Variations in the Brightness Variability Amplitude of A–M Stars

Аннотация

We study relations of the parameter R_var (amplitude of the brightness variability, stellar activity indicator) with respect to the effective temperature and the period of rotation of stars belonging to the A–M spectral types. A common dataset gives an idea of variations in this parameter for stars with convection. It was confirmed that the limit of applicability for the gyrochronological relation PtM does not correspond to the limiting value (B - V )₀ 0.47, and the relationship between a rotation period and color is likewise visible up to groups of stars with (B - V )₀ = 0.2. It is shown that the value of the parameter R_var for stars with effective temperatures of 6500–7500 K are about an order of magnitude lower than that for stars, which indicates a nonmonotonic change of the parameter and, therefore, does not allow us to conclude that there is a single dependence of this parameter in the entire temperature range. In the diagram R_var–P (the rotation period), A-type stars fall in the sequence of fast-rotating active objects, whereas stars with similar temperatures and belonging to the F spectral type (including objects from the cluster NGC 6866) lie in a different sequence corresponding to fast-rotating objects with a small amplitude in the brightness variability. The existence of a unified relationship between the properties of rotational activity in stars of spectral types A, F, and later, as well as possible physical processes leading to the implementation of a standard gyrochronological ratio and activity in A stars are discussed.