Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum

TL;DR

This study uses Bayesian modeling to estimate the intrinsic orbital period distribution of WZ Sge-type dwarf novae, revealing a spike near the period minimum and supporting their role as the missing population in CV evolution.

Contribution

It introduces a Bayesian approach to estimate the true population of WZ Sge stars and identifies a spike at the shortest orbital periods, aligning with theoretical predictions.

Findings

Estimated minimum orbital period ~70 min.

Detected a spike in the population at shortest periods.

Supported the existence of the missing CV population.

Abstract

WZ Sge-type dwarf novae are characterized by long recurrence times of outbursts (~10 yr) and short orbital periods (<~ 85 min). A significant part of WZ Sge stars may remain undiscovered because of low outburst activity. Recently, the observed orbital period distribution of cataclysmic variables (CVs) has changed partly because outbursts of new WZ Sge stars have been discovered routinely. Hence, the estimation of the intrinsic population of WZ Sge stars is important for the study of the population and evolution of CVs. In this paper, we present a Bayesian approach to estimate the intrinsic period distribution of dwarf novae from observed samples. In this Bayesian model, we assumed a simple relationship between the recurrence time and the orbital period which is consistent with observations of WZ Sge stars and other dwarf novae. As a result, the minimum orbital period was estimated to be ~70 min. The population of WZ Sge stars exhibited a spike-like feature at the shortest period regime in the orbital period distribution. These features are consistent with the orbital period distribution previously predicted by population synthesis studies. We propose that WZ Sge stars and CVs with a low mass-transfer rate are excellent candidates for the missing population predicted by the evolution theory of CVs.