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Paper: Glimpses into the Secret Lives of Hot Subdwarf Stars
Volume: 481, 6th Meeting on Hot Subdwarf Stars and Related Objects
Page: 161
Authors: Green, E.; Johnson, C.; Wallace, S.; O'Malley, C.; Amaya, H.; Biddle, L.; Fontaine, G.
Abstract: Compared to subdwarf B (sdB) stars, the evolutionary channels that produce subdwarf O (sdO) stars are not at all well understood. Various theories have been advanced to explain the existence of sdO stars and account for their observed properties. Masses of sdO stars are mostly unknown, because there seem to be very few sdO counterparts to the short-period binaries that are common among sdB stars and because none of the handful of known sdO pulsators are useful candidates for asteroseismic investigations. During a recent (negative) search for bright field counterparts of the rapid sdO pulsators in Omega Cen, we noticed that our precision light curves for many sdO stars sometimes exhibited very small, irregular variations on time scales of several minutes to several tens of minutes. Although the significance of the weakest variations is uncertain, repeated observations for two of these stars, one helium-rich sdO, PG 1427+196, and another helium-poor sdO, PG 1610+519, caught them unexpectedly dropping in luminosity by up to 0.05–0.10 mag for at least a couple of hours, during which their light flickered irregularly in a manner reminiscent of the light from accretion disks in cataclysmic variables. A similar luminosity decrease and flickering was observed in a helium-rich sdB binary, PG 1544+488. The light curves for these three stars suggest the presence of accretion disks analogous to those in in VY Scl stars. The small luminosity variations we observed in a number of other sdO stars indicate that they too could occasionally be capable of similar larger luminosity variations, and thus accretion disks might be associated with a significant number of sdO stars. However, the structure of such sdO systems is not at all clear.
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