Xizhen Lu

Context. Recently, a class of dormant black hole (BH) binaries with luminous companions (dBH-LCs) has been observed, such as G a i a BH1, BH2, and BH3. Unlike previously discovered X-ray BH binaries, the observed dBH-LCs have relatively long orbital periods (typically more than several tens to a few hundred days) and show very weak X-ray emission. Aims. Studying the formation and evolution of the whole dBH-LC population is a very interesting problem. Our aim is to study the contribution of massive stars to the dBH-LC population under different evolutionary models, namely, isolated binary evolution (IBE) and hierarchical triple evolution, and different formation channels (i.e., mass transfer and common envelope evolution). Methods. Using the Massive Objects in Binary Stellar Evolution code, the Triple Stellar Evolution code, and the latest initial multiple-star distributions, we modeled the populations of massive stars. We then calculated the orbital properties, mass distributions, and birth rates of the dBH-LC populations formed under these different conditions. Results. In the Milky Way, we calculate that the birth rate of dBH-LCs formed through IBE is about 4.35 × 10 ⁻⁵ yr ⁻¹ , while the birth rate through triple evolution is about 1.47 × 10 ⁻³ yr ⁻¹ . This means that the birth rate from triple evolution is one to two orders of magnitude higher than that from IBE. We find that in triple evolution, the main formation channel of dBH-LCs is post-merger binaries formed from inner binary mergers triggered by von Zeipel−Lidov−Kozai oscillations. In particular, if the merger product is formed from a central helium-burning star and a main-sequence star, the resulting star usually has a small core mass and a large envelope mass. The stars with this structure can form BHs in the pair-instability supernova range (about 60 M _⊙ ∼ 120 M _⊙ ), and they are about three times more massive than the maximum BH mass formed through IBE. Conclusions. Due to the presence of dynamical effects in triple evolution, the inner binaries in triples are more likely to interact than isolated binaries. As a result, dBH-LCs formed through triple evolution mainly come from the channel where the inner binary merges. The birth rate of dBH-LCs from triple evolution is about two orders of magnitude higher than that from IBE. In addition, some dBH-LCs with heavy BHs are also formed through the inner binary merger channel in triples. These results strongly indicate that the triple evolution can be the most important channel for dBH-LC formation.

Context. Intermediate-mass asymptotic giant branch (AGB) stars may have significant influence in the evolution of lithium (Li) in the Galaxy. During the AGB phase, stars eject surface material into interstellar matter (ISM) by stellar winds, and the Li content in their surfaces and winds decisively influences the AGB stars’ contribution of Li to ISM. Turbulent convection within stars - driven by internal gravity waves (IGWs) excited by convective motions - can transmit energy outward and induce mixing in non-convective regions, profoundly affecting the chemical composition of stellar surfaces and winds. Aims. We aim to investigate the effects of IGWs on Li production of the AGB stars. We intend to demonstrate the validity of extra mixing triggered by IGWs in the radiative zone between the convective thermal pulse and the convective envelope of intermediatemass AGB stars and investigate its impact on Li production in these stars. In this paper, we use this model combined with initial mass functions to derive the total Li production from intermediate-mass AGB stars. Methods. We used the Modules for Experiments in Stellar Astrophysics to construct stellar models from the zero-age main sequence to the end of the AGB. This simulation incorporated IGW-induced mixing coefficients and element diffusion effects. Grids were established to calculate the production of Li for AGB stars with different masses and metallicities. Subsequently, employing the method of population synthesis, we used linear interpolation in grids to estimate the contribution rate of Li production to the Galactic Li from a sample size of 10⁷ stars. Results. Simulated results demonstrate that IGWs triggered during each Helium-shell flash induces extra mixing in non-convective regions. In our model, an IGW induces extra mixing that transports material from the radiative zone between the convective thermal pulse and the convective envelope to the convective envelope, allowing ⁷Be originally located below the convective envelope to be transported into the convective envelope, where ⁷Be decays into ⁷Li. The positive effects of IGW mixing on the Li yield diminish with increasing initial mass. For models with the same initial mass, the positive impact of IGW mixing increases with metallicity. In our calculations, most AGB stars with initial masses between 3.5 M_⊙ and 7.5 M_⊙ can produce a positive Li yields. Using the synthesispopulation method, we estimate that the total Li yields produced by AGB stars with IGW mixing are of about 15 M_⊙, which is twice that without IGW mixing. The contribution of these models to total galactic Li is around 10%. It means that an AGB star may be a non-negligible source for Li production. Conclusions. Through this extra-mixing mechanism induced by IGWs, AGB stars can achieve a maximum A(Li) over 5, and intermediate-mass AGB stars significantly contribute to Li in the Galactic ISM. These findings underscore the crucial role of IGWs in stellar evolution, particularly in enhancing Li production.

The subsurface convective zones (CZs) of massive stars significantly influence many of their key characteristics. Previous studies have paid little attention to the impact of rotation on the subsurface CZ, so we aim to investigate the evolution of this zone in rapidly rotating massive stars. We use the Modules for Experiments in Stellar Astrophysics to simulate the subsurface CZs of massive stars during the main sequence phase. We establish stellar models with initial masses ranging from 5 M _⊙ to 120 M _⊙, incorporating four metallicities (Z = 0.02, 0.006, 0.002, and 0.0001) and three rotational velocities (ω/ω _crit = 0, ω/ω _crit = 0.50, and ω/ω _crit = 0.75). We find that rapid rotation leads to an expansion of the subsurface CZ, increases convective velocities, and promotes the development of this zone. Additionally, subsurface CZs can also emerge in stars with lower metallicities. Comparing our models with observations of massive stars in the Galaxy, the Large Magellanic Cloud, and the Small Magellanic Cloud, we find that rotating models better encompass the observed samples. Rotation significantly influences the evolution of the subsurface CZ in massive stars. By comparing with the observed microturbulence on the surfaces of OB stars, we propose that the subsurface CZs may be one of the sources of microturbulence.

Recently, an identified noninteracting black hole (BH) binary, Gaia ID 3425577610762832384 (hereafter G3425), contains a BH (∼3.6 M _⊙) falling within the mass gap and has a nearly circular orbit, challenging the classical binary evolution and supernova theory. Here, we propose that G3425 originates from a triple through a triple common envelope (TCE) evolution. The G3425 progenitor originally may consist of three stars with masses of 1.49, 1.05, and 21.81 M _⊙ and inner and outer orbital periods of 4.22 and 1961.78 days, respectively. As evolution proceeds, the tertiary fills its Roche lobe, leading to a TCE. We find that the orbital energy generated by the inspiral of the inner binary serves as additional energy imparted for ejecting the common envelope (CE), accounting for ∼97% of the binding energy in our calculations. This means that the outer orbit needs to expend only a small amount of the orbital energy to successfully eject the CE. The outcome of the TCE is a binary consisting of a 2.54 M _⊙ merger produced by the inner binary merger and a 7.67 M _⊙ helium star whose CE successfully ejected, with an orbital period of 547.53 days. The resulting post-TCE binary (PTB) has an orbital period that is 1–2 orders of magnitude greater than the orbital period of a successfully ejected classical binary CE. In subsequent simulations, we find that the successfully ejected helium star has a 44.2% probability of forming a BH. In the case of a noncomplete fallback forming a BH, with an ejected mass of 2.6 M _⊙ and a relatively low natal kick ( 1 1 − 5 + 16 km s⁻¹ to 4 9 − 39 + 39 km s⁻¹), this PTB can form G3425 in the Milky Way.

Based on astrometric measurements and spectral analysis from Gaia DR3, two quiescent black hole (BH) binaries, Gaia BH1 and BH2, have been identified. Their origins remain controversial, particularly for Gaia BH1. By considering a rapidly rotating (ω/ω _crit = 0.8) and strongly magnetized (B ₀ = 5000 G) merger product, we find that, at typical Galactic metallicity, the merger product can undergo efficient chemically homogeneous evolution. This results in the merger product having a significantly smaller radius during its evolution compared to that of a normally evolving massive star. Under the condition that the initial triple stability is satisfied, we use the Multiple Stellar Evolution code and the MESA code to identify an initial hierarchical triple that can evolve into Gaia BH1. It initially consists of three stars with masses of 9.03 M _⊙, 3.12 M _⊙, and 1 M _⊙, with inner and outer orbital periods of 2.21 days and 121.92 days, and inner and outer eccentricities of 0.41 and 0.45, respectively. This triple initially experiences triple evolution dynamics instability (TEDI) followed by Roche lobe overflow (RLOF). During RLOF, the inner orbit shrinks, and tidal effects gradually suppress the TEDI. Eventually, the inner binary undergoes a merger through contact (or collision). Finally, using models of rapidly rotating and strongly magnetic stars, along with standard core-collapse supernova (SN) or failed supernova (FSN) models, we find that a postmerger binary (PMB) consisting of an 12.11 M _⊙ merger product and a 1 M _⊙ companion star (originally an outer tertiary) can avoid RLOF. After an SN or FSN with a low ejected mass of ∼0.22 M _⊙ and a low kick velocity ( 46 − 33 + 25 km s − 1 or 9 − 8 + 16 km s − 1 ), the PMB can form Gaia BH1 in the Galactic disk.