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Theory of Stellar Coronae: An Interpretation of X-Ray Emission From Non-Degenerate Stellar Sources

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  • Jeffrey L. Linsky
  • Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, USA

I present arguments to demonstrate that the acoustic wave heating theory of stellar coronae is inadequate to explain Einstein observations of stellar coronae as well as previous ultraviolet and X-ray observations of the Sun and other stars. I outline the various lines of evidence that imply that magnetic fields, stellar rotation rates, and to some extent convection zone parameters are the important quantities in determining coronal heating and thus X-ray emission. These general results and the recent Einstein Observatory stellar observations suggest a speculative scenario of stellar coronae consisting of the following elements: Coronae in O-type stars are heated by the in-teraction of turbulent stellar winds with slowly decaying primordial magnetic fields or by radiative instabilities in the flow. The apparent absence of coronae in Ap stars, which have large but stable magnetic fields, is due to the absence of temporal changes in very stable atmospheres for which even weak convection is suppressed by the strong field. Weak X-ray emission in some normal A-type stars may be due to the beginnings of dynamo action, which is likely the dominant field regeneration process in cooler stars. In F- and later-type dwarfs, dynamo processes, which increase with stellar rotation velocity and convection zone depth, continually replenish surface magnetic fields. These in turn heat the corona through several possible mechanisms. The distinction between dMe and dM stars is probably a result of larger rotational velocities in the former, since these stars are very likely fully convective. For BY Draconis and RS CVn-type systems, which typically are close binaries, binarity is probably only important to the extent that tidal synchronism forces rapid rotation. K- and M-type giants and supergiants are very likely deficient in coronal material because in such stars strong chromospheric winds provide a cooling channel alternative to the conventional hot corona. This notion is reinforced by the “hybrid” character of early G supergiants, which provide an intermediate case. These stars show variable emission lines from hot lasma as well as strong chromosphere winds. The coexisting phenomena suggest the presence of two very distinct outer atmospheric structures, perhaps hot plasma entrained in closed magnetic loops and cool outflowing material in open field regions analogous to solar coronal holes.

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X-Ray Astronomy with the Einstein Satellite

Springer (e-books)
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  • BY Draconis stars
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