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  • 1
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    X-rays from RS Canum Venaticorum systems - A HEAO 1 survey and the development of a coronal model (1980)
    In:  Other Sources
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    Publikationsdatum: 2011-08-17
    Beschreibung: The paper presents HEAO 1 low-energy X-ray observations of 59 known or suspected RS CVn systems cited in the lists of Hall (1976), Eggen (1978), and the circulars of the Working Group on RS CVn systems of IAU Commission 42. The data are used to argue against the validity of the minimum flux coronal models. A coronal loop model is used to derive expressions for the loop parameters in terms of observable quantities, and find acceptable solutions for RS CVn systems. It is concluded that the difference between solar activity and that observed in RS CVn systems may be merely a matter of scale.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal; vol. 236
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  • 2
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    The chromospheric rotation activity relation in late type close binary systems (1982)
    In:  Other Sources
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    Publikationsdatum: 2011-08-18
    Beschreibung: The IUE data on 36 late type close binary stars are presented. It is shown that the chromospheric and TR line fluxes increase with decreasing stellar rotation period, though not as rapidly as does the X-ray flux. There is an increasing dependence upon rotation with increasing line temperature. The data are consistent with the hypothesis that there exists a critical rotation rate, which depends on temperature, below which the emission flux is independent of rotation and above which it increases linearly with increasing angular velocity omega.
    Schlagwort(e): ASTRONOMY
    Materialart: NASA. Goddard Space Flight Center Advan. in Ultraviolet Astron.; p 566-569
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  • 3
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    Observations of suspected low-mass post-T Tauri stars and their evolutionary status (1983)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: The results of a study of five X-ray discovered weak emission pre-main-sequence stars in the Taurus-Auriga star formation complex are presented. All are of spectral type K7-M0, and about 1-2 mag above the main sequence. One is a double-lined spectroscopic binary, the first spectroscopic binary PMS star to be confirmed. The ages, masses, and radii of these stars as determined by photometry and spectroscopy are discussed. The difference in emission strength between these and the T Tauri stars is investigated, and it is concluded that these 'post-T Tauri' stars do indeed appear more evolved than the T Tauri stars, although there is no evidence of any significant difference in ages.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 269; June 1
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  • 4
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    First observations of stellar coronal structure - The coronae of AR Lacertae (1983)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: In connection with the present investigation, X-ray, radio, and ultraviolet observations of AR Lacertae over one orbital cycle have been obtained. The X-ray light curve shows a deep primary minimum and a broad, shallow secondary minimum. The quiescent corona of the G2 IV component is small and asymmetric. This corona extends some 0.02 stellar radii above the photosphere; the leading, spotted hemisphere is brighter in X-rays than the trailing hemisphere. The K0 IV component has two coronal components. The extended, presumably hotter component extends to one stellar radius above the photosphere and exhibits a brighter leading hemisphere. There is an inner coronal component which is small relative to the stellar radius, but the lack of coverage past mid-eclipse severely limits the possibility to deduce the nature of this component. The UV observations also imply a nonuniform surface distribution of plage regions.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 267; April 15
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  • 5
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    The X-ray superbubble in Cygnus (1980)
    In:  Other Sources
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    Publikationsdatum: 2019-06-27
    Beschreibung: It is shown that Cyg X-6 and Cyg X-7 are part of a large X-ray ring, 13 degrees in diameter. At a distance of 2 kpc, this X-ray source lies behind the Great Rift of Cygnus, and as such has radiation from its central region absorbed. The diameter of the emitting region is 450 pc. The X-ray source is associated with a ring of elongated H-alpha filaments which enclose the Cygnus X radio source. At the center of this region lies the highly luminous Cygnus OB2 association.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal; vol. 238
    Format: text
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  • 6
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    The smothered coronae of T Tauri stars (1981)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: Einstein IPC observations of 10 fields containing pre-main-sequence stars are presented. The detection of significant soft X-ray flux from eight cataloged T Tauri stars plus the optical identification of two new T Tauri-like stars associated with X-ray sources are reported. It is demonstrated than an inverse correlation between the H-alpha equivalent width and the detection of X-ray flux exists. This is interpreted in terms of a model wherein the X-ray emission arises in a corona similar to that of active late-type stars, which is small compared to the extent of the circumstellar envelope. It is postulated that the lack of correlation of X-ray flux (or H-alpha) with A(v) is due to a lack of grains in the envelopes of the more active T Tauri stars.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal; vol. 250
    Format: text
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  • 7
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    HEAO A-2 soft X-ray source catalog (1983)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: The HEAO A-2 low energy X-ray detectors have surveyed over 95% of the sky in the spectral bands 0.18-0.44 and 0.44-2.8 keV, down to a typical limiting sensitivity of 1 x 10 to the -11th and 3 x 10 to the -11th ergs per sq cm per sec in each band, respectively. Using a significance criterion of 6 sigma for existence, 114 sources are cataloged, of which 54 were previously undiscovered and 32 remain unidentified. The catalog contains a listing of all counterpart identifications and a cross-reference to all HEAO 1 A-2 low energy detector team publications on the catalog sources complete through the end of 1981. Supplementary material allows the estimation of spectral parameters for the spectral models from the observed intensities. Simple graphs allow the extraction of energy fluxes at the earth, both with and without the effects of interstellar absorption. The angular distribution of the cataloged sources in the 0.44-2.8 keV band is concentrated toward the galactic plane and resembles that seen at higher energy. The distribution in the 0.18-0.44 keV band is more uniform and contains many more newly discovered and unidentified sources.
    Schlagwort(e): ASTRONOMY
    Materialart: Astrophysical Journal Supplement Series; 51; Jan. 198
    Format: text
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  • 8
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    X-ray photometry and spectroscopy of T Tauri stars (1984)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: Follow-up Einstein X-ray observations of four fields containing pre-main-sequence stars are presented. A search has been made for evidence of variability on time scales of hours and of 1 year. A large, slow flare in AS 205, and the X-ray disappearance of AA Tau are reported. Otherwise, T Tauri stars appear no more variable in X-rays than do active solar-like stars. An attempt has been made to determine spectral parameters for a number of the brighter sources. X-ray absorption column densities are consistent with those inferred from A(v), and T(x) of about 10-million K, in all cases. The importance of these observations for models of T Tauri stars and their coronae is considered.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 284; 194-201
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  • 9
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    HEAO 1 observations of active coronae in main-sequence and subgiant stars (1980)
    In:  Other Sources
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    Publikationsdatum: 2019-06-27
    Beschreibung: The HEAO 1 data has been searched for evidence of X-ray emission from 105 bright late-type stars of luminosity classes IV and V, selected on the basis of indirect optical evidence of the presence of a hot corona. Six of the target stars were detected at the 3-standard deviation level and 15 were coincident with 2-standard deviation X-ray sources. On a statistical basis no more than 5 of these 21 sources are spurious, and the probability that the identification with the class of active chromosphere stars is spurious is less than 0.00001. The sources lie near a line of X-ray/bolometric luminosity ratio = 0.0001, similar to a solar plage, and it is concluded that the most active coronae of late-type stars which are not members of close binary systems are being observed. The RS CVn systems discovered to date seem to form a distinct class of coronal X-ray sources, but the lowest X-ray luminosity members of the group, of which Capella may be the prototype, appear to overlap the domain of these single stars with active coronae. The data do not fit the coronal model of Gorenstein and Tucker (1976), but they are consistent with the coronal loop model of Rosner et al. (1978) as extended by Walter et al. (1980).
    Schlagwort(e): ASTRONOMY
    Materialart: PB80-227671 , Astrophysical Journal; vol. 236
    Format: text
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  • 10
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    On the coronae of rapidly rotating stars. II - A period-activity relation in G stars (1981)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: Observational evidence supporting the contention that coronal activity in G stars obeys the same relation with angular velocity as that observed in RS CVn systems is presented. It is found, however, that the level of the quiescent coronal activity, as measured by the ratio of the X-ray luminosity to the bolometric luminosity, at a given angular velocity is approximately one order of magnitude lower in G stars than in the RS CVn systems. Data on additional K stars indicate that these stars are indistinguishable from the K components of RS CVn systems. No evidence is found that the surface gravity has any appreciable effect on the level of coronal activity. The observation that the ratio of the X-ray luminosity to the bolometric luminosity is directly proportional to the product of the angular velocity and some function of the depth of the convective zone is seen as supporting a dynamo mechanism for the generation of the stellar magnetic flux.
    Schlagwort(e): ASTRONOMY
    Materialart: Astrophysical Journal; vol. 245
    Format: text
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