2018ApJ...867...47W

The slowly pulsating B-type (SPB) stars are the upper main-sequence stars on the HR diagram. Their oscillations are high-order, low-degree g-mode and can be used to probe the structure of the radiative zone near the outer boundary of the convective core and constrain the chemical mixing in stellar interiors. In SPB stars, the period spacing periodically varies with periods. It has been regarded as a signature of the chemical composition gradient beyond the convective core. Based on theoretical calculations, we find that the variation frequency of the period spacings (f_ΔP) is related to the width of the µ-gradient region on the buoyancy radius (Λ_µ) with the relation of f_ΔP ∼ 0.5Λ_µ. This indicates that the variation frequency f_ΔP is sensitive to the central hydrogen mass fraction X_C (i.e., the evolutionary status). Finally, we find that the variation frequency f_ΔP and the means of the period spacings <ΔP>can be used to construct a new C-D-like diagram (f_ΔP versus <ΔP>), which can be used to roughly decide the stellar evolutionary stages and to approximately determine stellar mass for SPB stars.