It is predicted that orbital decay by gravitational-wave radiation and tidal interaction will cause some close-binary stars to merge within a Hubble time. The merger of a helium-core white dwarf with a main-sequence star can produce a red giant branch star that has a low-mass hydrogen envelope when helium is ignited and thus become a hot subdwarf. Because detailed calculations have not been made, we compute post-merger models with a stellar evolution code. We find the evolutionary paths available to merger remnants and find the pre-merger conditions that lead to the formation of hot subdwarfs. We find that some such mergers result in the formation of stars with intermediate helium-rich surfaces. These stars later develop helium-poor surfaces owing to diffusion. Combining our results with a model population and comparing to observed stars, we find that some observed intermediate helium-rich hot subdwarfs can be explained as the remnants of the mergers of helium-core white dwarfs with low-mass main-sequence stars.
The mergers of double helium white dwarfs are believed to form isolated helium-rich hot subdwarfs. Observation shows that the helium-rich hot subdwarfs can be divided into two subgroups based on whether the surface is carbon-rich or carbon-normal. But it is not clear whether this distribution directly comes from binary evolution. We adopt the binary population synthesis (BPS) to obtain the population of single helium-rich hot subdwarfs according to the channel of double helium white dwarfs merger. We find that the merger channel can represent the two subgroups in the $T_{\rm{eff}}-\log g$ plan…
In the framework of our extensive modeling study of the spectral evolution of white dwarfs, we present here a new set of detailed calculations of the transport of residual hydrogen in helium-rich white dwarfs. First, we investigate the so-called float-up process at high effective temperature, whereby the upward diffusion of trace hydrogen leads to the formation of a hydrogen atmosphere. We examine the dependence of this phenomenon on the initial hydrogen abundance and on the strength of the radiative wind that opposes gravitational settling. Combined with our empirical knowledge of spectral ev…
Constraining transport of angular momentum in stars: Combining asteroseismic observations of core helium burning stars and white dwarfs
角动量的传输一直是恒星演化研究领域的一个具有挑战性的课题，尤其是在最近的星震调查之后。所有已发表的使用星震观测的自转研究都显示，观测到的自转速率和计算出的自转速率之间存在差异，这表明这些恒星中存在一个未确定的角动量传输过程。我们的目标是限制这一过程的效率，通过调查2.5 M $_{\odot}$恒星的旋转速率。首先，我们研究了泰勒-斯普鲁特发电机是否可以负责额外的传输角动量的恒星初始质量为2.5 M $_{\odot}$。然后，通过计算旋转模型，包括一个恒定的额外的人工粘度，我们确定了效率的丢失过程中的角动量输运比较模型的核心氦燃烧恒星的小行星地震观测。参数的研究进行调查的恒星演化代码，初始质量和演化阶段的效果。我们将我们的模型发展到了白矮星阶段，并提供了与白矮星旋转速率的比较。泰勒-斯普鲁特发电机不能提供足够的角动量传输，以达到本文所研究的核心氦燃烧星的观测值。我们发现，附加的人工粘度值$ν_{\rm{add}}$约10 $^7$ cm $^2$ s $^{-1}$提供了足够的角动量传输。然而，这些模型的旋转周期在白矮星阶段太高，无法与白矮星观测相匹配。从这个比较中，我们推断，在核心氦燃烧阶段，缺失过程的效率一定会降低。在排除核心氦燃烧阶段的$ν_{\rm{add}}$后，我们可以匹配核心氦燃烧星和白矮星的自转周期。
Stars on the ascending helium giant branch I. From white dwarf merger to helium giant: the evolutionary state of the rapidly rotating hot subdwarf HE 1518-0948
Universität Potsdam（波茨坦大学）；Instituto de Astrofísica de La Plata, Consejo Nacional de Investigaciones Científicas y Técnicas（拉普拉塔天体物理研究所，国家科学研究委员会）；Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天文与地球科学学院）；Hamburger Sternwarte, University of Hamburg（汉堡大学汉堡天文台）
通过光谱分析研究极端富氦热亚矮星 HE 1518-0948，发现其是双氦白矮星并合产物，目前处于氦壳燃烧并沿氦巨星分支上升阶段，为理解大质量热亚矮星在核心氦燃烧后的演化提供了重要实验室。
We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC~228782059, discovered from 55.1~days of {\em K2} photometry. The long duration, high quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of $34.1 \pm 0.4$~hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns $M_{*} = 0.685 \pm 0.003 M_{\odot}$, $T_{\rm{eff}}= 21{,}910 \pm 23$\,K and $\log g = 8.14 \pm0.01$\,dex. These values are comparable to those derived from spectroscopy by …
We present an analysis of all single white dwarf stars known to exhibit spectroscopic signatures of neutral helium line splitting due to the presence of a strong magnetic field. Using state-of-the-art models taking into account the effects of magnetic fields on the synthetic spectra, we determine effective temperatures, surface gravities and masses for the stars in our sample. Our analysis uses data from the second and third Gaia (early) data release, photometric data from diverse surveys such as the Sloan Digital Sky Survey and Pan-STARRS, and archived spectroscopic data. We are able to succe…
Ultra-massive white dwarfs (UMWDs) with masses larger than 1.05Msun are basically believed to harbour oxygen-neon (ONe) cores. Recently, Gaia data reveals an enhancement of UMWDs on Hertzsprung-Russell diagram (HRD), which indicates that extra cooling delay mechanism such as crystallization and elemental sedimentation may exist in the UMWDs. Further studies suggested that a portion of UMWDs should have experienced pretty long cooling delays, implying that they are carbon-oxygen (CO) WDs. However, the formation mechanism of these UMCOWDs is still under debate. In this work, we investigated whet…
White dwarf stars are subject to various element transport mechanisms that can cause their surface composition to change radically as they cool, a phenomenon known as spectral evolution. In this paper, we undertake a comprehensive theoretical investigation of the spectral evolution of white dwarfs. First, we introduce STELUM, a new implementation of the stellar evolutionary code developed at the Université de Montréal. We provide a thorough description of the physical content and numerical techniques of the code, covering the treatment of both stellar evolution and chemical transport. Then, we…
In binary systems, the helium accretion onto carbon-oxygen (CO) white dwarfs (WDs) plays a vital role in many astrophysical scenarios, especially in supernovae type Ia. Moreover, ignition density for accretion rate $\dot{M} \lesssim 10^{-9} M_{\odot}$ $yr^{-1}$ in helium accreting CO white dwarfs decides the triggering mechanism of a supernova explosion which could be either off-centre helium flash or central carbon flash. We aim to study the accretion of helium with a slow accretion rate $5 \times 10^{-10} M_{\odot}$ $yr^{-1}$ onto relatively cool and hot white dwarfs of different abundances …
In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with -10.0 < log(H/He) < -2.0, 7.5 < log(g) < 9.0 and 12000 K < T_eff < 34000 K, based on 282 3D atmospheric models calculated with the CO5BOLD radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing SDSS sample of DB and DBA white dwarfs, we find significant corrections both for the effective temperature and surface …
We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known DBVs, we use these objects to provide an update to the empirical extent of the DB instability strip. With our sample of new DBVs, the red edge is better constrained, as we nearly double the number of D…
Orbital decay mechanisms argue that double white dwarf mergers are inevitable, but extremely rare. Whilst some mergers result in explosions, the survivors re-ignite helium and burn brightly for tens of thousands or millions of years. Candidate survivors include extreme helium stars, R CrB variables and various classes of helium-rich subluminous star. Nuclear waste on the survivors' surfaces provides evidence of the stars' nuclear history prior to and their nucleosynthesis during the merger. Extensive and deep spectroscopic surveys offer rich prospects for future discoveries.
A Massive Magnetic Helium Atmosphere White Dwarf Binary in a Young Star Cluster
我们已经在盖亚DR 2目录中搜索了年轻疏散星团方向上以前未知的热白矮星。这个实验的目的是尝试将初始-最终质量关系（IFMR）扩展到更高的质量，可能提供一个与目前认为大约1.38 M $_{\odot}$的拉塞卡极限的张力。我们在年轻的$\sim$星系团Messier 47（NGC 2422）的方向上发现了一颗特别有趣的白矮星。所有的盖亚指标（自行、视差、在盖亚颜色星等图中的位置）都表明它是星团的成员。它的光谱，从双子座南部获得，产生了一些异常：它是一个DB（富氦大气）白矮星，它有很大的磁场（2.5 MG），质量高（$\sim$ 1.06 M $_\odot$），它的颜色非常特殊-特别是红色的（$r$，$i$，$z$和$y$），这表明它有一个晚型伴星。这是唯一的磁化，分离的双星白矮星与任何已知的光谱类型的星团内或星团外的非简并伴星。如果白矮星是星团的一员，正如所有指标所显示的那样，它的前身质量略高于6 M $_\odot$。然而，它可能讲述了一个比IFMR更有趣的故事，一个关于恒星磁场，I型超新星和来自低质量恒星系统的引力波的起源。
We explore changes in the adiabatic low-order g-mode pulsation periods of 0.526, 0.560, and 0.729 M$_\odot$ carbon-oxygen white dwarf models with helium-dominated envelopes due to the presence, absence, and enhancement of $^{22}$Ne in the interior. The observed g-mode pulsation periods of such white dwarfs are typically given to 6$-$7 significant figures of precision. Usually white dwarf models without $^{22}$Ne are fit to the observed periods and other properties. The root-mean-square residuals to the $\simeq$ 150$-$400 s low-order g-mode periods are typically in the range of $σ_{rms}$ $\less…
