 deg by 1 deg fields of the Pleiades region have been analyzed, and the results are presented. The maximum-likelihood X-ray luminosity functions for the Pleiades G and K stars in the cluster are derived, and it is shown that, for the G stars, the Pleiades X-ray luminosity function is significantly brighter than the corresponding function for Hyades G dwarf stars. This finding indicates a dependence of X-ray luminosity on stellar age, which is confirmed by comparison of the same data with median X-ray luminosities of pre-main sequence and local disk population dwarf G stars. It is suggested that the significantly larger number of bright X-ray sources associated with G stars than with K stars, the lack of detection of M stars, and the relatively rapid rotation of the Pleiades K stars can be explained in terms of the onset of internal differential rotation near the convective envelope-radidative core interface after the spin-up phase during evolution to the main sequence.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 292; 172-180
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  • 8
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    An Einstein Observatory X-ray survey of main-sequence stars with shallow convection zones (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The results of an X-ray survey of bright late A and early F stars on the main B-V sequence between 0.1 and 0.5 are presented. All the stars were observed with the Einstein Observatory for a period of at least 500 seconds. The survey results show significantly larger X-ray luminosities for the sample binaries than for the single stars. It is suggested that the difference is due to the presence of multiple X-ray sources in binaries. It is shown that the X-ray luminosities for single stars increase rapidly with increasing color, and that the relation Lx/Lbol is equal to about 10 to the -7th does not hold for A stars. No correlation was found between X-ray luminosity and projected equatorial rotation velocity. It is argued on the basis of the observations that X-ray emission in the sample stars originated from coronae. The available observational evidence supporting this view is discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 290; 307-320
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  • 9
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    Closed coronal structures. VI - Far-ultraviolet and X-ray emission from active late-type stars and the applicability of coronal loop models (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 289; 203-212
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  • 10
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    Magnetohydrodynamic thermal instabilities in cool inhomogeneous atmospheres (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The stability of magnetic loops to current-driven filamentation instabilities is investigated. The unperturbed atmosphere is assumed to be composed of an (upper) isothermal optically thin low-density portion and a (lower) higher-density portion which is in radiative equilibrium; in both cases, the atmosphere is in hydrostatic equilibrium, so that gravitational stratification is taken into account. In order to provide specific equilibrium conditions for evaluation of the dispersion relation, conditions appropriate for the surface of a solar-type star are adopted; i.e., a fairly low temperature (T = 5000 K) appropriate for a 'precoronal' state associated, for example, with magnetic flux emerging from photospheric levels under the action of magnetic buoyancy. A linear stability analysis is performed, and numerical results show that physically plausible current densities, which would be generated by typical loop-footpoint motions, are effective in driving MHD instabilities in such a plasma. The instability growth rates are strongly dependent on the assumed current density distribution and on the density scale height.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 291; 798-805
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