First Kepler results on compact pulsators - V. Slowly pulsating subdwarf B stars in short-period binaries

Abstract

The survey phase of the Kepler Mission includes a number of hot subdwarf B (sdB) stars to search for non‐radial pulsations. We present our analysis of two sdB stars that are found to be g‐mode pulsators of the V1093 Her class. These two stars also display the distinct irradiation effect typical of sdB stars with a close M‐dwarf companion with orbital periods of less than half a day. Because the orbital period is so short, the stars should be in synchronous rotation, and if so, the rotation period should imprint itself on the multiplet structure of the pulsations. However, we do not find clear evidence for such rotational splitting. Though the stars do show some frequency spacings that are consistent with synchronous rotation, they also display multiplets with splittings that are much smaller. Longer‐duration time series photometry will be needed to determine if those small splittings are in fact rotational splitting, or caused by slow amplitude or phase modulation. Further data should also improve the signal‐to‐noise ratio, perhaps revealing lower‐amplitude periodicities that could confirm the expectation of synchronous rotation. The pulsation periods seen in these stars show period spacings that are suggestive of high‐overtone g‐mode pulsations.

Asteroseismology revealing trapped modes in KIC 10553698A

“…In the first four quarters of the Kepler mission, a survey for pulsating stars was made, and a total of 113 compact-pulsator candidates were checked for variability by Østensen et al (Østensen et al 2010b;2011 = Paper I). This very successful survey revealed one clear V361-Hya pulsator (Kawaler et al 2010b) and one other transient short-period pulsator, and a total of thirteen V1093-Her stars (Reed et al 2010;Kawaler et al 2010a;Baran et al 2011, Paper II), including an sdB+dM eclipsing binary in which the hot primary shows an exceptionally rich pulsation spectrum (Østensen et al 2010a). Another three V1093-Her pulsators have been identified in the open cluster NGC 6791 (Pablo et al 2011;Reed et al 2012a), bringing the total number of sdBV stars in the Kepler field to eighteen.…”

Binaries discovered by the MUCHFUSS project

“…Interestingly, Pablo et al (2011) were able to infer a rotation period of 9.63 days of the primary from the characteristic even splitting of the pulsation periods, thereby demonstrating that the rotation of the primary is not synchronised with the orbital period. More recently Pablo et al (2012) have demonstrated that the same is the case for both of the long-period pulsators of Kawaler et al (2010a). Long-period pulsations have also been reported from ground-based studies of the sdB+dM binary GALEX J0321+4727, which has an orbital period of 0.266 d .…”

“…Recently, a fourth sdB+dM binary with a pulsating primary was revealed in Kepler data by Pablo et al (2011). This system with an orbital period of 0.399 d is very similar to the two systems presented by Kawaler et al (2010a), but is a member of the open cluster NGC 6791. Interestingly, Pablo et al (2011) were able to infer a rotation period of 9.63 days of the primary from the characteristic even splitting of the pulsation periods, thereby demonstrating that the rotation of the primary is not synchronised with the orbital period.…”

“…The most spectacular one is 2M1938+4603 (Østensen et al 2010a), which is an eclipsing binary with a period of 0.126 d, in which the primary pulsates with an unusually high number of low-amplitude modes covering both long and intermediate period ranges. Two non-eclipsing sdB+dM binaries in which the primaries are long-period pulsators of the V1093-Her type were presented by Kawaler et al (2010a), revealing orbital periods of 0.443 and 0.395 days, a range which is difficult to discover with ground based observations. Recently, a fourth sdB+dM binary with a pulsating primary was revealed in Kepler data by Pablo et al (2011).…”

The Impact of CoRoT and Kepler on Eclipsing Binary Science

“…Kawaler et al (2010) found two compact binaries with gravity-mode pulsations superposed to an irradiation effect typical of sdB stars with a close M-dwarf companion. The orbital periods are less than half a day such that seismic sounding should become possible in the future, once the noise level of the data can be brought down to such a level that tidal and/or rotational splitting can be disentangled from period spacings of the modes.…”