Radial and Nonradial Beating Modes for RR Lyrae Variable Star Blazhko Effect

At the eleventh conference in the Los Alamos pulsation series in 1992, this author suggested that the observed fundamental mode period of the variable star RR Lyrae can beat with the nonradial $l = 1,$ g4 mode to produce the observed Blazhko effect seen in that star. Two earlier papers were by Borkowski (1980) and Moskalik (1986), both using only radial modes. Consideration of nonradial modes for other RR Lyrae variables allows many modes, with some that are as close as one or two percent in period to the observed radial fundamental mode. My 1992 publication stated that the nonradial mode was weakly pulsationally unstable, but now with my improved codes, the indication is that the close nonradial modes are always just slightly stable. For this case then, the nonradial mode needs to be excited by nonlinear coupling to the radial mode which is often possible. See Diembowski and Mizerski (2004); Van Hoolst, Dziembowski, and Kawaler (1998); and Nowakowski and Dziembowski (2001) for these RR Lyrae type variables. Many other stars have these solar‐like, non self‐excited pulsations. For the observed double‐mode RR Lyrae variables, beating is possible with both the observed radial modes, and complicated nonlinear pulsations can be predicted. It is found that the cooler RR Lyrae variables have large nonradial mode damping, making coupling less likely, and so the Blazhko effect ones must be hotter, near the ab type fundamental mode blue edges, as observed.