2011MNRAS.414.2885R

We investigate the possibility of nearly equally spaced periods in 13 hot subdwarf B (sdB) stars observed with the Kepler spacecraft and one observed with CoRoT. Asymptotic limits for gravity (g-)mode pulsations provide relationships between equal-period spacings of modes with differing degrees l and relationships between periods of the same radial order n but differing degrees l. Period transforms, Kolmogorov–Smirnov tests and linear least-squares fits have been used to detect and determine the significance of equal-period spacings. We have also used Monte Carlo simulations to estimate the likelihood that the detected spacings could be produced randomly.

Period transforms for nine of the Kepler stars indicate l= 1 period spacings, with five also showing peaks for l= 2 modes. 12 stars indicate l= 1 modes using the Kolmogorov–Smirnov test while another shows solely l= 2 modes. Monte Carlo results indicate that equal-period spacings are significant in 10 stars above 99 per cent confidence, and 13 of the 14 are above 94 per cent confidence. For 12 stars, the various methods find consistent period spacings to within the errors, two others show some inconsistencies, likely caused by binarity, and the last has significant detections but the mode assignment disagrees between the methods.

We use asymptotic period spacing relationships to associate observed periods of variability with pulsation modes for l= 1 and 2. From the Kepler first-year survey sample of 13 multiperiodic g-mode pulsators, five stars have several consecutive overtones making period spacings easy to detect, six others have fewer consecutive overtones but period spacings are readily detected, and two stars show marginal indications of equal-period spacings. We also examine a g-mode sdB pulsator observed by CoRoT with a rich pulsation spectrum, and our tests detect regular period spacings.

We use Monte Carlo simulations to estimate the significance of the detections in individual stars. From the simulations, it is determined that regular period spacings in 10 of the 14 stars are very unlikely to be random, another two are moderately unlikely to be random and two are mostly unconstrained.

We find a common l= 1 period spacing spanning a range from 231 to 272 s allowing us to correlate pulsation modes with 222 periodicities and that the l= 2 period spacings are related to the l= 1 spacings by the asymptotic relationship {img}. We briefly discuss the impact of equal-period spacings which indicate low-degree modes with a lack of significant mode trappings.