arXiv subjects

Zhenwei Li

Publications and source records attributed to Zhenwei Li.

At least 19 recordsLinked to original sources

arXiv2026

A Helium-shell Burning Blue Horizontal Branch Star Produced from Common Envelope Evolution

Observationally, blue horizontal branch (BHB) stars are defined as hot stars occupying a characteristic region between the extreme blue horizontal branch and RR Lyrae variables in the Hertzsprung-Russell diagram. Most of them are interpreted as stripped core-helium-burning stars, but the role of binary interaction in their formation remains unclear. Here, we report the discovery of a metal-rich BHB star in a 0.82628-day binary system (Feige 64) comprising a $0.35\pm0.03\,M_{\odot}$ BHB star and a likely $1.26\pm0.17\,M_{\odot}$ white dwarf (WD). The BHB star has an effective temperature of $15{,}524\pm310$ K and a luminosity of $39.7\pm4.1\,L_{\odot}$. Stellar evolution modelling indicates that it is a helium-shell-burning star produced through the common-envelope channel, retaining a hydrogen-rich envelope that is more massive than previously thought for low-mass stars. This finding provides direct evidence for binary interaction in the formation of BHB stars, offering a fresh perspective on interpreting this emerging population.

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arXiv2026

Influence of mass-transfer stability on the formation of post-common-envelope binaries

Post-common-envelope binaries are the natural laboratories for constraining the physics of common envelope evolution, which is one of the most uncertain phases in binary stellar evolution. Traditional binary population synthesis models, adopting mass transfer stability criteria based on polytropic stellar models, systematically overpredict the number of post-common-envelope binaries with solar-type main-sequence companions. In this work, we present an updated binary population synthesis model using the rapid binary evolution code \textit{Binary Star Evolution}, incorporating a physically motivated mass transfer stability criterion and a self-consistent envelope binding energy prescription. We compile a comprehensive sample of classic white dwarf + main sequence post-common-envelope binaries with well-measured parameters, hosting both M-dwarf and A/F/G/K- stars. We find that the enhanced mass transfer stability is an additional mechanism responsible for the observed dearth of post-common-envelope binaries with solar-type main sequence companions; neither magnetic braking nor selection effects alone can fully account for this deficit, and a combination of all three processes is most likely required. Models with inefficient common envelope evolution ($\alpha_{\rm CE}=0.25$) provide the best overall match to the observed population. These results highlight the critical role of MT stability in shaping the observed post-common-envelope binaries population and provide new constraints on common envelope evolution.

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arXiv2026

Mass-Orbital Period Distribution of Massive White Dwarfs Formed Through Stable Mass Transfer

White dwarfs (WDs) in binaries can form through either the stable mass-transfer process or common envelope evolution (CEE). Compared to CEE, the stable mass-transfer process can lead to a distinct mass-orbital period ($M_{\mathrm{WD}}-P_{\mathrm{orb}}$) relation. Thus, this relation of WDs contains the information about the evolution channels. We can study the relation in WD binary systems to determine whether their progenitors undergo a CEE. We use the stellar evolution code MESA as our primary computational tool and adopt the quasi-adiabatic criterion to ensure that our models satisfy the conditions for stable mass transfer. Our study considers different mass-transfer schemes, varying metallicities, and the relation for both low-mass and intermediate-mass progenitors. Previous studies have focused on the relation for low-mass progenitors, which cannot explain some long-period, high-mass WD binaries. Our results show that the relations for intermediate-mass progenitors whose cores remain non-degenerate prior to central helium burning can account for the formation channels of long-period and massive WD binaries.

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arXiv2026

Mass distribution of neutron stars in binary systems

It is known that the mass distribution of the known neutron stars (NSs) exhibits a bimodal pattern. The origin of this distribution remains a subject of debate. We constructed a super-Eddington accretion model for accreting neutron stars and investigated the mass growth and distribution of these stars using the population synthesis method. We find, in our model, the mass growth of NSs depends on the binary orbital period and the mass of the donor star. Our results can successfully account for the bimodal distribution of NS masses. The peak distribution of NS masses at around ~ 1.8 Msun primarily originates from NS binary systems where the donor star mass is less than ~ 1.6 Msun and the orbital period is shorter than 20 days; while, NS systems that may undergo common envelope evolution and these NSs can account for the mass peak at 1.4 Msun.

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arXiv2026

Formation of Extremely Low-mass White Dwarf Binaries Undergoing Enhanced Angular Momentum Loss

Extremely low-mass white dwarfs (ELM WDs) are helium (He) WDs with masses below $\sim 0.3\ M_{\odot}$, mainly formed through binary interaction. ELM WD binaries typically are formed from two channels, namely the stable Roche lobe overflow (RLOF) channel and the common envelope ejection channel. For ELM WD binaries produced from RLOF channel, the ELM WD mass has a strong correlation with the orbital period, i.e., the so-called WD mass-orbital period relation. However, the observations in the ELM Survey show that the orbital periods of ELM WD binaries from the RLOF channel are typically shorter than the theoretically predicted values. Extra angular momentum loss (AML) may be needed to explain such a phenomenon. In this work, we assumed that part of the transferred mass from the donor is lost at the outer Lagrangian point and simulated the formation of ELM WD binaries. Enhanced AML enables more mass to be lost during thermal-timescale mass transfer, thereby affecting nuclear burning in the transfer phase and producing ELM WDs with distinct internal structures. These structural differences alter the (pre-)He WD mass-radius relation at the end of mass transfer, which in turn shifts the WD mass-orbital period relation downward. These adjustments enable our model to successfully reproduce the majority of observed systems from the relevant survey projects.

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arXiv2026

A New Pathway to Single Be Stars: Ejected Companions from Type Ia Supernovae

Be stars are rapid rotators generally produced by binary interactions. The single Be stars in the observations pose challenges to the Be star formation theory. In this paper, we propose a new pathway for the formation of single Be stars, in which the Be star is taken as the ejected companion star from a Type Ia supernova (SN Ia) explosion. Recent numerical simulations suggest that explosive oxygen burning, initiated via the convective Urca process in certain helium (He) stars near the Chandrasekhar mass limit, can set off a SN Ia. Based on this proposition, we further demonstrate that about $0.4\%$ of He star + main-sequence (MS) star binaries may evolve into single Be stars, where the MS star is spun up due to the mass accretion from the He star, and then the He star explodes as a SN Ia. We employ detailed binary evolutionary simulations and find the parameter space that would produce single Be stars via the SN Ia channel. Around $22\%$ of Be stars from the SN Ia progenitor channel exhibit peculiar tangential velocities exceeding $24\ \rm km/s$, classifying them as runaway stars. This suggests that the SN Ia channel plays a meaningful role in forming single Be stars, particularly within the runaway star population.

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arXiv2026

A Path to Constraints on Common Envelope Ejection in Massive Binaries: Full Evolutionary Reconstruction of Three Black Hole X-ray Binaries

The massive binary common envelope (CE) phase plays a pivotal role in the formation of close black hole/neutron star (BH/NS) binaries, yet significant uncertainties remain in our understanding of this process. In this study, we aim to constrain the massive binary CE phase by systematically reconstructing three observed BH X-ray binaries (BHXBs): GRO J1655-40, SAX J1819.3-2525, and 4U 1543-47. Through comprehensive binary evolution simulations and parametric supernova (SN) modeling, we establish lower limits for the CE efficiency parameters under different energy considerations within the standard energy formalism. Specifically, we derive minimum values for three cases: $\alpha_{\rm 0.5U}$ and $\alpha_{\rm U}$ representing CE efficiencies with half and all of the internal energy contributing to the envelope ejection, respectively, and $\alpha_{\rm H}$ accounting for the envelope's enthalpy. Our analysis reveals that the self-consistent formation of these three BHXBs requires CE efficiency parameters satisfying: $\alpha_{\rm 0.5U}\gtrsim 6.7$, $\alpha_{\rm U}\gtrsim 4.2$ and $\alpha_{\rm H}\gtrsim 1.7$. Notably, we find no viable solutions with CE efficiency values below unity, even when considering the most extreme scenarios in which the envelope binding energy is significantly reduced through enthalpy inclusion. {Our results strongly imply that either additional energy sources are required, or the formalism itself must be revised.} Furthermore, we quantitatively assess the impact of BH natal kicks on our results. A key finding is that 4U 1543-47's formation requires substantial natal kicks ($\gtrsim 50 \;\rm km/s$), as lower kick velocities are incompatible with isolated binary evolution.

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arXiv2026

Adiabatic Mass Loss In Binary Stars. VI. Massive Helium Binary Stars

The stability of binary mass transfer is a critical problem for binary evolution. We systematically calculate the adiabatic mass-loss model for naked helium stars with masses ranging from 10$M_{\odot}$ to 80$M_{\odot}$ to study the critical mass ratio ($q_\textrm{crit}$) of Wolf-Rayet binaries. We set up two prescriptions about Wolf-Rayet stellar wind and consider the isotropic re-emission effect during adiabatic mass loss. Results of the critical mass ratio for conserved dynamically unstable mass transfer show that most of the no-wind helium stars on the main sequence (HeMS) have $0.7<q_\textrm{crit}<3.0$ and on the Hertzsprung gap (HeHG) have $1.5<q_\textrm{crit}<27$. With the Wolf-Rayet star wind effect, the $q_\textrm{crit}$ gets lower on a certain evolutionary stage. With the isotropic re-emission effect, the $q_\textrm{crit}$ gets larger for early-evolutionary stage helium stars and lower for late-evolutionary stage helium stars. Based on fully non-conserved mass transfer, the criteria for HeMS stars are $1.0<q_\textrm{crit}<2.8$ and HeHG stars are $1.5<q_\textrm{crit}<5.0$. Compared with the widely used criterion $q_\textrm{crit}=3$ (HeMS) and $q_\textrm{crit}=4$ (HeHG), our result becomes more unstable for the HeMS stars and more stable for the HeHG stars. Our work could be applied to the binary mass transfer stage of massive helium binaries, such as Wolf-Rayet star binaries and high mass X-ray binaries with Wolf-Rayet star companions. It can be applied to the binary population synthesis studies for the formation of special objects, such as double black hole mergers.

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arXiv2025

Formation of Be stars via wind accretion: Case study on Black hole + Be star binaries

Be stars are rapidly rotating main-sequence (MS) stars that play a crucial role in understanding stellar evolution and binary interactions. In this letter, we propose a new formation scenario for black hole (BH) + Be star binaries (hereafter BHBe binaries), where the Be star is produced through the Wind Roche Lobe Overflow (WRLOF) mechanism. Our analysis is based on numerical simulations of the WRLOF process in massive binaries, building upon recent theoretical work. We demonstrate that the WRLOF model can efficiently form BHBe binaries under reasonable assumptions on stellar wind velocities. Using rapid binary population synthesis, we estimate the population of such systems in the Milky Way, predicting approximately $\sim$ {1800-3200} currently existing BHBe binaries originating from the WRLOF channel. These systems are characterized by high eccentricities and exceptionally wide orbits, with typical orbital periods exceeding 1000 days and a peak distribution around $\sim$10000 days. Due to their long orbital separations, these BHBe binaries are promising targets for future detection via astrometric {and interferometric} observations.

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arXiv2025

Formation of Extremely Low-Mass White Dwarfs in Wide Orbits

Helium white dwarfs (WDs) with masses less than 0.3 $\rm M_{\sun}$ are known as extremely low-mass WDs (ELM WDs), which cannot be produced by single stellar evolution in theory. Generally, these stars are believed to form through binary interactions. Recently, two ELM WDs in unusually wide orbits were reported, i.e., KIC 8145411 and HE 0430-2457. Their orbital separations are too wide to be produced by the binary evolution scenario. In this work, we study the formation of wide-orbit ELM WD binaries from hierarchical triple systems. In this scenario, an ELM WD is formed from the inner binary and subsequently forms a wide binary system with the third object. We find that the merger of an evolved star with a brown dwarf in the inner binary fails to produce single ELM WDs, but Type Ia supernovae (SNe Ia) explosions can successfully do so. Furthermore, we investigate the impact of the supernova explosion on the orbital distribution of the surviving binary and find that this channel may have a probability of reproducing the orbital parameters of HE 0430-2457, but fails to reproduce the observed features of KIC 8145411. This supports recent observational recalibrations suggesting that KIC 8145411 resides in a triple system rather than a binary.

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arXiv2025

A New Algol-type Binary with an Accretion disk

We present a comprehensive photometric and spectroscopic analysis of the Algol-type binary \textit{Gaia} DR3 1892576067672499328. We identified the system as a spectroscopic binary based on medium-resolution LAMOST spectra. Combined with \textit{TESS} photometry, we determine an orbital period of $ P = 2.47757 (1) $ days, a low mass ratio of $ q = 0.098 \pm 0.002 $, and an orbital inclination of $ i = 46.934^{+2.613}_{-1.11} $ degrees. The orbit is consistent with being circular ($ e = 0 $). The binary comprises a $ M_1 = 1.817 ^{ +0.106}_{-0.202} \,M_\odot $, $ R_1 = 1.265^{+0.121}_{-0.160}\,R_\odot $ A-type primary and a Roche-lobe-filling secondary of $ M_2 = 0.179 ^{ +0.011}_{-0.020} \,M_\odot $, $ R_2 = 1.994 ^{ +0.041}_{-0.077} \,R_\odot $. The double-peak H$\alpha$ emission line indicates the possible existence of a Keplerian accretion disc. We established a simple standard accretion disc model and modeled the geometric and dynamical properties of the accretion disc. The obtained outer disc radius $R_{\mathrm{out}} \approx 3.36 \pm 0.43\,R_\odot$ is consistent with the values inferred from the emission velocity of H$\alpha$. Systemic velocity variations observed over time suggest the possible presence of a tertiary companion, with a minimum mass of $M_3 > 0.369 \pm 0.024 \,M_\odot$. Given the low mass ratio, the secondary may evolve into a proto-helium white dwarf, forming an \text{EL CVn}-type system in the future. This system offers valuable insights into accretion dynamics and the formation of binaries.

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arXiv2025

A magnetic white dwarf formed through a binary merger within 35 million years

White dwarfs (WDs) represent the final evolutionary stage of most stars, typically originating from progenitor stars with masses below approximately 8 $M_{\odot}$ to 10 $M_{\odot}$. Formation through single-star evolution generally requires at least 25 Myr, with the youngest WDs often near the Chandrasekhar limit of 1.4 $M_{\odot}$. In contrast, WDs formed via binary channels, such as mergers or mass transfer, can develop smaller masses in a shorter timescale and may exhibit unique characteristics, including strong surface magnetic fields and rapid rotation. Accurately determining the ages of these WDs is essential for understanding their formation. A valuable method involves studying WDs in star clusters, where member stars share the same age and chemical composition, allowing for precise constraints on the formation times and metallicities of the WDs' progenitors. Here we report a WD found in the open cluster RSG 5, which is only 35 Myr old. The WD's mass is lower than 1.05 $M_{\odot}$, indicating it may not have formed through single-star evolution. The WD possesses an exceptionally strong surface magnetic field ($\ge 200$ MG), a short rotational period ($\sim 6.5$ min), and, most notably, a co-rotating half-ring of ionized circumstellar debris. This distinctive feature provides evidence for a binary merger origin, a scenario further substantiated by our stellar evolution models.

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arXiv2025

The binary fraction of Blue Horizontal-Branch (BHB) Stars

Blue horizontal-branch (BHB) stars are old, low-mass, metal-poor stars that serve as important tracers of the Galactic halo structure, kinematics, and evolution.Understanding their binary properties provides key insights into their formation channels and the role of binary interactions in the evolution of horizontal branch stars. We intend to investigate the intrinsic binary fraction $f_{\rm b}^{\rm in}$ of BHB stars and its dependencies on metallicity, kinematics, and effective temperature. We collect \GG{299} BHB stars from LAMOST with multiple radial velocity (RV) measurements and classify the sample into halo-like and disk-like BHBs based on their kinematics and metallicity, as well as into bluer and redder BHBs based on their \G{effective temperature}. We then investigate the observed binary fraction for each group based on the radial velocity variations and apply a set of Monte Carlo simulations, assuming distributions of $f(P) \propto P^\pi$ and $f(q) \propto q^\kappa$, to correct the observed binary fraction for observational biases and derive the intrinsic binary fraction. After correcting for observational biases, the intrinsic binary fraction increases to 31% for n > 2 and 32% for n > 3. A clear contrast is observed between halo-like and disk-like BHB stars, with halo-like BHBs exhibiting a lower intrinsic binary fraction (28% for n > 2 and 29% for n> 3) compared to disk-like BHBs (46% and 51%, respectively), indicating different formation pathways. Additionally, we find that bluer BHB stars exhibit a significantly higher binary fraction (42% for n > 2 and 45% for n> 3) than redder BHB stars (24% and 23%, respectively), which suggests a possible link between binarity and the effective temperature, although more samples are required to confirm this.

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arXiv2025

A Be Star + He Star Binary as an Indicator of a Binary Mass Transfer Phase

The rapid rotation of Be stars is supposed to mainly originate from binary evolution. In recent years, more and more Be stars with helium (He) star companions have been discovered, which provides a significant opportunity to study binary interaction physics. In this work, we perform binary population synthesis with an updated binary mass transfer stability criterion and try to understand the details of mass transfer processes by constructing a series of Be star + He star (BeHe) binary populations. We found that the simulations and the observations can be divided into two groups according to the masses of components, corresponding to the two distinct evolutionary processes during the mass transfer. In particular, we found that the mass ratios of BeHe binaries may be taken as a probe of the initial mass ratios of the primordial binaries. Moreover, the results suggest that a higher mass transfer efficiency ($\gtrsim 0.5$) supports the observations better. The simulations predicted too many Be star binaries experiencing Case B mass transfer, which conflicts with the observations. The reason is due to either observational selection effects or unclear physical factors.

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arXiv2025

The Initial Mass Function of the Galactic Early-type Field Stars Based on the LAMOST Survey

Research on the high-mass end of the initial mass function (IMF) has been limited due to a scarcity of samples. Recently, Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), as the most efficient spectroscopic telescope, has provided new opportunities for related research. In this study, based on approximately 70,000 main-sequence early-type stars from the LAMOST survey, we investigated the IMF of Galactic field stars at the high-mass end (1.5 < M/Msun < 7.1). First, we derived the slope of the present-day mass function (PDMF), finding after correcting for selection effect in the observed sample. We then accounted for the effects of stellar evolution and unresolved binaries to correct the PDMF back to the IMF, resulting in {\alpha}ini = 2.70 - 2.82. Notably, we corrected both stellar evolution and unresolved binary effects simultaneously by using binary-Star evolution code, which enhances the robustness of our results. Additionally, we investigated how different mass-ratio (q) distributions of binaries and different star formation histories of the Milky Way impact the IMF. Finally, we tested samples across different spatial scales and found that {\alpha}ini may exhibit a decreasing trend as the spatial scale increases, which could be attributed to variations in metallicity.

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arXiv2025

Orbital Parameters of 665 Double-lined Spectroscopic Binaries in the LAMOST Medium-Resolution Survey

The period, mass ratio, eccentricity, and other orbital parameters are fundamental for investigating binary star evolution. However, the number of binaries with known orbital parameters remains limited. Utilizing the LAMOST-MRS survey, we derived orbital solutions for 665 SB2 binaries by fitting the radial velocities of 1119 SB2 systems with at least six observations, employing a modified version of Thejoker optimized for SB2 binaries. To ensure the reliability of the results, four selection criteria were applied: reduced chi-square, normalized mean absolute error, maximum phase gap, and RV distribution metric. After applying these criteria, 665 reliable orbits were retained. Comparison with Kepler, TESS, and ZTF light curve data shows excellent agreement, with discrepancies in some cases attributed to shorter pulsation periods observed in light curves. Additionally, good consistency is found between our periods and those of SB1 systems in Gaia data. These orbital solutions contribute to understanding binary star evolution and the statistical properties of binary populations.

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arXiv2024

Discoveries and Properties of EL CVn-type Binaries in the TESS Survey

EL CVn-type systems represent a rare evolutionary stage in binary star evolution, providing ideal laboratories for investigating stable mass transfer processes and the formation of extremely low-mass white dwarfs (ELM WDs). The Transiting Exoplanet Survey Satellite (TESS) has delivered an extensive collection of high-precision time-domain photometric data, which is invaluable for studying EL CVn binaries. In this study, we identified 29 EL CVn systems from the TESS eclipsing binary catalogs (sectors 1-65), 11 of which are newly discovered. These systems consist of smaller, hotter pre-He white dwarfs and A/F main-sequence stars. The orbital periods of these binaries range from 0.64 to 2.5 days. Utilizing TESS light curves, Gaia distances, and multi-band photometric data (e.g., GALEX, 2MASS, WISE, SkyMapper), we modeled the light curves and spectral energy distributions to derive system parameters, including effective temperatures, masses, and radii. These systems were then compared with the white dwarf mass-period relation and the evolutionary tracks of ELM WDs. The comparison reveals that these binaries are consistent with the expected mass-period relation for white dwarfs and align well with the evolutionary tracks on the Teff-logg diagram for ELM WDs. This result suggests that these EL CVn systems likely formed through stable mass transfer processes. We provide a catalog of complete parameters for 29 EL CVn systems identified from the TESS survey. This catalog will serve as an essential resource for studying binary mass transfer, white dwarf formation, and pulsation phenomena in EL CVn-type systems.

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arXiv2024

LAMOST J171013+532646: a detached short-period non-eclipsing hot subdwarf + white dwarf binary

We present an analysis of LAMOST J171013.211+532646.04 (hereafter J1710), a binary system comprising a hot subdwarf B star (sdB) and a white dwarf (WD) companion. Multi-epoch spectroscopy reveals an orbital period of 109.20279 minutes, consistent with TESS and ZTF photometric data, marking it as the sixth detached system known to harbor a WD companion with a period less than two hours. J1710 is remarkably close to Earth, situated at a distance of only $350.68^{+4.20}_{-4.21} \, \mathrm{pc}$, with a GAIA G-band magnitude of 12.59, rendering it conducive for continuous observations. The spectral temperature is around 25164 K, in agreement with SED fitting results ($25301^{+839}_{-743} \, \mathrm{K}$). The TESS light curve displays ellipsoidal variation and Doppler beaming without eclipsing features. Through fitting the TESS light curve using the Wilson-Devinney code, we determined the masses for the sdB ($M_1 = 0.44^{+0.06}_{-0.07} \, M_{\odot}$) and the compact object ($M_2 = 0.54^{+0.10}_{-0.07} \, M_{\odot}$), with the compact object likely being a WD. Furthermore, MESA models suggest that the sdB, with a helium core mass of 0.431 $M_{\odot}$ and a hydrogen envelope mass of $1.3 \times 10^{-3}\, M_{\odot}$, is in the early helium main-sequence phase. The MESA binary evolution shows that the J1710 system is expected to evolve into a double white dwarf system, making it an important source of low-frequency gravitational waves.

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