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

Keisuke Isogai, Taichi Kato, Tomohito Ohshima, Kiyoshi Kasai, Arto Oksanen, Kazunari Masumoto, Daiki Fukushima, Kazuki Maeda, Miho Kawabata, Risa Matsuda, Naoto Kojiguchi, Yuki Sugiura, Nao Takeda, Katsura Matsumoto, Hiroshi Itoh, Elena P. Pavlenko, Kirill Antonyuk, Oksana Antonyuk, Nikolai Pit, Aleksei Sosnovskij, Alex Baklanov, Julia Babina, Aleksandr Sklyanov, Seiichiro Kiyota, Franz-josef Hambsch, Colin Littlefield, Yutaka Maeda, Lewis M. Cook, Gianluca Masi, Pavol A. Dubovsky, Rudolf Novak, Shawn Dvorak, Akira Imada, Daisaku Nogami

Published 2016-05-18Version 1

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.