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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
    Details
    Publication Date: 2011-08-17
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 236
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  • 2
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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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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 269; June 1
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  • 3
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    First observations of stellar coronal structure - The coronae of AR Lacertae (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 267; April 15
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  • 4
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    The X-ray superbubble in Cygnus (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 238
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  • 5
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    The smothered coronae of T Tauri stars (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 250
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  • 6
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    X-ray photometry and spectroscopy of T Tauri stars (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 284; 194-201
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  • 7
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    Discovery of a 50 minute binary period and a likely 22 magnitude optical counterpart for the X-ray burster 4U 1915-05 (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Absorption dips which recur with a period of 2985 s have been observed from the X-ray burst source 4U 1915-05. It is suggested that the dips are caused by obscuration of the X-ray source by material at the point where the gas stream from the companion meets the accretion disk, and that the 2985 s periodicity is the orbital period of the binary system. These observations represent the first direct evidence of the binary nature of X-ray burst sources. In addition, a new optical identification of 4U 1915-05 is suggested: a 22 magnitude candidate observed in a CCD image of the optical field at the arcsec-accurate HRI X-ray position.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 253
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  • 8
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    On the coronae of rapidly rotating stars. IV - Coronal activity in F dwarfs and implications for the onset of the dynamo (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: X-ray observations of 14 early F dwarfs are reported and these stars are used, together with a complete sample from the literature, to examine how the characteristics of coronal X-ray emission vary from dwarfs of spectral type A through G. Evidence for a rotation-activity relation in stars redder than B - V = 0.45 is found. Stellar duplicity and age, except insofar as they influence the rotation rate, do not appear to be important in determining the coronal X-ray flux level in F dwarfs. It is suggested that the appearance of a relation between rotation and activity at B - V = 0.45 indicates the turn-on of a solar-like dynamo. The high X-ray surface fluxes and small variance thereof for dwarfs with B - V = 0.3-0.45 are also discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 274; 794-800
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  • 9
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    Period-activity relations in close binary systems (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The question as to whether the amount of activity observed in stars with the same fundamental stellar parameters depends on rotational velocity is considered in view of close binary systems, making use of their orbital rather than their rotational period. It is noted that the main sequence G star binaries lie systematically lower than the K subgiants for the same rotational periods, so that relations including a variety of spectral classes cannot be used to directly deduce correlations between magnetic fields and activity without suitable correction. The behavior of the present sample suggests that activity increases with later spectral type and with evolution off the main sequence. The chromosphere shows a minor reaction to rotational velocity increase.
    Keywords: ASTROPHYSICS
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  • 10
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    Stellar chromospheres and coronae in the Ursa Major Cluster stars (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: IUE spectra of 18 proposed members of the Ursa Major Cluster and Einstein X-ray images of 11 of these stars are discussed. Thirteen stars, six in the nucleus and seven in the extended comoving stream, are probably true members of the UMa Cluster in that their bright ultraviolet and X-ray surface fluxes indicate youth. Four stars, one in the nucleus and three in the stream, exhibit weak ultraviolet and/or X-ray emission suggesting that they are old field stars that have the same space motion as the UMa Cluster. The X-ray surface fluxes of the UMa Cluster stars appear to be brighter than those of the Hyades Cluster stars, consistent with their relative ages. It is argued that chromospheres and transition regions could be present in dwarf stars hotter than B-V of about 0.30, but are unobservable in IUE spectra due to the rapid increase in photospheric flux in hotter stars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 281; 815-825
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