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Electron-capture-supernovae (EC-SNe) provide an alternative channel for producing neutron stars (NSs). They play an important role in the formation of double NS (DNS) systems and the chemical evolution of galaxies, and contribute to the NS mass distribution in observations. It is generally believed that EC-SNe originate from e -captures on |$\rm ^{24}Mg$| and |$\rm ^{20}Ne$| in the massive degenerate oxygen–neon (ONe) cores with masses close to the Chandrasekhar limit (M Ch). However, the origin of EC-SNe is still uncertain. In this paper, we systematically studied the EC-SNe in NS + He star systems by considering the explosive oxygen burning that may occur in the near- M Ch ONe core. We provided the initial parameter spaces for producing EC-SNe in the initial orbital period − initial He star mass (log |$P_{\rm orb}^{\rm i}-M_{\rm He}^{\rm i}$|⁠) diagram, and found that both |$M_{\rm He}^{\rm i}$| and minimum |$P_{\rm orb}^{\rm i}$| for EC-SNe increase with metallicity. Then, by considering NS kicks added to the newborn NS, we investigated the properties of the formed DNS systems after the He star companions collapse into NSs, such as the orbital periods, eccentricities, and spin periods of recycle pulsars (P spin), etc. The results show that most of the observed DNS systems can be produced by NS kicks of |$\lesssim$| 50 km s−1. In addition, we found that NSs could accrete more material if the residual H envelope on the He star companions is considered, which can form the mildly recycled pulsars (P spin ∼ 20 ms) in DNS systems. [ABSTRACT FROM AUTHOR]

The disc instability mechanism (DIM) is widely accepted to account for the transient behaviour of dwarf novae (DNe), which experience short outbursts separated by long quiescence. The duty cycle (the ratio between the outburst duration and the recurrence time) determines the amount of accreted mass by the white dwarf (WDs) during outbursts, thus playing an important role in the long-term binary evolution. Employing the code of Modules for Experiments in Stellar Astrophysics, we systemically investigate the influence of the duty cycles on the evolution of DNe and the mass growth of accreting carbon-oxygen (CO) WDs. Our calculations show that, while the DIM can considerably influence the accretion process, efficient WD-mass growth requires a particular range of the duty cycle. For WDs with the initial masses of 0.6, 0.7, and 1.1 M⊙, these duty cycles are 0.006 |$\, \le$| d |$\, \le$| 0.007, d   =  0.005, and , d   = 0.003 and the accumulated mass of the WDs can reach 0.1, 0.13, and 0.21 M⊙, respectively. In all of our simulations, no CO WDs can grow their masses to the explosion mass of Type Ia supernovae of about 1.38 M⊙. Because of a much short time-scale of the outburst state, the final donor-star masses and orbital periods are insensitive to the duty cycles. Therefore, we propose that the DIM in DNe could alleviate the WD mass problem to some extent. [ABSTRACT FROM AUTHOR]

Ultracompact X-ray binaries (UCXBs) are low-mass X-ray binaries with hydrogen-deficient mass donors and ultrashort orbital periods. They have been suggested to be the potential Laser Interferometer Space Antenna (LISA) sources in the low-frequency region. Several channels for the formation of UCXBs have been proposed so far. In this paper, we carried out a systematic study on the He star donor channel, in which a neutron star (NS) accretes matter from a He main-sequence (MS) star through Roche lobe overflow, where the mass transfer is driven by the gravitational wave radiation. First, we followed the long-term evolution of the NS+He MS binaries by employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), and thereby obtained the initial parameter spaces for the production of UCXBs. We then used these results to perform a detailed binary population synthesis approach to obtain the Galactic rates of UCXBs through this channel. We estimate the Galactic rates of UCXBs appearing as LISA sources to be |${\sim} 3.1\!-\!11.9\, \rm Myr^{-1}$| through this channel, and the number of such UCXB- LISA sources in the Galaxy can reach about 1–26 calibrated by observations. This work indicates that the He star donor channel may contribute significantly to the Galactic UCXB formation rate. We found that the evolutionary tracks of UCXBs through this channel can account for the location of the five transient sources with relatively long orbital periods quite well. We also found that such UCXBs can be identified by their locations in the mass-transfer rate versus the orbital period diagram. [ABSTRACT FROM AUTHOR]

PSR J0636+5128 is a 2.87 ms binary millisecond pulsar (MSP) discovered by the Green Bank Northern Celestial Cap Pulsar Survey, and possesses the third shortest orbital period (P  = 1.6 h) among confirmed binary pulsars. Recent observations reported that this source is experiencing a rapid orbital expansion at a rate of |$\dot{P}=(1.89\pm 0.05)\times 10^{-12}\,\rm s\, s^{-1}$|⁠. The evaporation winds of the companion induced by the spin-down luminosity of the MSP may be responsible for such a positive orbital period derivative. However, our calculations show that the winds ejecting from the vicinity of the companion or the inner Lagrangian point cannot account for the observation due to an implausible evaporation efficiency. Assuming that the evaporation winds eject from the vicinity of the MSP in the form of asymmetric disc winds or outflows, the evaporation efficiency can be constrained to be ∼0.1. Therefore, the rapid orbital expansion detected in PSR J0636+5128 provides evidence of outflows and accretion disc around the MSP. [ABSTRACT FROM AUTHOR]

The Hubble Space Telescope observation of the double white dwarf (WD) binary SDSS J125733.63+542850.5 reveals that the massive WD has a surface gravity log g1 ~ 8.7 (which implies a mass of M1 ~ 1.06 M⊙) and an effective temperature T1 ~ 13 000 K, while the effective temperature of the low-mass WD (M2 < 0.24 M⊙) is T2 ~ 6400K. Therefore, the massive and the low-mass WDs have a cooling age τ1 ~ 1 Gyr and τ2 ≥ 5 Gyr, respectively. This is in contradiction with traditional binary evolution theory. In this paper, we propose a strange dwarf (SD) scenario to explain the formation of this double WD binary. We assume that the massive WD is a SD originating from a phase transition (PT) in a ~1.1 M⊙ WD, which has experienced accretion and spin-down processes. Its high effective temperature could arise from the heating process during the PT. Our simulations suggest that the progenitor of SDSS J125733.63+542850.5 can be a binary system consisting of a 0.65 M⊙ WD and a 1.5 M⊙ main-sequence star in a 1.492 d orbit. Especially, the secondary star (i.e. the progenitor of the low-mass WD) is likely to have an ultra-low metallicity of Z = 0.0001. [ABSTRACT FROM AUTHOR]

Recent timing analysis reveals that the orbital period of the first-discovered accreting millisecond pulsar SAX J1808.4-3658 is increasing at a rate Ṗorb = (3.89 ± 0.15) × 10-12 s s-1, which is at least one order of magnitude higher than the value arising from the conservative mass transfer. An ejection of mass-loss rate of 10-9Mʘ yr-1 from the donor star at the inner Lagrangian point during the quiescence state could interpret the observed orbital-period derivative. However, it is unknown whether this source can offer such a high mass-loss rate. In this work, we attempt to investigate an evolutionary channel towards SAX J1808.4-3658. Once the accretion disc becomes thermally and viscously unstable, the spin-down luminosity of the millisecond pulsar and the X-ray luminosity during outbursts are assumed to evaporate the donor star, and the resulting winds carry away the specific orbital angular momentum at the inner Lagrangian point. Our scenario could yield the observed orbital period, the orbital-period derivative, and the peak X-ray luminosity during outbursts. Low-mass X-ray binaries with a 1.0Mʘ donor star, and an orbital period in the range of 0.8-1.5 d, may be the progenitor of SAX J1808.4-3658. Our numerical calculations propose that the current donor-star mass is 0.044Mʘ, which is approximately in agreement with the minimum mass of the donor star. In addition, our scenario can also account for the formation of black widows or the diamond planets like PSR J1719-1438. [ABSTRACT FROM AUTHOR]

It is difficult for the intermediate-mass X-ray binaries (IMXBs) evolutionary channel to form intermediate-mass binary pulsars (IMBPs) with a short orbital period (less than 3 d) via stable mass transfer. The main reason is that the magnetic braking mechanisms are generally thought not to work for donor stars with a mass of greater than 1.5 M⊙ in the canonical model. However, some intermediate-mass stars have anomalously strong magnetic fields (about 100–10 000 G), i.e. so-called Ap or Bp stars. With the coupling between the magnetic field and the irradiation-driven wind from the surface of Ap/Bp stars, a plausible magnetic braking mechanism should be expected. In this work, we attempt to investigate if IMXBs with Ap/Bp stars can produce IMBPs with a short orbital period (less than 3 d) by such an anomalous magnetic braking mechanism. Using a stellar evolution code, we have simulated the evolution of a large number of IMXBs consisting of a neutron star (NS) and an Ap/Bp star. For the spin evolution of the NS, we consider the accretion torque, the propeller torque and the spin-down torque caused by the interaction between the magnetic field and the accretion disc. The calculated results show that, employing anomalous magnetic braking of Ap/Bp stars, IMXBs can evolve into compact IMBPs with short orbital periods of less than 3 d. However, there exists significant discrepancy between the spin periods of IMBPs in our simulated results and those observed. [ABSTRACT FROM AUTHOR]