Superoutburst of CR Bootis: Estimation of Mass Ratio of a typical AM CVn star by Stage A Superhumps

TL;DR

This study observed superoutbursts in the AM CVn star CR Boo, detecting stage A superhumps to estimate the system's mass ratio, supporting the dwarf nova instability model over alternative explanations.

Contribution

First detection of precursor outbursts and stage A superhumps in CR Boo, enabling mass ratio estimation and supporting the thermal-tidal instability model.

Findings

Mass ratio of CR Boo estimated as 0.101(4).

Detection of precursor outbursts and stage A superhumps.

Supports the dwarf nova instability model for AM CVn stars.

Abstract

We report on two superoutbursts of the AM CVn-type object CR Boo in 2014 April--March and 2015 May--June. A precursor outburst acompanied both of these superoutbursts. During the rising branch of the main superoutburst in 2014, we detected growing superhumps (stage A superhumps) whose period was $0.017669(24)$ d. Assuming that this period reflects the dynamical precession rate at the radius of the 3:1 resonance, we could estimate the mass ratio ($q=M_2/M_1$) of 0.101(4) by using the stage A superhump period and the orbital one of 0.0170290(6) d. This mass ratio is consistent with that expected by the theoretical evolutionary model of AM CVn-type objects. The detection of precursor outbursts and stage A superhumps is the second case in AM CVn-type objects. There are two interpretations of the outbursts of AM CVn-type objects. One is a dwarf nova (DN) outbursts analogy, which is caused by thermal and tidal instabilities. Another is the VY Scl-type variation, which is caused by the variation of the mass-transfer rate of the secondary. This detection of the superhump variations strongly suggests the former interpretation.