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Stellar chromospheres and coronae in the Ursa Major Cluster stars
Walter, F. M.; Linsky, J. L.; Simon, T.; Golub, L.; Vaiana, G. S.
AA(Hawaii, University, Honolulu, HI), AB(Harvard-Smithsonian Center for Astrophysics, Cambridge, MA), AC(Harvard-Smithsonian Center for Astrophysics, Cambridge, MA)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 281, June 15, 1984, p. 815-825. Research supported by the Smithsonian Institution. (ApJ Homepage)
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NASA/STI Keywords:
Galactic Clusters, Stellar Atmospheres, Stellar Coronas, Ultraviolet Spectra, X Ray Sources, Chromosphere, Dwarf Stars, Eclipsing Binary Stars, Emission Spectra, Iue, Line Spectra, Radiant Flux Density, Spectral Energy Distribution, Stellar Evolution
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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.

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