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Search Results: 1 - 10 of 102457 matches for " Wen-Cong Chen "
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Can angular momentum loss cause the period change of NN Ser?
Wen-Cong Chen
Physics , 2009, DOI: 10.1051/0004-6361/200911638
Abstract: NN Ser is a non mass-transferring pre-cataclysmic variable containing a white dwarf with a mass of $\sim 0.5 M_{\odot}$ and an M dwarf secondary star with a mass of $\sim 0.2 M_{\odot}$. Based on the data detected by the high-speed CCD camera ULTRACAM, it was observed that the orbital period of NN Ser is decreasing, which may be caused by a genuine angular momentum loss or the presence of a third body. However, neither gravitational radiation and magnetic braking can ideally account for the period change of NN Ser. In this Letter, we attempt to examine a feasible mechanism which can drain the angular momentum from NN Ser. We propose that a fossil circumbinary disk (CB disk) around the binary may have been established at the end of the common envelope phase, and the tidal torques caused by the gravitational interaction between the disk and the binary can efficiently extract the orbital angular momentum from the system. We find that only if M dwarf has an ultra-high wind loss rates of $\sim 10^{-10} M_{\odot} \rm yr^{-1}$, and a large fraction ($\delta\sim 10 %$) of wind loss is fed into the CB disk, the loss rates of angular momentum via the CB disk can interpret the period change observed in NN Ser. Such a wind loss rate and $\delta$-value seem to be incredible. Hence it seems that the presence of a third body in a long orbit around the binary might account for the changing period of NN Ser.
On the origin of orbital period change in WY Cancri: a genuine angular momentum loss?
Wen-Cong Chen
Physics , 2013, DOI: 10.1093/pasj/65.3.L4
Abstract: WY Cancri is a short-period ($P$=0.829 d) eclipsing RS Canum Venaticorum stars, and both components are late-type stars. Recently, observations provided by photometric observations and light time minima show that the orbital period of WY Cancri is experiencing a secular decrease at a rate of ${\rm d} P/{\rm d}t=-1.2\times10^{-7}\rm d yr^{-1}$. In this Letter, we attempt to investigate if the period change of WY Cancri can originate from the angular momentum loss. In calculation, we assume that this source has a high wind loss rate of $\sim10^{-10} \rm M_{\odot} yr^{-1}$. To account for the observation, magnetic braking demands a strong surface magnetic field of $\gtrsim 10000$ G like Ap/Bp stars. Furthermore, if this source may be surrounded by a circumbinary disk, and 6% of the wind loss feeds the disk, tidal torque between the disk and the binary can offer the observed angular momentum loss rate. Such a strong magnetic field or an extremely high wind input fraction seem to be highly unlikely.
The progenitor of binary millisecond radio pulsar PSR J1713+0747 (Research Note)
Wen-Cong Chen,Jorge A. Panei
Physics , 2011, DOI: 10.1051/0004-6361/201014833
Abstract: PSR J1713+0747 is a binary system comprising millisecond radio pulsar with a spin period of 4.57 ms, and a low-mass white dwarf (WD) companion orbiting the pulsar with a period of 67.8 days. Using the general relativistic Shapiro delay, the masses of the WD and pulsar components were previously found to be $0.28\pm 0.03 M_{\odot}$ and $1.3\pm 0.2 M_{\odot}$ (68% confidence), respectively. Standard binary evolution theory suggests that PSR J1713+0747 evolved from a low-mass X-ray binary (LMXB). Here, we test this hypothesis. We used a binary evolution code and a WD evolution code to calculate evolutionary sequences of LMXBs that could result in binary millisecond radio pulsars such as PSR J1713+0747. During the mass exchange, the mass transfer is nonconservative. Because of the thermal and viscous instabilities developing in the accretion disk, the neutron star accretes only a small part of the incoming material. We find that the progenitor of PSR J1713+0747 can be modelled as an LMXB including a donor star with mass $1.3-1.6 M_{\odot}$ and an initial orbital period ranging from 2.40 to 4.15 days. If the cooling timescale of the WD is 8 Gyr, its present effective temperature is between 3870 and 4120 K, slightly higher than the observed value. We estimate a surface gravity of ${\rm Log} (g) \approx 7.38 - 7.40$.
On the progenitors of super-Chandrasekhar mass type Ia supernovae
Wen-Cong Chen,Xiang-Dong Li
Physics , 2009, DOI: 10.1088/0004-637X/702/1/686
Abstract: Type Ia supernovae (SNe Ia) can be used as the standard candlelight to determine the cosmological distances because they are thought to have a uniform fuel amount. Recent observations of several overluminous SNe Ia suggest that the white dwarf masses at supernova explosion may significantly exceed the canonical Chandrasekhar mass limit. These massive white dwarfs may be supported by rapid differential rotation. Based on single degenerate model and the assumption that the white dwarf would differentially rotate when the accretion rate $\dot{M}>3\times10^{-7}M_{\odot}\rm yr^{-1}$, we have calculated the evolutions of close binaries consisting of a white dwarf and a normal companion. To include the effect of rotation, we introduce an effective mass $M_{\rm eff}$ for white dwarfs. For the donor stars with two different metallicities $Z=0.02$ and 0.001, we present the distribution of the initial donor star masses and the orbital periods of the progenitors of super-Chandrasekhar mass SNe Ia. The calculation results indicate that, for an initial massive white dwarf of $1.2M_{\odot}$, a considerable fraction of SNe Ia may result from super-Chandresekhar mass white dwarfs, but very massive ($>1.7 M_{\sun}$) white dwarfs are difficult to form, and none of them could be found in old populations. However, super-Chandrasekhar mass SNe Ia are very rare when the initial mass of white dwarfs is $1.0M_{\odot}$. Additionally, SNe Ia in low metallicity environment are more likely to be homogeneous.
On the progenitors of millisecond pulsars by the recycling evolutionary channel
Wei-Min Liu,Wen-Cong Chen
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19202.x
Abstract: The recycling model suggested that low-mass X-ray binaries (LMXBs) could evolve into binary millisecond pulsars (BMSPs). In this work, we attempt to investigate the progenitor properties of BMSPs formed by the recycling evolutionary channel, and if sub-millisecond pulsars can be produced by this channel. Using Eggleton's stellar evolution code, considering that the dead pulsars can be spun up to a short spin period by the accreting material and angular momentum from the donor star, we have calculated the evolution of close binaries consisting of a neutron star and a low-mass main-sequence donor star, and the spin evolution of NSs. In calculation, some physical process such as the thermal and viscous instability of a accretion disk, propeller effect, and magnetic braking are included. Our calculated results indicate that, all LMXBs with a low-mass donor star of 1.0 - 2.0 $M_\odot$ and a short orbital period ($\la 3-4 \rm d$) can form millisecond pulsars with a spin period less than 10 ms. However, it is difficult to produce sub-millisecond pulsars by this evolutionary channel. In addition, our evolutionary scenario cannot account for the existence of BMSPs with a long orbital period ($P_{\rm orb}\ga 70-80\rm d$).
Magnetic braking of Ap/Bp stars: an alternative formation mechanism of compact intermediate-mass binary pulsars
Wei-Min Liu,Wen-Cong Chen
Physics , 2014, DOI: 10.1093/mnras/stu823
Abstract: 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 $\rm M_{\odot}$ in the canonical model. However, some intermediate-mass stars have anomalously strong magnetic fields (about 100 -- 10000 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 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.
Evolution of neutron star + He star binaries: an alternative evolutionary channel to intermediate-mass binary pulsars
Wen-Cong Chen,Wei-Min Liu
Physics , 2013, DOI: 10.1093/mnrasl/slt043
Abstract: It is difficult for intermediate-mass X-ray binaries to form compact intermediate-mass binary pulsars (IMBPs) with a short orbital-period ($\la 3 \rm d$), which have a heavy ($\ga 0.4 M_{\odot}$) CO or ONeMg white dwarf companions. Since neutron star + He star binaries may experience common-envelope evolution, they have some advantage to account for the formation of short orbital-period IMBPs. In this work, we explore the probability of IMBPs formed by this evolutionary channel. Using Eggleton's stellar evolution code, considering that the dead pulsars were spun up by the accreting material and angular momentum from the He star companions, we have calculated the evolution of a large number of neutron star + He star binaries. Our simulated results indicate that, the NS + He star evolutionary channel can produce IMBPs with a WD of $\sim0.5 - 1.1 M_{\odot}$ and an orbital period of $0.03 - 20$ d, in which pulsars have a spin-period of $1.4 - 200$ ms. Comparing the calculated results with the observational parameters (spin period and orbital period) of 9 compact IMBPs, the NS + He star evolutionary channel can account for the formation of 4 sources. Therefore, NS + He star binaries offer an alternative evolutionary channel to compact IMBPs.
Low braking index of PSR J1734-3333: an interaction between fall-back disk and magnetic field?
Wen-Cong Chen,Xiang-Dong Li
Physics , 2015, DOI: 10.1093/mnrasl/slv152
Abstract: Recent timing observation reported that the radio pulsar PSR J1734 - 3333 with a rotating period $P=1.17~\rm s$ is slowing down with a period derivative $\dot{P}=2.28\times 10^{-12}\rm s\,s^{-1}$. Its derived braking index $n=0.9 \pm 0.2$ is the lowest value among young radio pulsars with the measured braking indices. In this Letter, we attempt to investigate the influence of the braking torque caused by the interaction between the fall-back disk and the strong magnetic field of the pulsar on the spin evolution of PSR J1734 - 3333. Analytical result show that this braking torque is obviously far more than that by magnetic dipole radiation for pulsars with spin period of $> 0.1$ s, and play an important role during the spin-down of the pulsars. Our simulated results indicate that, for some typical neutron star parameters, the braking index and the period derivative approximately in agreement with the measured value of PSR J1734 - 3333 if the material inflow rate in the fallback disk is $2 \times 10^{17} \rm g\,s^{-1}$. In addition, our scenario can account for the measured braking indices of four young pulsars. However, our predicted X-ray luminosity are 1 -2 order of magnitude higher than the observation. We proposed that this discrepancy may originate from the instability of fall-back disk.
Orbital period decay of compact black hole x-ray binaries: the influence of circumbinary disks?
Wen-Cong Chen,Xiang-Dong Li
Physics , 2015, DOI: 10.1051/0004-6361/201526524
Abstract: Recently, compact black hole X-ray binaries XTE J 1118+480 and A0620-00 have been reported to be experiencing a fast orbital period decay, which is two orders of magnitude higher than expected with gravitational wave radiation. Magnetic braking of an Ap/Bp star has been suggested to account for the period change when the surface magnetic field of the companion star $B_{\rm s}\ga 10^{4}$ G. However, our calculation indicates that anomalous magnetic braking cannot significantly contribute to the large orbital period decay rates observed in these two sources even if $B_{\rm s}\ga 10^{4}$ G. Observations have provided evidence that circumbinary disks around two compact black hole X-ray binaries may exist. Our analysis shows that, for some reasonable parameters, tidal torque between the circumbinary disk and the binary can efficiently extract the orbital angular momentum from the binary, and result in a large orbital period change rate. Based on the circumbinary disk model, we simulate the evolution of XTE J 1118+480 via a stellar evolution code. Our computations are approximatively in agreement with the observed data (the masses of two components, donor star radius, orbital period, and orbital period derivative). The mass transfer rate and circumbinary disk mass are obviously far greater than the inferred values from observations. Therefore, it seems that the circumbinary disk is unlikely to be the main cause of the rapid orbital decay observed in some compact black hole X-ray binaries.
Ultralong distance coupling between asymmetric resonant microcavities
Fang-Jie Shu,Chang-Ling Zou,Wen-Cong Chen,Fang-Wen Sun
Physics , 2013, DOI: 10.1364/JOSAB.31.000478
Abstract: The ultralong distance coupling between two Asymmetric Resonant Microcavities (ARCs) is studied. Different from traditional short distance tunneling coupling between microcavities, the high efficient free space directional emission and excitation allow ultralong distance energy transfer between ARCs. In this paper, a novel unidirectional emission ARC, which shows directionality I40 = 0.54, is designed for materials of refractive index n = 2.0. Compared with regular whispering gallery microresonators, the coupled unidirectional emission ARCs show modulations of resonance frequency and linewidth even when the distance between cavities is much longer than wavelength. The performances and properties of the ultralong distance interaction between ARCs are analyzed and studied by coupling mode theory in details. The ultralong distance interaction between ARCs provides a new way to free-space based optical interconnects between components in integrated photonic circuits.
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