New Method of Estimating Binary's Mass Ratios by Using Superhumps

Abstract

We propose a new dynamical method of estimating binary's mass ratios by using the period of superhumps in SU UMa-type dwarf novae during the growing stage (the stage A superhumps). This method is based on the working hypothesis that the period of superhumps in the growing stage is determined by the dynamical precession rate at the 3W1 resonance radius, and is suggested in our new interpretation of the superhump period evolution during a superoutburst (2013, PASJ, 65, 95). By comparing objects having known mass ratios, we show that our method can provide sufficiently accurate mass ratios comparable to those obtained by eclipse observations in quiescence. One of the advantages of this method is that it requires neither an eclipse nor any experimental calibration. It is particularly suitable for exploring the low mass-ratio end of the evolution of cataclysmic variables, where the secondary is not detectable by conventional methods. Our analysis suggests that previous determinations of the mass ratio by using superhump periods during a superoutburst were systematically underestimated for low mass-ratio systems, and we provided a new calibration. It reveals that most WZSge-type dwarf novae have either secondaries close to the border of the lower main-sequence or brown dwarfs, and most of the objects have not yet reached the evolutionary stage of period bouncers. Our results are not in contradiction with an assumption that an observed minimum period ($\sim 77$ min) of ordinary hydrogen-rich cataclysmic variables is indeed the minimum period. We highlight how important the early observation of stage A superhumps is, and propose an effective future strategy of observation.

Period Variations of Superhumps in Su Uma Stars

“…We have analyzed more than 500 superoutbursts of SU UMa-type dwarf novae. These results have been published by T. Kato et al (see Kato et al (2009);Kato et al (2010); Kato et al (2012); Kato et al (2013); Kato et al (2014)). …”

Accretion Processes in CV AM CVn SDSS J090221.35+381941.9

“…For the white dwarf channel, we chose the relation Table 2 lists the simulated positive superhump periods for each simulated stage and each evolutionary channel. As shown in Table 2, the white dwarf channel produces a simulated Stage A positive superhump period that is shorter than that of Stage B, which has not been observed to date in SU UMa CV systems (Kato & Osaki, 2013). As such, this channel is eliminated as a possibility for J0902 as discussed in .…”

OT J002656.6+284933 (CSS101212:002657+284933): an SU UMa-type dwarf nova with the longest superhump period

“…Since stage A was not very well observed and the resultant period may have already been affected by stage B superhumps, we independently estimated the period of stage A superhumps by using an empirical relation that the period of stage A superhumps is 1.0-1.5% longer than that of averaged stage B superhumps (Kato et al 2009) [This relation has been confirmed in Kato, Osaki (2013) for well-observed systems]. This method gives a period of 0.13356-0.13422 d. Since the lower limit is close to our direct estimate, we used the upper limit as our upper-limit estimate of the period of stage A superhumps.…”