Other
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.4404s&link_type=abstract
American Astronomical Society, 193rd AAS Meeting, #44.04; Bulletin of the American Astronomical Society, Vol. 30, p.1315
Other
Scientific paper
We explore non-dimensional relationships between the magnetic dynamo cycle period P_cyc, the rotational period P_rot, the activity level (as observed in Ca ii HK), and other stellar properties using a large stellar sample including Ca 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_cyc, with P_rot/P_cyc ~ 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_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_rot/P_cyc branch is primarily composed of inactive stars. The expanded dataset suggests that for t ga 1 Gyr, stars can have P_cyc on one or both branches (double P_cyc stars), though among older stars, those with higher mass (F) tend to have their primary P_cyc on the low P_rot/P_cyc branch, while lower mass (K) stars occupy the high P_rot/P_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_cyc ~ 50-100 yr) cycles not yet resolved by the data. Most extremely active stars (P_rot < 3 days) appear to occupy a third branch with P_rot/P_cyc ~ Ro(0.4) , 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_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.
Brandenburg Axel
Saar Steven H.
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