A New Explanation of the Orbital Period Gap of the Cataclysmic Variables

S.-B. QIAN; L. Y. ZHU; Z. DAI; E. FERNÁNDEZ-LAJÚS

Río de Janeiro, Brasil

Simposio; IAU Symposium 262: "Stellar Populations, Planning for the Next Decade”; 2009

IAU XXVII General Assembly

Orbital period variations of two types of cataclysmic variable stars (CVs), i.e., SW Sextantis-type CVs and dwarf novae, were investigated by including some new times of light minimum obtained by several telescopes in the world. A relation between the orbital period and its rate of change (dP/dt) is discovered for SW Sextantis-type CVs, which can be explained by enhanced mass transfer from the secondary components to the white dwarfs. This caused them to have the highest luminosity among CVs and was the reason that results in the observed SW Sextantis phenomena. We also found that all of the SW Sextantis-type CVs, with orbital periods between 3 and 4 h (just at the upper limit of the 2-3 h period gap), show their orbital period increasing. Since SW Sextantis-type stars are the dominant population of CVs with orbital periods between 3 and 4 h, this result suggests that the cluster of CVs between 3 and 4 h in the orbital period distribution can be explained by enhanced mass transfer. Meanwhile, long-term decreases are discovered in a few dwarf novae with period below 2 hours, which can be plausibly interpreted bye a combination of mass transfer and angular momentum loss via gravitational radiation and magnetic braking of the completely convective component stars. We found that the rates of magnetic braking is several times the gravitational radiation rate, which is consistent with the theoretical requirement. These detections suggest that the evolution of CVs below period gap is also driven by magnetic braking and magnetic braking does not stop when the secondary becomes a completely convective star.