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Session 81 - Variable Cool and Late Type Stars.
Display session, Wednesday, January 15
On the Sun, magnetic activity is essentially restricted to latitudes within 45 degrees of the equator. However, active stars, such as RS CVn systems, and T Tauri stars generally show polar activity. I model the evolution of a toroidal magnetic flux tube as it rises fromthe base of the convection zone in various lower main sequence stars, and show that, on the main sequence, polar activity is more likely for less massive stars. For example, a 1 M_\sun star must rotate with ømega > 5 ømega_\sun to develop significant polar magnetic flux, while a 0.4 M_\sun star will generate predominantly polar flux for ømega \le 0.5 ømega_\sun. This leads to the expectation that essentially all M stars should display polar, rather than equatorial, magnetic activity. A similar effect is expected for RS CVn and T Tauri systems, and for giant stars.
An important consequence of this polar activity is the inhibition of angular momentum loss from these systems. On the lower main sequence, angular momentum is lost via the interaction of the stellar magnetic field with the outflowing stellar wind. However, if regions of strong magnetic field are confined to the poles, stellar spindown rates can be dramatically reduced, by up to a factor of three or more for early M stars. Examination of open clusters should show the presence of rapidly-rotating M stars at ages beyond those previously expected.
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