Other
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...414l..33s&link_type=abstract
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 414, no. 1, p. L33-L36.
Other
27
Chromosphere, Periodic Variations, Stellar Activity, Stellar Evolution, Dynamo Theory, Stellar Magnetic Fields, Stellar Rotation
Scientific paper
We propose (P(cyc)/P(rot))-squared, the square of the ratio of the characteristic oscillatory timescales of stellar chromospheric activity to the rotation period, as a useful parameterization of the stellar activity cycle and as the observational equivalent of the theoretical dynamo number, N(D). (P(cyc)/P(rot))-squared can be obtained observationally from 25-yr activity records of stellar Ca II H and K chromospheric emission fluxes of the Mount Wilson Observatory HK Project. Using that parameterization, we study the relationships between the period of the activity cycle and mass or age (estimated from the average level of chromospheric emission and its calibration with age). The quantity (P(cyc)/P(rot))-squared increases as B-V decreases, in qualitative agreement with the expectation that as the fractional depth of convective zone decreases (i.e., toward higher mass stars), N(D) increases (i.e., the variability of stellar activity tends to be more irregular). (P(cyc)/P(rot))-squared seems independent on age for young stars but has a well-defined dependence on age for the older stars. The difference in the behavior of (P(cyc)/P(rot))-squared with age is another aspect of chromospheric activity that changes as a star ages; the time of the transition depends on mass but occurs roughly near stellar age of about 1-3 billion yr.
Baliunas Sallie L.
Soon Willie W.-H.
Zhang Qian
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