Dynamo Model for Magnetic Activity Saturation in Rapidly Rotating Solar-Type Stars Stellar magnetic activity: dependence on rotation rate (observations) Activity saturation for rapid rotation A possible reason for the saturation L. L. Kitchatinov & S. V. Olemskoy Stellar dynamo model showing the saturation Special dynamo regime in rapid rotators Seoul – 2015 Institute for Solar-Terrestrial Physics, Irkutsk;

Magnetic Activity - Rotation Coronal activity (Wright et al. 2011) Chromospheric activity (Noyes et al. 1984) Unsigned magnetic flux (Vidotto et al. 2014) Ro = P rot /  is the Rossby number  is the convective turnover time P rot is the rotation period

Dynamo = differential rotation + alpha-effect Two versions of the alpha-effect: 1) Canonical alpha-effect of turbulent convection by Parker (1955). 2) The Babcock-Leighton mechanism (Babcock 1961; Leighton 1969) Latitude Joy’s law (Hale et al. 1919) Saturation of the Babcock-Leighton mechanism for rapid rotation: Rotation Alpha-effect ~ sin (tilt angle)

Dynamo model Equations: Design: Differential rotation according to the helioseismology Single-cell meridional flow Non-local alpha-effect

Dependence of the alpha-effect on (co)latitude and rotation rate The dependence of the mean tilt-angle on latitude was inferred from the extensive literature on the Joy’s law (Hale et al. 1919; Dasi-Espuig et al. 2010; Ivanov 2011; Stenflo & Kosovichev 2012; Wang et al. 2015; …): The alpha-effect dependence on latitude and rotation rate was prescribed as follows:

Vidotto et al. (2014) Toroidal magnetic flux as the function of rotation period:

P rot = 1day Magnetic field oscillations about a (latitude-dependent) nonzero mean imply a double-periodic oscillations in magnetic energy. Brandenburg et al. (1998)

Conclusions Saturation of magnetic activity in rapidly rotating solar-type stars can be attributed to the saturation of the BL-type dynamos for the tilt angles approaching 90  with increasing rotation rate. The dynamo model for the saturation of magnetic activity predicts a special dynamo regime with double-periodic oscillations in magnetic energy for rapid rotators.