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Session 43 - Variable Stars.
Display session, Thursday, January 08
Chromospheric activity in lower main sequence stars has long been tied to the stellar rotation rate (Kraft 1967, Skumanich 1972, Noyes et al 1984). More recently, activity levels have been observed to saturate at a maximum value for rapidly rotating stars (see, e.g., Vilhu 1984, Saar 1991, Mathioudakis et al. 1995), and this maximum value is achieved for a rotation rate on the order of 10Ømega _\sun. At the present time, researchers disagree as to whether the observed saturation effect is caused by an actual limit on magnetic field production by the stellar dynamo or is instead due to a simple filling factor effect.
Stellar magnetic activity is generally measured using proxy indicators such as Ca II H+K line emission and soft X-ray flux, and such proxies arise from magnetic flux tubes at the stellar surface. Recently, Buzasi (1997) has demonstrated that, in rapidly rotating lower main sequence stars, flux tubes rising from the convective overshoot region are forced towards the polar regions. This effect, due to the Coriolis force, increases towards later spectral types, due to the increasing depth of the convection zone. The result is that rapidly-rotating low-mass stars, such as M dwarfs, have smaller fractions of the stellar surface available to display activity than do rapidly-rotating G dwarfs. The ramifications of this effect for explanations of saturation which are based on filling factors are discussed, along with their observational consequences at visible, ultraviolet, and X-ray wavelengths.
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Program listing for Thursday