Scaling of oscillation frequencies in rotating stars

Abstract

Properties of stars undergoing pulsation such as the well-known root-mean-density scaling relation can be useful when trying to match the observed properties of a particular star. It is often assumed that this relation is valid for p-mode frequencies in rotating stars. To examine the change in frequency with rotation and mass, we have studied oscillation frequencies of two-dimensional uniformly rotating zero-age main-sequence stellar models in the δ Scuti mass range. We identified axisymmetric p and g modes for non-rotating models and then traced them as the rotational velocity was increased. We considered a rotation sequence of ten models for four different masses, with the largest rotation rate being about 200 km s-1. The models were required to have the same surface shape between all masses for a given rotation rate. We find that scaling relationships exist among the oscillation frequencies of the same mode for different masses when the models have the same shape. For p modes, this scaling closely follows the period-root-mean-density relation found in spherical stars. The g modes also scale between models of the same shape, with the scaling reflecting the change in properties outside the convective core as the stellar mass increases. These scaling relationships can be particularly useful in finding specific stellar models to match the oscillation frequencies of individual stars. We also find that the large separation scales approximately with the root mean density as the rotation rate increases, although the individual mode frequencies do not.