Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae

We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of superhump period are found to be composed of three distinct stages: early evolutionary stage with a longer superhump period, middle stage with systematically varying periods, final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods less than 0.08 d show positive period derivatives. Contrary to the earlier claim, we found no clear evidence for variation of period derivatives between superoutburst of the same object. We present an interpretation that the lengthening of the superhump period is a result of outward propagation of the eccentricity wave and is limited by the radius near the tidal truncation. We interpret that late stage superhumps are rejuvenized excitation of 3:1 resonance when the superhumps in the outer disk is effectively quenched. Many of WZ Sge-type dwarf novae showed long-enduring superhumps during the post-superoutburst stage having periods longer than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently, mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives and are excellent candidate for the systems around or after the period minimum of evolution of cataclysmic variables (abridged).Comment: 239 pages, 225 figures, PASJ accepte

X-ray response to disk evolution in two γ Cas stars

International audienceAbstract The Hα emission of a set of southern γ Cas stars was monitored since 2019, with the aim of detecting transition events and examining how their peculiar X-ray emission would react in such cases. Two stars, HD 119682 and V767 Cen, were found to display slowly decreasing disk emissions. These decreases were not perfectly monotonic and several temporary and limited re-building events were observed. For HD 119682, the emission component in Hα disappeared in mid-July 2020. In X-rays, the X-ray flux was twice smaller than recorded two decades ago but of a similar level as observed a decade ago. The X-ray flux decreased over the campaign by 30 per cent, but the hardness remained similar in datasets of all epochs. In particular, the γ Cas character remained as clear as before even when there was no trace of disk emission in the Hα line. For V767 Cen, the full disappearance of disk emission in Hα never occurred. We followed closely a disk rebuilding event, but no significant change in flux or hardness was detected. These behaviours are compared to those of other γ Cas stars and their consequences on the X-ray generation are discussed