We study the formation of ultra-compact binaries (AM CVn stars and ultra-compact X-ray binaries) with emphasis on the surface chemical abundances of the donors in these systems. Hydrogen is not convincingly detected in the spectra of these systems. Three different proposed formation scenarios involve different donor stars, white dwarfs, helium stars or evolved main-sequence stars. Using detailed evolutionary calculations we show that the abundances of helium WD donors and evolved main-sequence stars are close to equilibrium CNO-processed material, and the detailed abundances correlate with the core temperature and thus mass of the MS progenitors. Evolved MS donors typically have traces of H left. For hybrid or CO white dwarf donors, the carbon and oxygen abundances depend on the temperature of the helium burning and thus on the helium core mass of the progenitors. For helium star donors in addition to their mass, the abundances depend strongly on the amount of helium burnt before mass transfer starts and can range from unprocessed and thus almost equal to CNO-processed matter, to strongly processed and thus C/O rich and N-deficient. We briefly discuss the relative frequency of these cases for helium star donors, based on population synthesis results. Finally we give diagnostics for applying our results to observed systems and find that the most important test is the N/C ratio, which can indicate the formation scenario as well as, in some cases, the mass of the progenitor of the donor. In addition, if observed, the N/O, O/He and O/C ratios can distinguish between helium star and WD donors. Applied to the known systems we find evidence for WD donors in the AM CVn systems GP Com, CE 315 and SDSS J0804+16 and evidence for hybrid WD or very evolved helium star donors in the UCXBs 4U 1626-67 and 4U 0614+09. [Abridged]
In spite of their importance for modern astronomy, we do not fully understand how close binary stars containing at least one white dwarf form from main sequence binary stars. The discovery of EL CVn binaries, close pre-white dwarfs with A/F main sequence star companions, offers now the unique possibility to test models of close compact binary star formation. Binary evolution theories predict that these EL CVn stars descend from very close main sequence binaries with orbital periods shorter than 3 days. If this is correct, nearly all EL CVn stars should be inner binaries of hierarchical triples…
We model cataclysmic variables (CVs) with solar metallicity donors ($X=0.7,\:Z=0.02$) that evolve to form AM CVn stars through the Evolved CV formation channel using various angular momentum loss mechanisms by magnetic braking ($\mathrm{AML_{MB}}$). We find that the time-scale for $\mathrm{AML_{MB}}$ in our double-dynamo (DD) model is shorter than that of previously used empirical formulae. Owing to the shorter time-scales, a larger parameter space of initial conditions evolves to form AM CVn stars with the DD model than with other models. We perform an analysis of the expected number of AM CV…
AM CVn systems are ultra-compact, helium-rich, accreting binaries with degenerate or semi-degenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 minutes. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) lightcurves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra of two long-…
JWST observations of three long-period AM CVn binaries: detection of the donors and hints of magnetically truncated disks
California Institute of Technology（加州理工学院）；University of Oklahoma（俄克拉荷马大学）；JILA, University of Colorado and National Institute of Standards and Technology（JILA，科罗拉多大学与美国国家标准与技术研究院）；Massachusetts Institute of Technology（麻省理工学院）
The physical properties of AM CVn stars: new insights from Gaia DR2
AM CVn双星是轨道周期在5-65分钟范围内的贫氢致密双星，被预测为持续引力波辐射的强源。使用盖亚数据发布2，我们提出了41个已知的56个系统的自行车和自行。与使用HST精细制导传感器确定的视差相比，我们发现原型恒星AM CVn比以前认为的要近得多。这解决了模型难以解释的高光度和质量吸积率。利用Pan-STARRS 1数据，我们确定了AM CVn星的绝对星等。有证据表明，捐赠恒星的质量和半径比预期的白矮星更大，或者捐赠恒星不是白矮星。使用已知的AM CVn恒星的距离，我们发现了强有力的证据，大量的AM CVn恒星仍有待发现。由于该值设置了LISA的引力波信号的背景，因此引起了广泛的兴趣。我们确定的质量传递速率为15 AM CVn星，并发现大多数有一个显着大于预期的速率从标准模型。这进一步证明了施主星的尺寸比预期的要大。
Accretion Disk Winds in AM CVn Binaries - a Monte Carlo Approach
AM CVn系统是类似激变变星（CV）的相互作用双星，但更紧凑，轨道周期小于80分钟。主星是一颗白矮星，而主星的性质还不完全清楚。通过分析吸积盘（由Nagel等人在这些会议上提出），可以发现次级星系外层的丰度和成分。从高状态AM CVn系统的光谱不仅显示典型的签名的吸积盘，但也P天鹅座线轮廓，流出的迹象存在于系统中。在这里，我们提出了第一个定量的光谱分析的吸积盘风在AM CVn系统。用我们的三维蒙特卡罗辐射转移程序WOMPAT模拟了出现的风谱。我们表明，P天鹅座配置文件可以复制与我们的风模型。
The space gravitational wave (GW) detector Laser Interferometer Space Antenna (LISA) that was planed to launch in the early 2030s is to detect the low-frequency GW signals in the Galaxy. AM CVn stars were generally thought to be important low-frequency GW sources. Employing the MESA code, in this work we calculate the evolution of a great number of binary systems consisting of a white dwarf (WD) and a main sequence (MS) star, and diagnose whether their descendants-AM CVn stars can be visible by the LISA. The simulated results show that the progenitors of these LISA sources within a distance of…
The evolution and stability of mass transfer of CO+He WD binaries are not well understood. Observationally they may emerge as AM CVn binaries and are important gravitational wave (GW) emitters. In this work, we have modeled the evolution of double WD binaries with accretor masses of $0.50 - 1.30\;M_{\odot}$ and donor masses of $0.17\; - 0.45\;M_{\odot}$ using the detailed stellar evolution code MESA. We find that the evolution of binaries with same donor masses but different accretor masses is very similar and binaries with same accretor masses but larger He donor masses have larger maximum ma…
Ultracompact cataclysmic variables (CVs) of the AM CVn type are deemed to be important verification sources for the future space gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA). We model the present-day Galactic population of AM CVn stars with He-star donors. Such a population has long expected to exist, though only a couple of candidates are known. We applied the hybrid method of binary population synthesis (BPS) which combines a simulation of the population of immediate precursors of AM CVn stars by a fast BPS code with subsequent tracking of their evolutio…
The New RS CVn Binary V1034 Her Revisited and the orbital period - Activity relation of Short-period RS CVn binaries using photometric distortion amplitude
We consider initial stage of the evolution of AM CVn type stars with white dwarf donors, which is accompanied by thermonuclear explosions in the layer of accreted He. It is shown that the accretion never results in detonation of He and accretors in AM CVn stars finish their evolution as massive WDs. We found, for the first time, that in the outbursts the synthesis of n-rich isotopes, initiated by the ${\mathrm{^{22}{Ne}(α,n)^{25}Mg}}$ reaction becomes possible.
New light curves and photometric solutions of FP Lyn, FV CVn and V354 UMa are presented. It is found that these three systems are W-subtype shallow contact binaries. In addition, it is obvious that the light curves of FP Lyn and V354 UMa are asymmetric. Therefore, a hot spot was added on the primary star of FP Lyn and a dark spot was added on the secondary star of V354 UMa. At the same time, we added a third light to the photometric solution of FP Lyn for the final result. The obtained mass ratios and fill-out factors are $q=1.153$ and $f=13.4\%$ for FP Lyn, $q=1.075$ and $f=4.6\%$ for FV CVn,…
We calculate the stellar evolution of both white dwarfs (WDs) in AM CVn binaries with orbital periods of $P_{\mathrm{orb}} \approx 5-70$ minutes. We focus on the cases where the donor starts as a $M_{\mathrm{He}} < 0.2 \, M_{\odot}$ Helium WD and the accretor is a $M_{\mathrm{WD}} > 0.6 \, M_{\odot}$ WD. Using Modules for Experiments in Stellar Astrophysics (MESA), we simultaneously evolve both WDs assuming conservative mass transfer and angular momentum loss from gravitational radiation. This self-consistent evolution yields the important feedback of the properties of the donor on the mass tr…
No Period Change in Two Long-Period AM CVn Binaries
University of Oklahoma（俄克拉荷马大学）；University of Colorado and National Institute of Standards and Technology（科罗拉多大学和国家标准与技术研究院）；University of Warwick（华威大学）；Institute of Science and Technology Austria（奥地利科学技术研究所）；University of Amsterdam（阿姆斯特丹大学）；Carnegie Institution for Science（卡内基科学学会）；Yunnan Observatories, CAS（中国科学院云南天文台）；University of Cambridge（剑桥大学）；University of Hamburg（汉堡大学）；Liverpool John Moores University（利物浦约翰摩尔斯大学）
研究双长周期AM CVn双星Y Z LMi和Gaia14aae的食时变化，发现其轨道周期变化率一致为零，排除了额外角动量损失机制，限制了超高速引力波发射的可能。
AM CVn systems are a rare type of accreting binary that consists of a white dwarf and a helium-rich, degenerate donor star. Using the Zwicky Transient Facility (ZTF), we searched for new AM CVn systems by focusing on blue, outbursting stars. We first selected outbursting stars using the ZTF alerts. We cross-matched the candidates with $Gaia$ and Pan-STARRS catalogs. The initial selection of candidates based on the $Gaia$ $BP$-$RP$ contains 1751 unknown objects. We used the Pan-STARRS $g$-$r$ and $r$-$i$ color in combination with the $Gaia$ color to identify 59 high-priority candidates. We obta…
A study of transients from ground-based surveys reveals new ultra-compact accreting white dwarf binaries
University of Texas Rio Grande Valley（德克萨斯大学里奥格兰德河谷分校）；South Texas Space Science Institute, University of Texas Rio Grande Valley（德克萨斯大学里奥格兰德河谷分校南德克萨斯空间科学研究所）；Texas Tech University（德克萨斯理工大学）；Universidad Nacional Autónoma de México, Instituto de Astronomía（墨西哥国立自治大学天文学研究所）；University of Michigan（密歇根大学）；Institute of Science and Technology Austria（奥地利科学技术研究所）
We model helium-rich stars with solar metallicity ($X=0.7,\:Z=0.02$) progenitors that evolve to form AM Canum Venaticorum systems through a helium-star formation channel, with the aim to explain the observed properties of Gaia14aae and ZTFJ1637+49. We show that semi-degenerate, H-exhausted ($X\leq 10^{-5}$), He-rich ($Y\approx0.98$) donors can be formed after a common envelope evolution (CEE) phase if either additional sources of energy are used to eject the common envelope, or a different formalism of CEE is implemented. We follow the evolution of such binary systems after the CEE phase using…
We investigate the evolutionary properties of AM~CVn stars with massive white dwarf donors and accretors. As a representative of them we consider a binary initially composed by a 0.30 M$_\odot$, He WD and a 1.02 M$_\odot$, CO WD. We evaluate the time-dependent mass transfer rate from the donor and compute the evolution of the accretor, accounting for the effects of mass exchange on the evolution of orbital parameters. We model the thermal response of the accreting CO WD with the FUN evolutionary code coupled to a full nuclear network, from H to Bi, including more than 700 isotopes linked by ab…
SDSS J080710+485259 is the longest period outbursting ultracompact white dwarf binary. Its first ever detected superoutburst started in November of 2018 and lasted for a year, the longest detected so far for any short orbital period accreting white dwarf. Here we show that the superoutburst duration of SDSS J080710+485259 exceeds the 2 months viscous time of its accretion disk by a factor of about 5. Consequently it follows neither the empirical relation nor the theoretical relation between the orbital period and the superoutburst duration for AM CVn systems. Six months after the end of the su…
We present LAMOST J0140355+392651 (hereafter J0140), a close ($P_{\rm orb} = 3.81$ hours) binary containing a bloated, low-mass ($M \approx 0.15 M_{\odot}$) proto-white dwarf (WD) and a massive ($M\approx 0.95\,M_{\odot}$) WD companion. The system's optical light curve is dominated by large-amplitude ellipsoidal variability but also exhibits additional scatter, likely driven by pulsations. The proto-WD is cooler ($T_{\rm eff} = 6800\pm 100$ K) and more puffy ($\log\left[g/\left({\rm cm\,s^{-2}}\right)\right]=4.74\pm0.07$) than any known extremely low mass (ELM) WD, but hotter than any known ca…
The evolutionary history of ultra-compact accreting binaries. I. Chemical abundances and formation channel of the eclipsing AM CVn system ZTF J225237.05-051917.4 from HST spectroscopy
Hamburg Observatory, University of Hamburg（汉堡天文台，汉堡大学）；European Southern Observatory（欧洲南方天文台）；Department of Physics and Astronomy, Texas Tech University（德克萨斯理工大学物理与天文学系）；Department of Physics, University of Warwick（华威大学物理系）；Institut für Theoretische Physik und Astrophysik, University of Kiel（基尔大学理论物理与天体物理研究所）；São Paulo State University (UNESP), School of Engineering and Sciences（圣保罗州立大学（UNESP）工程与科学学院）；The Observatories of the Carnegie Institution for Science（卡内基科学机构天文台）；Department of Physics, University of California, Santa Barbara（加州大学圣塔芭芭拉分校物理系）；Departamento de Física, Universidad Técnica Federico Santa María（费德里科·圣玛丽亚技术大学物理系）；Institute of Science and Technology Austria（奥地利科学技术研究所）；Anton Pannekoek Institute for Astronomy, University of Amsterdam（阿姆斯特丹大学安东·潘农克天文研究所）
Ultracompact accreting binary systems each consist of a stellar remnant accreting helium-enriched material from a compact donor star. Such binaries include two related sub-classes, AM CVn-type binaries and helium cataclysmic variables, in both of which the central star is a white dwarf. We present a spectroscopic and photometric study of six accreting binaries with orbital periods in the range of 40--70 min, including phase-resolved VLT spectroscopy and high-speed ULTRACAM photometry. Four of these are AM CVn systems and two are helium cataclysmic variables. For four of these binaries we are a…
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 f…
Coherent radio emission from stars can be used to constrain fundamental coronal plasma parameters, such as plasma density and magnetic field strength. It is also a probe of chromospheric and magnetospheric acceleration mechanisms. Close stellar binaries, such as RS Canum Venaticorum (RS CVn) systems, are particularly interesting as their heightened level of chromospheric activity and possible direct magnetic interaction make them a unique laboratory to study coronal and magnetospheric acceleration mechanisms. RS CVn binaries are known to be radio-bright but coherent radio emission has only con…
Metallicity is known to significantly affect the radial expansion of a massive star: the lower the metallicity, the more compact the star, especially during its post-MS evolution. We study this effect in the context of binary evolution. Using the stellar-evolution code MESA, we computed evolutionary tracks of stars at different metallicities, exploring variations of factors known to affect the radial expansion (e.g. semiconvection, overshooting, rotation). We find observational support for an evolution in which already at metallicity $0.2Z_{\odot}$ massive stars remain relatively compact durin…
Radio Properties of RS Canum Venaticorum Variables in VLASS and RACS
University of Alberta（阿尔伯塔大学）；Clemson University（克莱姆森大学）；University of Sydney（悉尼大学）；University of Wisconsin-Milwaukee（威斯康星大学密尔沃基分校）；Australian Telescope National Facility, CSIRO Astronomy and Space Science（澳大利亚望远镜国家设施，CSIRO天文学与空间科学）
