ASASSN-18 aan：一颗轨道周期为3.6小时的旋转SU UMa型激变变星和一颗晚G型副星
ASASSN-18aan: An Eclipsing SU UMa-type Cataclysmic Variable with a 3.6-hour Orbital Period and a Late G-type Secondary Star
摘要：
本文报道了食SU UMa型矮新星ASASSN-18 aan的光度和光谱观测结果。我们观测到2018年的超级爆发，亮度为2.3等，发现轨道周期（$P_{\rm orb}$）为0.149454（3）d，或3.59 hr。这是周期间隔的长度，确定ASASSN-18 aan是少数长$P_{\rm orb}$ SU UMa型矮新星之一。估算的质量比（$q=M_2/M_1 = 0.278(1)$）与3：1共振的潮汐不稳定上限几乎相同。从日食观测中我们发现，在超爆发开始时的吸积盘可能达到3：1的共振半径，说明ASASSN-18 aan的超爆发是由潮汐不稳定性引起的。考虑到长$P_{\rm orb}$ WZ Sge型矮新星的情况，我们认为在潮汐截断半径处的潮汐耗散足以在相对高的$q$系统（如SU UMa型矮新星）中诱导类SU UMa行为，但这在低$q$系统（如WZ Sge型矮新星）中不再有效。该系统的不寻常的性质延伸到第二颗恒星，我们发现它的光谱类型为G9，远早于轨道周期的典型值，第二颗恒星的质量$M_2$约为0.18 M $_{\odot}$，小于轨道周期和第二颗恒星光谱类型的预期值。我们还看到了在次级光谱中钠丰度增加的迹象。在少量其他CV和相关对象中可以看到异常热的二次曲线。这些系统显然经历了显着的核演化开始之前的质量转移。在ASASSN-18 aan的情况下，这显然导致了比通常在系统的$P_{\rm orb}$中发现的质量比更低，这可能是在这个相对较长的时期内发生超级爆发的原因。
英文摘要：
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.
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