Abstract A population of anomalous ultramassive white dwarfs discovered with Gaia, often referred to as the Q branch, show high (multi-Gyr) cooling delays produced by exotic physical mechanisms. They are believed to be the products of stellar mergers, but the exact origin and formation channel remain unclear. We obtained a spectroscopically complete, volume-limited sample of the Q branch region within 100 pc and found significant differences in atmospheric composition and rotation rates as a function of tangential velocity. In particular, we discover that stellar remnants with the longest cooling delays do not show strong magnetism nor detectable short-period rotational variability, as opposed to what is generally believed for double-degenerate mergers. This indicates that either these white dwarfs arise from a formation channel with no strong magnetism induced, or that the magnetism produced from the merger dissipates over the cooling delay timescales. Our follow-up photometry has also discovered pulsations in the second and third hydrogen-dominated DAQ white dwarfs, one hotter than 15,500 K, possibly extending the boundaries of the DAV instability strip for white dwarfs with thin hydrogen layers.

Abstract Early excess emission observed in Type Ia supernovae (SNe Ia) within ∼1 day of explosion provides a critical window into their progenitor systems. In the present study, we investigate formation of the circumstellar matter (CSM) in double white-dwarf (WD) mergers. We further study the interaction between the CSM and the SN ejecta. We first model the orbital evolution and super-Eddington mass transfer/ejection in the double WD systems. We then conduct hydrodynamical and light-curve (LC) simulations of the SN–CSM interaction, assuming a prompt SN Ia explosion in the context of the carbon-ignited violent merger (C-ignited VM). Our simulations show that at the moment of the merger, the binary system has the CSM distribution following ρ CSM ≃ D ( r / 1 0 14 cm ) − 3.5 ( D ≃ 1 0 − 14 – 1 0 − 13 g cm − 3 ) . The simulated LCs reproduce the early flux excesses across optical to UV bands, as well as their color evolution, observed in the VM candidates, i.e., 03fg/02es-like SNe Ia. This supports that 03fg/02es-like objects originate from the VM explosions. We also discuss the case of the helium-ignited VM, which might be realized in some WD–WD mergers depending on the He content in the system. Focused here is the timing when the explosion is initiated, and we find that the explosion is initiated after the companion WD is, at least partially, tidally disrupted also in this case; we thus expect the formation of the CSM through the mass transfer phase also for the helium-ignited VM scenario.

Abstract Common envelope evolution is a critical but still poorly understood phase in binary evolution. It plays a key role in forming close binaries such as hot subdwarfs, double white dwarfs (WDs), X-ray binaries, and double neutron stars. However, its outcomes remain highly uncertain. Depending on the efficiency of envelope ejection, a system may either survive as a close binary or undergo a complete merger. In this work, we investigate the postmerger evolution of systems where a CO WD merges with the core of an asymptotic giant branch (AGB) star. A grid of merger remnant models with various core and envelope masses is constructed. At the onset of evolution, the CO core contracts and undergoes off-center carbon ignition, producing an inwardly propagating carbon flame. For remnants with a relatively low mass of CO core, the flame phase is followed by core contraction and subsequent H shell burning. For more massive CO cores, the carbon flame reaches the center and is soon followed by off-center neon burning, which is expected to eventually lead to core-collapse supernovae. The merger remnants occupy nearly the same region on the H-R diagram as ordinary AGB or super-AGB stars, exhibiting similar surface properties. Although their surface abundance may differ slightly from those of normal AGB stars, depending on the initial core and envelope masses, these differences are strongly reduced once mass loss is taken into account. We suggest that some giant-like stars, including candidates for Thorne–Żytkow objects (e.g., HV 2112), might alternatively be explained as AGB–WD merger remnants.

Binary systems of two carbon-oxygen white dwarfs are one of the most promising candidates for the progenitor systems of Type Ia supernovae. Violent mergers, where the primary white dwarf ignites when the secondary white dwarf smashes into it while being disrupted on its last orbit, were the first double degenerate merger scenario proposed that ignites dynamically. However, violent mergers likely contribute only a few percent to the total Type Ia supernova rate and do not yield normal Type Ia supernova light curves. Here we revisit the scenario, simulating a violent merger with better methods and, in particular, a more accurate treatment of the detonation. We find good agreement with previous simulations but with one critical difference: The secondary white dwarf being disrupted and accelerated towards the primary white dwarf and impacted by its explosion does not fully burn, and its core survives as a bound object. The explosion leaves behind a 0.16, star travelling 2800, making it an excellent (and so far the only) candidate to explain the origin of the fastest observed hypervelocity stars. We also show that before the explosion, 5 , of material predominantly consisting of helium, carbon, and oxygen had already been ejected at velocities above 1000, -3 M_⊙ Finally, we argue that if a violent merger made the hypervelocity stars D6-1 and D6-3 and violent mergers require the most massive primary white dwarfs in binaries of two carbon-oxygen white dwarfs, there has to be a much larger population of white dwarf mergers with slightly lower mass primary white dwarfs. Because this population likely represents ≫ 10% of the Type Ia supernovae rate, it can essentially only give rise to normal Type Ia supernovae.

Many white dwarfs are observed in compact double white dwarf binaries, and through the emission of gravitational waves, a large fraction are destined to merge. The merger remnants that do not explode in a Type Ia supernova are expected to initially be rapidly rotating and highly magnetized. In this work, we present our discovery of the variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF,J2008+4449, as a likely merger remnant showing signs of circumstellar material without a stellar or substellar companion. The nature of ZTF,J2008+4449 as a merger remnant is supported by its physical properties: it is hot (35,500 K) and massive (1.12 M_⊙), rapidly rotating with a period of ≈,6.6 minutes, and likely possesses exceptionally strong magnetic fields (∼,400--600,MG) at its surface. Remarkably, we detect a significant period derivative of (1.80 . The unusual variability of the Balmer emission on the spin period of the white dwarf is consistent with the trapping of a half ring of ionized gas in the magnetosphere of the white dwarf. indicating that the white dwarf is spinning down, and a soft X-ray emission that is inconsistent with photospheric emission. As the presence of a mass-transferring stellar or brown dwarf companion is excluded by infrared photometry, the detected spin-down and X-ray emission could be tell-tale signs of a magnetically driven wind or of interaction with circumstellar material, possibly originating from the fallback of gravitationally bound merger ejecta or from the tidal disruption of a planetary object. We also detect Balmer emission, which requires the presence of ionized hydrogen in the vicinity of the white dwarf, showing Doppler shifts as high as ≈,2000,km,s -1

Abstract 2MASS J05513444+4135297 (hereafter J0551+4135) is the only pulsating DAQ white dwarf (WD) known with a carbon and hydrogen atmosphere. Its unusual atmospheric composition and kinematics indicate a WD merger origin. We present time-series photometry of J0551+4135 obtained using the Apache Point Observatory 3.5 m, Gemini North 8 m, and Gran Telescopio Canarias 10 m telescopes. J0551+4135 exhibits variations in pulsation amplitude and frequency over time. We detect 10 significant recurring peaks across different subsets of observations, with frequencies ranging from 987 to 1180 μ Hz, consistent with nonradial gravity ( g )-mode oscillations. We present new evolutionary models suitable for spectroscopic characterization of DAQ WDs, and derive a mass of 1.13 ± 0.01 M ⊙ and a cooling age of 1.7 ± 0.1 Gyr for a CO core, and 1.12 ± 0.01 M ⊙ and 1.6 ± 0.1 Gyr for an ONe-core WD, respectively. However, detailed asteroseismology of this unique pulsator has to wait until fully consistent DAQ evolutionary models are available. Further observations, including multisite campaigns to reduce daily aliasing and to improve the signal-to-noise ratio, would be helpful for the identification of additional modes and constraining the internal structure of this unique pulsator.

Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch (RGB). They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). The mass distribution of canonical hot subdwarfs is expected to peak at the core mass required for helium ignition under degenerate conditions in the 0.45 to 0.5 range. However, non-degenerate helium ignition from intermediate-mass progenitors and non-canonical pathways, such as the merger of helium white dwarfs and delayed helium flashes, are also expected to contribute to the hot subdwarf population. Using high-quality, homogeneous spectra of 335 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. Our focus is on the mass distribution of the different types of hot subdwarfs and their connections to the various formation scenarios. We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature ( surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the Gaia parallax was used to compute the stellar parameters radius, luminosity, and mass. Our spectroscopic sample mostly consists of H-rich sdBs and sdOs, but also contains 41 He-rich sdOs. Additionally, the sample includes 11 intermediate-helium stars and 19 horizontal branch objects with ≳ 14 kK. We detected the presence of helium stratification in six sdB stars with around 30 kK, making them good candidates for also showing enrichment in their atmospheres. Our sdB distribution along the EHB shows a gap near 33 kK, visible in both the Kiel (łogg - diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1 This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass ($<$0.45 sdBs located below the EHB that might have originated from more massive progenitors. These low-mass sdBs preferentially show low helium abundances. Finally, we identified more than 80 pulsating stars in our sample and found that they fall into well-defined p- and g-mode instability regions.

Abstract We present the first deep radio continuum observations of Pa 30, a nebula hosting a unique optical source driven by an ultrafast outflow with a velocity of 16,000 km s−1. The nebula was proposed to be the remnant of a white dwarf merger that occurred in 1181CE. We report no detection of the radio diffuse emission from Pa 30 or radio emission from the central source, setting 3σ upper limit flux densities of 0.84 and 0.29 mJy at 1.5 and 6 GHz, respectively, for Pa 30. The radio surface brightness of Pa 30 is ∼3 orders of magnitude smaller than that of typical supernova remnants (SNRs) with comparable angular size. If Pa 30 is an SNR, our observations show it to be the faintest known in the radio band. Considering that 10% of the supernova (SN) kinetic energy is transferred to cosmic rays (CRs), the absence of radio synchrotron emission suggests that the SN kinetic energy ≲3 × 1047(B/10 μG)−1.65 erg, which is 3–4 orders of magnitude lower than that of typical SNRs and the lowest measured among Galactic SNRs. There is also an indication of inefficient CR acceleration for this source. The low SN kinetic energy either implies the potential existence of many more radio-faint, subenergetic SNRs in our Galaxy or challenges the SNR interpretation of Pa 30.

Abstract We explore the potential of lunar-based gravitational-wave detectors to broaden the multimessenger astrophysics landscape by detecting mergers of massive (M 1, M 2 > 1M ⊙) double white dwarf (WD) binaries. These systems are potential progenitors of Type Ia supernovae and could serve as independent probes of cosmic expansion. We examine two proposed lunar gravitational-wave detector concepts operating in the subhertz band (0.1–1 Hz): the Gravitational-Wave Lunar Observatory for Cosmology (a proxy for suspended test mass detectors) and the Lunar Gravitational-Wave Antenna (a proxy for seismic array detectors). We estimate that these detectors could reach distances of up to ∼1 Gpc for the most massive mergers. We show that lunar detectors could observe up to dozens of massive WD mergers annually, including those originating from globular clusters. Lunar detectors would constrain the masses of these WDs with an unprecedented accuracy of one part in a million. Furthermore, these detectors would provide early warnings weeks before mergers, including sky-localization of square arcminute resolution, enabling a new era of coordinated multimessenger follow-up of electromagnetic transients—whether they evolve into Type Ia supernovae or accretion-induced collapse events.

We carried out NLTE (non local thermodynamic equilibrium) radiative transfer simulations to determine whether an explosion during the merger of a carbon-oxygen (CO) white dwarf (WD) with a helium (He) WD can reproduce the characteristic Ca II/[Ca II] and He I lines observed in Ca-rich transients. Our study is based on a 1D representation of a hydrodynamic simulation of a 0.6 M⊙ CO+0.4 M⊙ He WD merger. We calculated both the photospheric and nebular-phase spectra, including treatment for non-thermal electrons, as is required for accurate modelling of He I and [Ca II]. Consistent with Ca-rich transients, our simulation predicts a nebular spectrum dominated by emission from [Ca II] 7291, 7324 Å and the Ca II near-infrared (NIR) triplet. The photospheric-phase synthetic spectrum also exhibits a strong Ca II NIR triplet, prominent optical absorption due to He I 5876 Å and He I 10830 Å in the NIR, which is commonly observed for Ca-rich transients. Overall, our results therefore suggest that CO+He WD mergers are a promising channel for Ca-rich transients. However, the current simulation overpredicts some He I features, in particular both He I 6678 and 7065 Å, and shows a significant contribution from Ti II, which results in a spectral energy distribution that is substantially redder than most Ca-rich transients at peak. Additionally, the Ca II nebular emission features are too broad. Future work should investigate if these discrepancies can be resolved by considering full 3D models and exploring a range of CO+He WD binary configurations.

Abstract The majority of merging white dwarfs leave behind a white dwarf remnant. Hot/warm DQ white dwarfs with carbon-rich atmospheres have high masses and unusual kinematics. All evidence points to a merger origin. Here, we demonstrate that far-UV (FUV) and optical photometry provides an efficient way to identify these merger remnants. We take advantage of this photometric selection to identify 167 candidates in the Galaxy Evolution Explorer All-Sky Imaging Survey footprint, and provide follow-up spectroscopy. Out of the 140 with spectral classifications, we identify 75 warm DQ white dwarfs with T eff > 10,000 K, nearly tripling the number of such objects known. Our sample includes 13 DAQ white dwarfs with spectra dominated by hydrogen and (weaker) carbon lines. Ten of these are new discoveries, including the hottest DAQ known to date, with T eff ≈ 23,000 K and M = 1.31M ⊙. We provide a model atmosphere analysis of all warm DQ white dwarfs found, and present their temperature and mass distributions. The sample mean and standard deviation are T eff = 14,560 ± 1970 K and M = 1.11 ± 0.09M ⊙. Warm DQs are roughly twice as massive as the classical DQs found at cooler temperatures. All warm DQs are found on or near the crystallization sequence. Even though their estimated cooling ages are of order 1 Gyr, their kinematics indicate an origin in the thick disk or halo. Hence, they are likely stuck on the crystallization sequence for ∼10 Gyr due to significant cooling delays from distillation of neutron-rich impurities. Future all-sky FUV surveys like Ultraviolet Explorer have the potential to significantly expand this sample.

Abstract Studying compact object binary mergers in star clusters is crucial for understanding stellar evolution and dynamical interactions in galaxies. Open clusters in particular are more abundant over cosmic time than globular clusters. However, previous research on low-mass clusters with ≲103 M ⊙ has focused on binary black holes (BBHs) or black hole–neutron star (BH–NS) binaries. Binary mergers of other compact objects, such as white dwarfs (WDs), are also crucial as progenitors of transient phenomena such as Type Ia supernovae and fast radio bursts (FRBs). We present simulations of three types of open clusters with masses of 102, 103, and 104 M ⊙. In massive clusters with ≳104 M ⊙, BBHs are dynamically formed; however, less massive compact binaries such as WD–WDs and WD–NSs are perturbed inside the star clusters, causing them to evolve into other objects. We further find BH–NS mergers only in 103 M ⊙ clusters. Considering star clusters with a typical open cluster mass, we observe that WD–WD merger rates slightly increase for 103 M ⊙clusters but decrease for 102 M ⊙ clusters. Since the host clusters are tidally disrupted, most of them merge outside of the clusters. Our WD–WD merger results have further implications for two classes of transients. Super-Chandrasekhar WD–WD mergers are present in our simulations, demonstrating potential sources of FRBs at a rate of 70–780 Gpc−3 yr−1, higher than the rate estimated for globular clusters. Additionally, we find that carbon–oxygen WD–WD mergers in our open clusters (34–640 Gpc−3 yr−1) only account for 0.14%–2.6% of the observed Type Ia supernova rate in our local Universe.

Context. Fast X-ray transients (FXTs) are bright X-ray flashes with durations of a few minutes to hours, peak isotropic luminosities of LX, peak ∼ 1042 − 1047 erg s−1, and total isotropic energies of E ∼ 1047 − 1050 erg. They have been detected with space-based telescopes such as Chandra, XMM-Newton, Swift-XRT, and Einstein Probe in the soft X-ray band. Einstein Probe detected > 50 in its first year of operation. While several models have been proposed, the nature of many FXTs is currently unknown. One model predicts that FXTs are powered by the spin-down energy of newly formed millisecond magnetars. In this context, they are usually thought to form in a binary neutron star (BNS) merger. However, the rates seem to be in tension: the BNS volumetric rate is estimated to be ∼102 Gpc−3 yr−1, which barely overlaps with the estimated FXT volumetric rate of 103 − 104 Gpc−3 yr−1; thus, even in the small range of overlap, BNS mergers would need to produce FXTs with nearly 100% efficiency. Aims. We explore the maximum volumetric formation rate of millisecond spin period magnetars, including several possibilities beyond the BNS channel, comparing it with the volumetric rate of FXTs to determine what fraction of FXTs could have a millisecond magnetar origin. Methods. We compiled the estimated rate densities for several different suggested formation channels of rapidly spinning magnetars, including the accretion-induced collapse of white dwarfs, binary white dwarf mergers, neutron star–white dwarf mergers, and the collapse of massive stars. We converted the Milky Way event rates to volumetric rates, wherever necessary, by considering either the star formation rate or the stellar mass density distributions as a function of redshift. Results. We find that the highest possible rates among these possibilities come from binary white dwarf mergers and the collapse of massive stars. However, both scenarios may be unfavourable for FXT production due to uncertainties in the resultant spin and magnetic field distributions of the newly formed neutron stars and several observational constraints. Moreover, in all the scenarios, we find that the fraction of neutron stars that meet both criteria of rapid rotation and a strong magnetic field is either very low or highly uncertain. We conclude that millisecond magnetars are not the most viable progenitors of FXTs and can account for at most 10% of the entire FXT population.

Abstract Long-duration GRB 211211A, which lacked an associated supernova at very a low redshift z = 0.076 but was associated with a possible kilonova emission, has attracted great attention. The neutron star–white dwarf (NS–WD) merger is proposed as a possible progenitor of GRB 211211A, and it could naturally explain the long duration of the prompt emission. However, the NS–WD merger is not an ideal site for producing heavy elements via r-process nucleosynthesis. In this Letter, we investigate the heavy elements produced in NS–WD mergers based on numerical simulations of nucleosynthesis via SkyNet and then calculate the resulting kilonova-like emission to compare with the solidly observed case of possible kilonova emission associated with GRB 211211A. By adopting three models (i.e., Model-A, Model-B, and Model-C) from M. A. R. Kaltenborn et al. at different temperatures (T = 4, 5, and 6 GK), which are treated as free parameters, we find that the mass number of the heaviest element produced in our simulations is less than 90 (A < 90). Moreover, by comparing the calculated kilonova-like emission with the afterglow-subtracted observations of the possible kilonova associated with GRB 211211A, it is found that the merger of an NS and WD cannot be ruled out as the origin of GRB 211211A to produce the possible kilonova emission if the remnant of the WD–NS merger is a supramassive or stable magnetar. Otherwise, it is difficult to explain the early possible kilonova emission following GRB 211211A by invoking the merger of a WD and an NS.

Mergers of double white dwarfs (DWDs) are considered significant potential progenitors of type Ia supernovae (SNe Ia), which serve as “standard candles” in cosmology to measure the expansion rate of the Universe and explore the nature of dark energy. Although there is no direct observational evidence to definitively determine the formation pathways of SNe Ia, studying the physical properties of DWDs provides valuable insights into their evolutionary processes, interaction modes, and merger mechanisms, which are essential for understanding the explosion mechanisms of SNe Ia. This study aims to identify DWD candidates through spectroscopic radial velocity (RV) measurements and analyze their physical properties based on DESI EDR. We crossmatched DESI EDR with the Gaia EDR3 white dwarf (WD) catalog to select DA spectra. We measured the spectroscopic RV using the cross-correlation function (CCF) and assessed the significance of RV variability using a chi-squared-based variability method. Spectroscopic T_ and log g were derived by fitting the hydrogen Balmer lines, with 3D convection corrections applied. Orbital periods and semi-amplitudes were obtained through a Lomb-Scargle analysis of the RV time series. We interpolated WD cooling models and applied Monte Carlo simulations to calculate masses, cooling ages, radii, and their associated uncertainties. Additionally, we analyzed their photometric and spectral energy distribution properties to derive photometric temperatures and radii, which were then compared with the corresponding spectroscopic parameters. We identified 33 DA DWD candidates with significant RV variability, including 28 new discoveries. Among them, we found an extremely low-mass DWD candidate and a potential triple system. For these candidates, we measured key physical parameters including T_ log g, mass, and radius, and estimated the orbital periods based on the available data. Of these, 17 candidates exhibit relatively clear periodic RV variability in the current data, and we report their best-fitting periods and RV semi-amplitudes.

SN Ia CSM 2020aeuh: A Massive Binary C/O White Dwarf Merger?

Abstract We present an analysis of three near-infrared (NIR; 1.0–2.4 μm) spectra of the SN 2003fg–like/“super-Chandrasekhar” Type Ia supernovae (SNe Ia) SN 2009dc, SN 2020hvf, and SN 2022pul at respective phases of +372, +296, and +294 days relative to the epoch of B-band maximum. We find that all objects in our sample have asymmetric, or “tilted,” [Fe ii] 1.257 and 1.644 μm profiles. We quantify the asymmetry of these features using five methods: velocity at peak flux, profile tilts, residual testing, velocity fitting, and comparison to deflagration–detonation transition models. Our results demonstrate that, while the profiles of the [Fe ii] 1.257 and 1.644 μm features are widely varied between 2003fg-likes, these features are correlated in shape within the same SNe. This implies that line blending is most likely not the dominant cause of the asymmetries inferred from these profiles. Instead, it is more plausible that 2003fg-like SNe have aspherical chemical distributions in their inner regions. These distributions may come from aspherical progenitor systems, such as double white dwarf mergers, or off-center delayed-detonation explosions of near-Chandrasekhar mass carbon–oxygen white dwarfs. Additional late-phase NIR observation of 2003fg-like SNe and detailed 3D non-LTE modeling of these two explosion scenarios are encouraged.

Abstract Colliding of two high Mach-number plasma jets is one of the methods for generating shocks, offering valuable insights for inertial confinement fusion (ICF) and astrophysics. Especially, quantum degeneracy and kinetic effects appear when the colliding plasma jets are of high density and high Mach number, which play critical roles in the resulting dynamics. Recently, the double-cone ignition (DCI) project provides an unprecedented experimental platform for investigating the collision of plasma to explore many new physics. In this work, a theoretical analysis was conducted using the newly developed hybrid particle-in-cell (PIC) LAPINS code, covering a vast range of plasma densities and temperatures. The results reveal that electron degenerate pressure in the upstream region suppresses the growth of the downstream density. Moreover, under extremely high Mach number collisions of moderately dense plasma, a significant non-equilibrium kinetic effect, namely cross penetration, is observed, along with an anomalous decrease in the downstream density compression ratio, contrary to expectations of monotonic growth. These findings provide valuable suggestions for the baseline design of the DCI scheme and related ICF shock studies, and offer potential analogies to astrophysical processes such as white dwarf mergers and shocks in dense nebulae.

ABSTRACT Merger of two white dwarfs (WDs) has been proposed to form an isolated WD having high magnetization and rapid rotation. We study the influence of the magnetohydrodynamic (MHD) wind on spin evolution of the newly formed merger product. We consider the scenario that the merger product appears as a giant-star-like object with a radius of ${>}10^{10}\,{\rm cm}$ and a luminosity of the order of an Eddington value. We solve a structure of the merger product under the hydrostatic equilibrium and identify the position of the slow-point in the hot envelope. It is found that if such a giant-star-like object is spinning with an angular speed of the order of the Keplerian value, the MHD wind can be produced. The mass-loss rate is estimated to be of the order of ${\sim} 10^{20-21}\,{\rm g\,s^{-1}}$, and the time-scale of the spin down is ${\sim} 10\rm{{\!-\!}}10^{3}$ yr, which depends on stellar magnetic field. We discuss that the final angular momentum when the MHD wind is terminated is related to the magnetic flux and initial radiation luminosity of the merger product. We apply our model to three specific magnetic WD sources ZTF J190132.9+145808.7, SDSS J221141.8+113604.4, and PG 1031+234 by assuming that those WDs were as a result of the merger product. We argue that the current periods of ZTF J190132.9+145808.7 and PG 1031+234 that are strongly magnetized WDs are related to the initial luminosity at the giant phase. For SDSS J221141.8+113604.4, which is mildly magnetized WD, its angular momentum was almost determined when the spin-down time-scale due to MHD wind is comparable to the cooling time-scale in the giant phase.

Abstract Hydrogen-deficient Carbon (HdC) stars are a class of supergiants with anomalous chemical compositions, suggesting that they are remnants of CO–He white dwarf (WD) mergers. This class comprises two spectroscopically similar subclasses—dusty R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars. Both subclasses have a stark overabundance of 18O in their atmospheres, but spectroscopic differences between them remain poorly studied. We present high-resolution (R ≈ 75,000) K-band spectra of six RCB and six dLHdC stars, including four newly discovered dLHdC stars, making this the largest sample to date. We develop a semi-automated fitting routine to measure 16O/18O ratios for this sample, tripling the number of dLHdC stars with oxygen isotope ratios measured from high resolution spectra. All six dLHdC stars have 16O/18O < 1, while the RCB stars have 16O/18O > 4. Additionally, for the first time, we find a trend of decreasing 16O/18O ratios with increasing effective temperature for HdC stars, consistent with predictions of theoretical WD merger models. However, we note that current models overpredict the low 16O/18O ratios of dLHdC stars by two orders of magnitude. We also measure abundances of C, N, O, Fe, S, Si, Mg, Na, and Ca for these stars. We observe a correlation between the abundances of 14N and 18O in our sample, suggesting that a fixed fraction of the 14N is converted to 18O in these stars via α-capture. Our results affirm the emerging picture that the mass ratio/total mass of the WD binary determine whether an RCB or dLHdC is formed post-merger.