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,andthe1.3mMDMtelescopeatKittPeakrendered � 250hrofsimultaneousU/Rtimeseriesphotometry,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 encouragingresults.Fixingtheeffectivetemperatureand surfacegravitytothespectroscopicestimates, wesuccessfully isolate just one family of optimal models that can reproduce the measured periods to better than 1%. While the stellar parametersthusinferredmustberegardedaspreliminary,theachievedfitbodeswellforfutureasteroseismicanalyses of long-period variable subdwarf B stars. Subject headingg stars: individual (PG 1338+481) — stars: interiors — stars: oscillations — subdwarfs