Pulsational instabilities in the hot sdO star SDSS J1600+0748: The key role of radiative levitation

SDSS J1600+0748 is the only hot sdO star for which unambiguous multiperiodic luminosity variations have been reported so far. These rapid variations, with periods in the range from 60 s to 120 s, are best qualitatively explained in terms of pulsational instabilities, but the exact nature of the driving mechanism has remained a puzzle. Models of sdO stars with uniform metallicity, even those with artificially-boosted values of the metallicity Z, are unable to excite pulsation modes in the range of interest as demonstrated most eloquently by Cristina Rodríguez-López in her Ph.D. thesis at the Universidad de Vigo in 2007. We confirm her results here, but also show that the inclusion of radiative levitation in the equilibrium models changes the picture dramatically. We find indeed that p-mode pulsations with periods overlapping with the observed ones in SDSS J1600+0748 can be excited in models in which radiative levitation of iron is taken into account. This process provides the needed boost to the opacity driving mechanism. We conclude that radiative levitation is a likely essential ingredient in the excitation physics at work in this unique star.