OT J075418.7+381225 and OT J230425.8+062546: Promising candidates for the period bouncer

Abstract

We report on photometric observations of two dwarf novae, OT J075418.7+381225 and OT J230425.8+062546, which showed superoutbursts in 2013 (OT J075418) and in 2011 (OT J230425). Their mean periods of the superhump were 0.0722403(26) d (OT J075418) and 0.067317(35) d (OT J230425). These objects showed a very long growth stage of the superhump (stage A) and a large period decrease in the stage A–B transition. The long stage A suggests slow evolution of the superhump due to the very small mass ratio of these objects. The declining rates during the plateau phase in the superoutburst of these objects were lower than those of SU UMa-type dwarf novae (DNe) with a similar superhump period. These properties were similar to those of SSS J122221.7−311523, the most likely candidate for the period bouncer. Therefore, these two DNe are regarded as likely candidates for the period bouncer. We estimated the number density of period bouncers roughly from our observations for the last five years. There is a possibility that these WZ Sge-type DNe with unusual outburst properties might account for the missing population of the period bouncer suggested by the evolutionary scenario.

ASASSN-16dt and ASASSN-16hg: Promising candidate period bouncers

“…The details of the classification with the morphology of plateau stages are described in Kimura et al (2016). In addition, the duration of stage A superhumps is normally long in the candidates (Kato et al 2013b;Nakata et al 2014), and also in ASASSN-16dt, the stage A superhumps continued for a long interval, ∼4 days.…”

“…Variation of superhump amplitudes in the SU UMa-type objects with 0.06 d < P orb ≤ 0.07 d. The diamonds and circles represent the candidates for a period bouncer and ordinary SU UMa-type DNe, respectively. The data of the period-bouncer candidates are derived from Nakata et al (2013); Kato et al (2013b); Nakata et al (2014); ; Kato et al (2016); Kimura et al (2016); Kato et al (2017), and those of ordinary SU UMa-type DNe are derived from Kato et al (2009);Kato et al (2010); Kato et al (2012); Kato et al (2013a); Kato et al (2014b); Kato et al (2014a); ; Kato et al (2016); Kato et al (2017).…”

“…one of these candidates, the evidence suggesting a browndwarf companion has also been found (Neustroev et al 2017). Nakata et al (2014) also discussed that the detected fraction of these candidates can account for the theoretically expected population of period bouncers. In addition, Kimura et al (2016) reported that one of WZ Sge-type DNe, which showed a peculiar main superoutburst, may have a small mass ratio.…”

“…Recently, several period-bouncer candidates possibly filling the gap between the theories and observations have been discovered among WZ Sge-type DNe via photometric observations (Kato et al 2013b;Nakata et al 2014). These objects showed peculiar rebrightenings, and have very small mass ratios and relatively long orbital periods as WZ Sge-type DNe (more than 0.06 d).…”

Short Orbital Period Cataclysmic Variables

“…The main characteristics of the prototype of the subclass (WZ Sge) are summarized below: the short orbital period of 81.6min, close to the predicted period minimum; the spectra in quiescence show the strong double-peaked Balmer emission lines from the accretion disk surrounded by broad absorptions, formed by the primary white dwarf (see for an example [25]); infrequent ∼20-30 years and the large-amplitude (∼ 8 m ; 1913,1946,1978,2001) superoutbursts succeeded by echo outbursts, there are not normal outbursts; optical light curves during "s" -during super-outbursts; "q" -during quiescence; "-" absent of double-humps in LCs. Recent bounce-back candidates: OT J075418.7+381225, OT J230425.8+062546 [35], SSS J122221.7-311523 [36,37] a super-outburst show long-lasting super-humps [26]; there are double-humped in the light curves during super-outbursts and in quiescence [27,26]; there are evidence of forming of spiral arms in the disk during super-outbursts [28,29]; the accretion disc is asymmetric in quiescence, and the bright spot region is to be extended along the mass transfer stream [30,31]; the outer layers of the accretion disc is a low density and a low temperature of ∼3000K [32]; there is some evidence that a cavity was formed in the inner part of the disk during quiescence implying an annulus-shaped accretion disc [33]. The number of WZ Sge-type stars increases with decreasing orbital periods and they seem to be a continuation of the evolution of SU UMa-type stars when mass transfer rates begin to drop closely to the period minimum (Fig.…”