2002MNRAS.330..855T

The photometric characteristics of slowly pulsating B stars are investigated using a numerical approach. Stability calculations are performed for a set of stellar models representative of the mid-B type, using a non-radial non-adiabatic pulsation code. The results from these calculations are used to synthesize photometry, in several common systems, for unstable modes of harmonic degrees ℓ=1…4. Focusing on the Geneva system for illustrative purposes, a variety of techniques are employed to analyse and visualize the synthetic data, including the use of multicolour-amplitudes and amplitude-phase diagnostic diagrams. One outstanding aspect of the analysis is the discovery, for the ℓ=2…4 modes, of `inter-term cancellation' (ITC) - the process of destructive interference between the flux variations originating from surface temperature perturbations and those arising from radius perturbations.

The ITC can be severe enough that a mode may be excited to a significant amplitude, and yet exhibit levels of photometric variability that fall below typical observational detection thresholds. Furthermore, it can affect not only the light variations in a given photometric passband, but also the variations of the bolometric flux. However, the cancellation is dependent on wavelength, and will not occur to the same degree in more than one passband. Therefore, simultaneous observation in a multitude of passbands represents the best approach to ensuring that no modes are overlooked during searches for variability in B-type stars.

A consequence of ITC is that ratios between the variability amplitude, in differing passbands, become very sensitive towards mode-to-mode changes in the pulsation. This increased sensitivity will tend to complicate any attempts at identifying the harmonic degrees of the modes responsible for observed variability. However, the cancellation also introduces significant phase differences between the light variations in each passband, especially for the ℓ=3 and ℓ=4 modes. On the grounds that correspondingly large phase differences are not seen in observational data, it is argued that the variability seen in slowly pulsating B stars can tentatively be attributed to ℓ=1 and ℓ=2 modes.