Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...524..295s&link_type=abstract
The Astrophysical Journal, Volume 524, Issue 1, pp. 295-310.
Astronomy and Astrophysics
Astronomy
107
Stars: Novae, Cataclysmic Variables, Stars: Activity, Stars: Late-Type, Stars: Magnetic Fields, Stars: Rotation
Scientific paper
We further explore nondimensional 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 by expanding the stellar sample studied in the first paper in this series. We do this by adding photometric and other cycles seen in active stars and the secondaries of CV systems and by selectively adding less certain cycles from the Mount Wilson HK survey; evolved stars, long-term HK trends and secondary P_cyc are also considered. We confirm that most stars with age t>~0.1 Gyr occupy two roughly parallel branches, separated by a factor of ~6 in P_cyc, with the ratio of cycle and rotational frequencies ω_cyc/Ω~Ro^-0.5, where Ro is the Rossby number. Using the model of the first paper in this series, this result implies that the α effect increases with mean magnetic field (contrary to the traditional α-quenching concept) and that α and ω_cyc decrease with t. Stars are not strictly segregated onto one or the other branch by activity level, though the high-ω_cyc/Ω branch is primarily composed of inactive stars. The expanded data set suggests that for t>~1 Gyr, stars can have cycles on one or both branches, though among older stars, those with higher (lower) mass tend to have their primary P_cyc on the lower (upper) ω_cyc/Ω branch. The Sun's ~80 yr Gleissberg cycle agrees with this scenario, suggesting that long-term activity ``trends'' in many stars may be segments of long (P_cyc~50-100 yr) cycles not yet resolved by the data. Most very active stars (P_rot<3 days) appear to occupy a new, third branch with ω_cyc/Ω~Ro^0.4. Many RS CVn variables lie in a transition region between the two most active branches. We compare our results with various models, discuss their implications for dynamo theory and evolution, and use them to predict P_cyc for three groups: stars with long-term HK trends, stars in young open clusters, and stars that may be in Maunder-like magnetic minima.
Brandenburg Axel
Saar Steven H.
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