A Driving Mechanism for the Newly Discovered Long‐Period Pulsating Subdwarf B Stars

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Abstract
We present the results of a stability survey carried out for a sequence of representative models of subdwarf B stars spanning the range of effective temperature 22,000 K ≤ Teff ≤ 38,000 K. We show that long-period, high-order g-modes are excited in the cooler models through the same κ-mechanism that successfully explains the presence of short-period, low-order p-modes in the hotter EC 14026 pulsators. This is analogous to the case of the β Cep/slowly pulsating B stars on the main sequence. We stress that radiative levitation is needed to boost the iron abundance in the driving region for both types of pulsating subdwarf B stars. And indeed, we find that pulsation modes cannot be excited in B subdwarf models if the metallicity is assumed to be uniform and solar. On the basis of our current models, we propose that the pulsation modes detected in long-period pulsating subdwarf B stars have values of the degree index l = 3 and/or 4, not the canonical values l = 1, 2, a suggestion that is, in principle, testable through multicolor photometry or time-resolved spectroscopy. In this way, we are able to explain quite well, at least at the qualitative level, the main observed characteristics of these pulsators. On the first account, the excited high-order g-modes with l = 3 and 4 in our models have periods that overlap with the range of quasi-periods observed in these stars. On the second account, if the observable modes in these pulsators have indeed such "high" values of l as we suggest, we find a natural explanation for the fact that their amplitudes are distinctly and systematically smaller than the amplitudes observed in EC 14026 stars. Finally, our results are also consistent with the observed fact that the long-period pulsators appear systematically cooler than the short-period EC 14026 stars. We point out, however, that our analysis suggests effective temperatures for the long-period B subdwarf pulsators that are somewhat lower than current spectroscopic estimates. The solution to this problem may come from future improvements in the models, the establishment of an effective temperature scale for subdwarf B stars that is free of systematic effects, or both.