RZ Leonis Minoris bridging between ER Ursae Majoris-type dwarf nova and nova-like system

Abstract

We observed RZ LMi, which is renowned for its extremely short (∼19 d) supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, and 60 d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the nova-like state (permanent superhumper). This observed behavior reproduced the prediction of the thermal-tidal instability model extremely well. We detected a precursor in the 2016 superoutburst and detected growing (stage A) superhumps with a mean period of 0.0602(1) d in 2016 and in 2013. Combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in WZ Sge-type dwarf novae, having orbital periods similar to RZ LMi. By using least absolute shrinkage and selection operator (Lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1) d. We estimated an orbital period of 0.05792 d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above that of ordinary SU UMa-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in RZ LMi may be a result of a stripped secondary with an evolved core in a system evolving toward an AM CVn-type object.

An update on the AM CVn family

“…It is observed to spend ∼ 60% of its time in a high accretion state, with outbursts every few days. This behaviour is not well understood within the standard disc instability model, but it is similar to the frequently outbursting RZ LMi stars among CVs [49,50]. The observation of such frequent outbursts in a helium binary provides a strong link between the physical processes operating in the accretion discs of hydrogen and helium binaries.…”

MASTER OT J172758.09+380021.5: a peculiar ER UMa-type dwarf nova, probably a missed nova in the recent past

“…MASTER 1727 is the next after DI UMa object on the standard CV evolutionary path. The evolutionary state of RZ LMi may be different (Kato et al, 2016b).…”

“…However, the tendency of the secular increase in the supercycle, which means a gradual increase in Ṁ , suggests that these objects (at least some of them) could experience a transition between DNe and nova-likes. Thus Kato et al (2016b) applied this idea for a systematic increase of the supercycle length in RZ LMi.…”

The bridge between ER Ursae Majoris-type dwarf nova and nova-like system

“…RZ LMi is one of the most enigmatic SU UMa-type DNe with an ultrashort (19 d) supercycle and unusually regular outburst pattern. This object was originally discovered as an ultravioletexcess variable star [3], and was confirmed as a member of the SU UMa-type DNe by the detection of superhumps ( [4]; [5]) after the discovery of another SU UMa-type dwarf nova ER UMa with a short supercycle of 43 d ( [6]) [for more details of a history of this object, see [7]]. Currently, six SU UMa-type stars including RZ LMi and ER UMa are usually called ER UMa-type stars (cf.…”

“…We also estimated P orb from periods of positive and negative superhumps, and calculated the mass ratio using P orb and a period of stage A superhumps. Note that this paper is a summary of [7] in which more details of the results of our campaigns are published.…”