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

TL;DR

This paper reports the first superoutburst observation of a WZ Sge-type dwarf nova with an orbital period below the theoretical period minimum, providing insights into its properties and possible evolutionary scenarios.

Contribution

It presents the discovery and analysis of ASASSN-15po, the first dwarf nova with an orbital period between 67-76 minutes, challenging existing theories on the period minimum of hydrogen-rich cataclysmic variables.

Findings

Measured superhump periods and estimated mass ratio of 0.0699.

Identified ASASSN-15po as the first dwarf nova below the theoretical period minimum.

Discussed four possible evolutionary scenarios for the object.

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 $P_{\rm orb}$ and a superhump period $P_{\rm SH}$ of stage A. ASASSN-15po [$P_{\rm orb} \sim$ 72.6 min] is the first DN with the orbital period between 67--76 min. Although the theoretical predicted period minimum $P_{\rm min}$ 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.