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Session 81 - Variable Cool and Late Type Stars.
Display session, Wednesday, January 15
We have analyzed HST and IUE ultraviolet spectra of five dwarfs which have extremely low, non-variable levels of Ca II HK emission. These stars appear to be in the stellar analog of the solar ``Maunder minimum'' - a period when the normal cyclic magnetic dynamo went into temporary quiescence. The stars all have very low levels of chromospheric and transition region (TR) emission. The ratio of TR (Si IV and C IV) to chromospheric emission (C II) is smaller than expected from published estimates of the ``basal'' emission, and increases with decreasing T_eff. This is in contrast to the lack of such a trend in dwarfs with variable Ca II HK (``normal'' operating magnetic dynamos) and the reverse of the trend expected if the emission was acoustic in origin. The existence of significant TR emission in ``Maunder minimum'' stars suggests that they are still generating magnetic flux, but it is apparently in some form which enhances chromospheric emission relative to the TR. A possible explanation for the observations is that a residual ``Maunder minimum'' magnetic flux is a generated by a non-cyclic distributed-type dynamo (a less efficient mechanism which operates throughout the convective zone, and thus grows with increased convective zone depth). If this scenario is correct, our data are the first observational evidence of such a dynamo operating in a star which is not fully convective.
Program listing for Wednesday