Superoutburst of SDSS J090221.35+381941.9: First Measurement of Mass Ratio in an AM CVn-Type Object using Growing Superhumps

TL;DR

This paper reports the first measurement of the mass ratio in an AM CVn-type object using the stage A superhump method during a superoutburst, providing new insights into the system's properties and evolution.

Contribution

It introduces the first successful application of the stage A superhump method to measure the mass ratio in an AM CVn-type system, validating theoretical models.

Findings

Measured the mass ratio q = 0.041(7) using superhump periods.

Detected growing superhumps during the superoutburst.

Identified the longest orbital period among outbursting AM CVn systems.

Abstract

We report on a superoutburst of the AM CVn-type object SDSS J090221.35+381941.9 [J0902; orbital period 0.03355(6) d] in 2014 March-April. The entire outburst consisted of a precursor outburst and the main superoutburst, followed by a short rebrightening. During the rising branch of the main superoutburst, we detected growing superhumps (stage A superhumps) with a period of 0.03409(1) d. During the plateau phase of the superoutburst, superhumps with a shorter period (stage B superhumps) were observed. Using the orbital period and the period of the stage A superhumps, we were able to measure the dynamical precession rate of the accretion disk at the 3:1 resonance, and obtained a mass ratio (q) of 0.041(7). This is the first successful measurement of the mass ratio in an AM CVn-type object using the recently developed stage A superhump method. The value is generally in good agreement with the theoretical evolutionary model. The orbital period of J0902 is the longest among the outbursting AM CVn-type objects, and the borderline between the outbursting systems and systems with stable cool disks appears to be longer than had been supposed.