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

Taichi Kato, Tomohito Ohshima, Denis Denisenko, Pavol A. Dubovsky, Igor Kudzej, William Stein, Enrique de Miguel, Arne Henden, Ian Miller, Kirill Antonyuk, Oksana Antonyuk, Nikolaj Pit, Aleksei Sosnovskij, Alex Baklanov, Julia Babina, Elena P. Pavlenko, Kazunari Matsumoto, Daiki Fukushima, Megumi Takenaka, Miho Kawabata, Daisuke Daisuke, Kazuki Maeda, Risa Matsuda, Katsura Matsumoto, Colin Littlefield, Arto Oksanen, Hiroshi Itoh, Gianluca Masi, Francesca Nocentini, Patrick Schmeer, Roger D. Pickard, Seiichiro Kiyota, Shawn Dvorak, Joseph Ulowetz, Yutaka Maeda, Raul Michel, Sergey Yu. Shugarov, Drahomir Chochol, Rudolf Novak

Published 2014-07-16Version 1

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.