标题： ELMs的诞生：一项ZTF调查，研究演化过的灾变变星转变为极低质量白矮星 显示英文标题

标题： Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs

摘要： 我们对具有演化的次星的质量转移和最近分离的灾变变星（CVs）进行了系统性调查，这些是极端低质量白矮星（ELM WDs）、AM CVn系统和分离的超紧凑双星的前身。 我们在盖亚颜色-星等图中选择主序星以下的目标，其ZTF光曲线显示大振幅的椭球形变化和轨道周期$P_{\rm orb} < 6$小时。 这产生了51个G=18更亮的候选者，其中我们已获得21个的多历元光谱。 我们确认全部21个都是完全或几乎充满洛希瓣的密近双星。 13个显示出持续质量转移的证据，而其他8个的质量转移可能刚刚停止。 大多数次星温度高于任何以前已知的CV供体，温度为$4700<T_{{\rm eff}}/{\rm K}<8000$。 值得注意的是，所有$T_{\rm eff} \gtrsim 7000\,\rm K$的次星似乎都是分离的，而所有较冷的次星仍在进行质量转移。 这种转变可能标志着磁制动由于供体对流包层的损失而变得无效的温度。 大多数原白矮星次星的质量接近$0.15\,M_{\odot}$；它们的伴星质量接近$0.8\,M_{\odot}$。 我们推断空间密度为$\sim 60\,\rm kpc^{-3}$，大约比正常CV低80倍，比ELM WDs低3倍。 所暗示的银河系出生率，$\mathcal{R}\sim 60\,\rm Myr^{-1}$，是AM CVn双星的一半。 大多数系统可以用CV的MESA模型很好地描述，在这些模型中，质量转移只有在供体离开主序星后才开始。 所有都预测将在哈勃时间内达到最小周期$5\lesssim P_{\rm orb}/{\rm min}\lesssim30$，届时它们将变成AM CVn双星或合并。 这个样本使已知的演化CV群体增加三倍，并为改善对紧凑双星群体的理解提供了广泛的机会。 显示英文摘要

摘要： We present a systematic survey for mass-transferring and recently-detached cataclysmic variables (CVs) with evolved secondaries, which are progenitors of extremely low-mass white dwarfs (ELM WDs), AM CVn systems, and detached ultracompact binaries. We select targets below the main sequence in the Gaia color-magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period $P_{\rm orb} < 6$ hr. This yields 51 candidates brighter than G=18, of which we have obtained many-epoch spectra for 21. We confirm all 21 to be completely-- or nearly--Roche lobe filling close binaries. 13 show evidence of ongoing mass transfer, which has likely just ceased in the other 8. Most of the secondaries are hotter than any previously known CV donors, with temperatures $4700<T_{{\rm eff}}/{\rm K}<8000$. Remarkably, all secondaries with $T_{\rm eff} \gtrsim 7000\,\rm K$ appear to be detached, while all cooler secondaries are still mass-transferring. This transition likely marks the temperature where magnetic braking becomes inefficient due to loss of the donor's convective envelope. Most of the proto-WD secondaries have masses near $0.15\,M_{\odot}$; their companions have masses near $0.8\,M_{\odot}$. We infer a space density of $\sim 60\,\rm kpc^{-3}$, roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, $\mathcal{R}\sim 60\,\rm Myr^{-1}$, is half that of AM CVn binaries. Most systems are well-described by MESA models for CVs in which mass transfer begins only as the donor leaves the main sequence. All are predicted to reach minimum periods $5\lesssim P_{\rm orb}/{\rm min}\lesssim30$ within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population and offers broad opportunities for improving understanding of the compact binary population.