Asteroseismological studies of long-period variable subdwarf B stars. II. Two-color photometry of PG 1338+481

We present the results of an observational campaign for the long-period variable subdwarf B star PG 1338+481. Seven continuous weeks of observing time at the Steward Observatory 1.55 m Kuiper telescope on Mount Bigelow, Arizona, and the 1.3 m MDM telescope at Kitt Peak rendered ∼250 hr of simultaneous U/R time series photometry, as well as an extra ∼70 hr of R-band-only data. The analysis of the combined light curves resulted in the extraction of 13 convincing periodicities in the 2100-7200 s range, with amplitudes up to ∼0.3% and ∼0.2% in the U and R, respectively. Comparing the ratios of amplitudes in the two wave bands to those predicted from theory suggests the presence of dipole modes, a notion that is further supported by the period spacing between the highest amplitude peaks. If confirmed, this poses a challenge to current nonadiabatic theory. At the quantitative level, we find that the distribution of the observed period spectrum is highly nonuniform and much sparser than that predicted from a representative model. We provide a possible interpretation in the text. The asteroseismological analysis attempted for PG 1338+481 on the basis of six observed periodicities believed to constitute consecutive dipole modes renders encouraging results. Fixing the effective temperature and surface gravity to the spectroscopic estimates, we successfully isolate just one family of optimal models that can reproduce the measured periods to better than 1 %. While the stellar parameters thus inferred must be regarded as preliminary, the achieved fit bodes well for future asteroseismic analyses of long-period variable subdwarf B stars.