학술논문
ASASSN-18aan: An Eclipsing SU UMa-type Cataclysmic Variable with a 3.6-hour Orbital Period and a Late G-type Secondary Star
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Working Paper
Author
Wakamatsu, Yasuyuki; Thorstensen, John R.; Kojiguchi, Naoto; Isogai, Keisuke; Kimura, Mariko; Ohnishi, Ryuhei; Kato, Taichi; Itoh, Hiroshi; Sugiura, Yuki; Sumiya, Sho; Matsumoto, Hanami; Ito, Daiki; Nikai, Kengo; Akitaya, Hiroshi; Ishioka, Chihiro; Oide, Kohei; Kanai, Takahiro; Uzawa, Yoshinori; Oasa, Yumiko; Tordai, Tamás; Vanmunster, Tonny; Shugarov, Sergey Yu.; Yamanaka, Masayuki; Sasada, Mahito; Takagi, Kengo; Nishinaka, Yuki; Yamazaki, Yuina; Otsubo, Ikki; Nakaoka, Tatsuya; Murata, Katsuhiro L.; Ohsawa, Ryou; Morita, Masahiro; Ichiki, Makoto; Dufoer, Sjoerd; Mizutani, Masanori; Horiuchi, Takashi; Tozuka, Miyako; Takayama, Masaki; Ohshima, Tomohito; Saito, Tomoki; Dubovsky, Pavol A.; Stone, Geoff; Miller, Ian; Nogami, Daisaku
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Abstract
We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period ($P_{\rm orb}$) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-$P_{\rm orb}$ SU UMa-type dwarf novae. The estimated mass ratio, ($q=M_2/M_1 = 0.278(1)$), is almost identical to the upper limit of tidal instability by the 3:1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3:1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-$P_{\rm orb}$ WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-$q$ systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-$q$ systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass $M_2$ of around 0.18 M$_{\odot}$, smaller than expected for the orbital period and the secondary's spectral type. We also see indications of enhanced sodium abundance in the secondary's spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system's $P_{\rm orb}$, which may account for the occurrence of a superoutburst at this relatively long period.
Comment: 17 pages, 14 figures, accepted for publication in PASJ
Comment: 17 pages, 14 figures, accepted for publication in PASJ