Spectroscopic and photometric confirmation of ASASSN-21au = ZTF20acyxwzf as the best candidate for a long-orbital period AM CVn star

ATel #14390; Keisuke Isogai, Yusuke Tampo, Naoto Kojiguchi, Kenta Taguchi, Junpei Itoh, Masaaki Shibata, Taichi Kato (Kyoto University), Hiroshi Itoh, Katsuaki Kubotera (VSOLJ)
on 12 Feb 2021; 22:58 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report on spectral features seen in the transient ASASSN-21au = ZTF20acyxwzf on 2021-02-10.85 UT and 2021-02-12.85 UT obtained by using a fiber-fed integral field spectrograph (KOOLS-IFU; Matsubayashi et al. 2019) attached to one of the Nasmyth focuses of the 3.8-m telescope Seimei (Kurita et al. 2020) at Okayama Observatory of Kyoto University.

Firstly, the ASAS-SN team reported that this object brightened to g = 13.3 mag on 2021-02-05.422 and that this object is a large amplitude CV. The ASAS-SN Sky Patrol has not detected any past outbursts. There is an optical counterpart of g=20.96(6), r=21.04(9), i=21.35(4), and z=20.72(17) in Pan-STARRS1. Thus, the outburst amplitude is about 8 mag, and hence this object was a good candidate for a WZ Sge-type dwarf nova. However, the outburst profile looks like a precursor + main superoutburst, which is not seen in WZ Sge stars (see ZTF). As already reported to vsnet-alert 25342 and 25369, we detected optical variations with a period of 0.04 d. This period suggests that this object is not an ordinary hydrogen-rich CV. In other words, this object can be an AM CVn star or EI Psc one.

We secured an optical spectrum at V ~ 16 mag on 2021-02-10.85 UT. The spectrum (left panels) shows only a blue continuum and no clear lines. For convenience, we indicate the position of Balmer lines and He I ones as pink dashed lines and blue dotted ones, respectively. Two days later, we obtained another optical spectrum at V ~ 13.7 mag on 2021-02-12.85 UT. The spectrum (right panels) shows a blue continuum with He I absorption lines. We indicate the position of Balmer lines as pink dashed lines, but there are no clear signals.

Although the chemical composition should be estimated by using a spectrum in quiescence, considering our photometric and spectroscopic results, this object is the best candidate for an AM CVn star. If this object is certainly an AM CVn star, this object has the longest orbital period in outbursting AM CVn stars. Continuous time-series photometry is very important.

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