ASASSN-18aan: An eclipsing SU UMa-type cataclysmic variable with a 3.6-hr orbital period and a late G-type secondary star
ASASSN-18aan: An eclipsing SU UMa-type cataclysmic variable with a 3.6-hr orbital period and a late G-type secondary star
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DOI：
10.1093/pasj/psab003
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发表时间：
2021-02
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影响因子：
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通讯作者：
Yasuyuki Wakamatsu;J. Thorstensen;N. Kojiguchi;K. Isogai;M. Kimura;Ryuhei Ohnishi;Taichi Kato;H. It
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作者：
Yasuyuki Wakamatsu;J. Thorstensen;N. Kojiguchi;K. Isogai;M. Kimura;Ryuhei Ohnishi;Taichi Kato;H. It
我们报道了食SU-UMA型矮新星ASASSN-18aan的光度和光谱观测结果。我们观测到了2018年2.3MAG变亮的超暴，发现其轨道周期(PORB)为0.149454(3)d，或3.59小时。这是周期间隔的延长，确立了ASASSN-18aan是为数不多的Long-Porb Su Uma型矮新星之一。估算的质量比(Q=M2/M1=0.278(1))与3：1共振的潮汐不稳定上限基本一致。从日食我们发现，超爆发开始时的吸积盘可能达到3：1的共振半径，这表明ASASSN-18AAN的超爆发是潮汐不稳定的结果。考虑到Long-Porb WZSGE型矮新星的情况，我们认为潮汐截断半径处的潮汐耗散足以在相对高Q的系统如SU UMatype矮新星中诱导类SU-Uma行为，但对于低Q系统如WZSGE型矮新星则不再有效。该系统的不同寻常的性质延伸到了次星，我们在次星上发现了G9的光谱类型，比轨道周期的典型光谱类型要早得多，次级质量M2约为0.18M⊙，小于轨道周期的预期和次级恒星的光谱类型。我们还看到次星光谱中钠丰度增强的迹象。在少量其他CV和相关对象中可以看到异常热的次要对象。在传质开始之前，这些系统显然经历了显著的核演化。在ASASSN-18aan的例子中，这显然导致了一个比该系统PORT B通常发现的质量比更低的质量比，这可能解释了在这个相对较长的时间段发生超爆发的原因。
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 (Porb) 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-Porb SU UMa-type dwarf novae. The estimated mass ratio, (q = M2/M1 = 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-Porb 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 UMatype 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 M2 of around 0.18 M⊙, 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 Porb, which may account for the occurrence of a superoutburst at this relatively long period.