Superoutburst of WZ Sge-type Dwarf Nova Below the Period Minimum: ASASSN-15po

Abstract

We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed early superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary ($q$) is 0.0699(8) by using $P_{\mathrm{orb}}$ and a superhump period $P_{\mathrm{SH}}$ of stage A. ASASSN-15po [$P_{\mathrm{orb}}\sim$ 72.6 min] is the first DN with the orbital period between 67--76 min. Although the theoretical predicted period minimum $P_{\min }$ of hydrogen-rich cataclysmic variables (CVs) is about 65--70 min, the observational cut-off of the orbital period distribution at 80 min implies that the period minimum is about 82 min, and the value is widely accepted. We suggest the following four possibilities: the object is (1) a theoretical period minimum object (2) a binary with a evolved secondary (3) a binary with a metal-poor (Popullation II) seconday (4) a binary which was born with a brown-dwarf donor below the period minimum.

18 pages, 9 figures, accepted for publication in PASJ

References in corpus (4)

• SDSS J150722.30+523039.8: a CV formed directly from a detached white dwarf/brown dwarf binary?
• SDSS 1507+02: A Halo Cataclysmic Variable?
• MOCCA-SURVEY database I. Accreting white dwarf binary systems in globular clusters -- I. cataclysmic variables -- present-day population
• Are Superhumps Good Measures of the Mass Ratio for AM CVn Systems?

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• Discovery of Standstills in the SU UMa-Type Dwarf Nova NY Serpentis
• Spectroscopic and Photometric Observations of Dwarf Nova Superoutbursts by the 3.8 m Telescope Seimei and the Variable Star Network
• Photometric Analysis of TCP J20171288$+$1156589 -- WZ Sge Type Dwarf Nova with Delayed Ordinary Superhumps Emergence