AAS Meeting #193 - Austin, Texas, January 1999
Session 44. Modeling Stellar Characteristics
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibits Hall 1

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[44.04] Time Evolution of the Magnetic Activity Cycle Period: Results for an Expanded Stellar Sample

S.H. Saar (SAO), A. Brandenburg (U.~of Newcastle upon Tyne)

We explore non-dimensional relationships between the magnetic dynamo cycle period P\rm cyc, the rotational period P\rm rot, the activity level (as observed in Ca {\sc ii} HK), and other stellar properties using a large stellar sample including Ca {\sc ii}, photometric and other cycles in dwarfs, evolved stars, active binaries, and the secondaries of CV systems. We confirm that most stars with age t \ga 0.3 Gyr occupy two roughly parallel branches, separated by a factor of ~6 in P\rm cyc, with P\rm rot/P\rm cyc \propto Ro-0.5, where Ro is the Rossby number. Using the simple model of Brandenburg, Saar, & Turpin (1998 ApJ 498, L51), this result implies that the \alpha effect increases with mean magnetic field B, and that \alpha and \omega\rm cyc decrease with age along these branches. Stars are not strictly segregated onto one or the other branch by activity level, though the high P\rm rot/P\rm cyc branch is primarily composed of inactive stars. The expanded dataset suggests that for t \ga 1 Gyr, stars can have P\rm cyc on one or both branches (double P\rm cyc stars), though among older stars, those with higher mass (F) tend to have their primary P\rm cyc on the low P\rm rot/P\rm cyc branch, while lower mass (K) stars occupy the high P\rm rot/P\rm cyc branch. The sun's ~80 year Gleissberg cycle agrees with this scenario, suggesting that long-term ``trends" in many stars may be segments of long (P\rm cyc ~ 50-100 yr) cycles not yet resolved by the data. Most extremely active stars (P\rm rot < 3 days) appear to occupy a third branch with P\rm rot/P\rm cyc \propto Ro0.4, where some kind of \alpha-quenching may be operating. Many RS CVn variables lie in a transition region between the two most active branches. We discuss implications for dynamo theory and evolution, and compare our results with various other models. The results are also used to predict P\rm cyc for stars with long-term HK trends, a sample of young open cluster members, and a group which may be in the stellar analog of the Maunder minimum.

The author(s) of this abstract have provided an email address for comments about the abstract: saar@cfa.harvard.edu

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