arXiv subjects

Dong Lai

Publications and source records attributed to Dong Lai.

At least 181 records · Page 10Linked to original sources

arXiv2012

High-Frequency QPOs and Overstable Oscillations of Black-Hole Accretion Disks

The physical origin of high-frequency QPOs (HFQPOs) in black-hole X-ray binaries remains an enigma despite many years of detailed observational studies. Although there exists a number of models for HFQPOs, many of these are simply "notions" or "concepts" without actual calculation derived from fluid or disk physics. Future progress requires a combination of numerical simulations and semi-analytic studies to extract physical insights. We review recent works on global oscillation modes in black-hole accretion disks, and explain how, with the help of general relativistic effects, the energy stored in the disk differential rotation can be pumped into global spiral density modes in the disk, making these modes grow to large amplitudes under certain conditions ("corotational instability"). These modes are robust in the presence of disk magnetic fields and turbulence. The computed oscillation mode frequencies are largely consistent with the observed values for HFQPOs in BH X-ray binaries. The approximate 2:3 frequency ratio is also expected from this model. The connection of HFQPOs with other disk properties (such as production of episodic jets) is also discussed.

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arXiv2012

Dynamical Tides in Compact White Dwarf Binaries: Helium Core White Dwarfs, Tidal Heating, and Observational Signatures

Tidal dissipation in compact white dwarf (WD) binary systems significantly influences the physical conditions (such as surface temperature and rotation rate) of the WDs prior to mass transfer or merger. In these systems, the dominant tidal effects involve the excitation of gravity waves and their dissipation in the outer envelope of the star. We calculate the amplitude of tidally excited gravity waves in low-mass (0.3M_\odot) helium-core (He) WDs as a function of the tidal forcing frequency ω. Like carbon-oxygen (CO) WDs studied in our previous paper, we find that the dimensionless tidal torque F(ω) (inversely proportional to the effective tidal quality factor) has an erratic dependence on ω. On average, F(ω) scales approximately as ω^6, and is several orders of magnitude smaller for He WDs than for CO WDs. We find that tidal torques can begin to synchronize the WD rotation when the orbital period is less than about a hour, although a nearly constant asynchronization is maintained even at small periods. We examine where the tidally excited gravity waves experience non-linear breaking or resonant absorption at a critical layer, allowing us to estimate the location and magnitude of tidal heating in the WD envelope. We then incorporate tidal heating in the MESA stellar evolution code, calculating the physical conditions of the WD as a function of orbital period for different WD models. We find that tidal heating makes a significant contribution to the WD luminosity for short-period (~10 min) systems such as SDSS J0651+2844. We also find that for WDs containing a hydrogen envelope, tidal heating can trigger runaway hydrogen shell burning, leading to a nova-like event before the onset of mass transfer.

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arXiv2012

Simulations of Overstable Inertial-acoustic Modes in Black-Hole Accretion Discs

We present two-dimensional inviscid hydrodynamic simulations of overstable inertial-acoustic oscillation modes (p-modes) in black-hole accretion discs. These global spiral waves are trapped in the inner-most region of the disc, and are driven overstable by wave absorption at the corotation resonance ($r_c$) when the gradient of the background disc vortensity (vorticity divided by surface density) at $r_c$ is positive and the disc inner boundary is sufficiently reflective. Previous linear calculations have shown that the growth rates of these modes can be as high as 10% of the rotation frequency at the disc inner edge. We confirm these linear growth rates and the primary disc oscillation frequencies in our simulations when the mode amplitude undergoes exponential growth. We show that the mode growth saturates when the radial velocity perturbation becomes comparable to the disc sound speed. During the saturation stage, the primary disc oscillation frequency differs only slightly (by less than a few percent) from the linear mode frequency. Sharp features in the fluid velocity profiles at this stage suggest that the saturation results from nonlinear wave steepening and mode-mode interactions.

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arXiv2012

Rossby Wave Instability in Accretion Discs with Large-Scale Poloidal Magnetic Fields

We study the effect of large-scale magnetic fields on the non-axisymmetric Rossby wave instability (RWI) in accretion discs. The instability develops around a density bump, which is likely present in the transition region between the active zone and dead zone of protoplanetary discs. Previous works suggest that the vortices resulting from the RWI may facilitate planetesimal formation and angular momentum transport. We consider discs threaded by a large-scale poloidal magnetic field, with a radial field component at the disc surface. Such field configurations may lead to the production of magnetic winds or jets. In general, the magnetic field can affect the RWI even when it is sub-thermal (plasma $β\sim 10$). For infinitely thin discs, the instability can be enhanced by about 10 percent. For discs with finite thickness, with a radial gradient of the magnetic field strength, the RWI growth rate can increase significantly (by a factor of $\sim 2$) as the field approaches equipartition ($β\sim 1$). Our result suggests that the RWI can continue to operate in discs that produce magnetic winds.

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arXiv2012

Tidal Novae in Compact Binary White Dwarfs

Compact binary white dwarfs (WDs) undergoing orbital decay due to gravitational radiation can experience significant tidal heating prior to merger. In these WDs, the dominant tidal effect involves the excitation of outgoing gravity waves in the inner stellar envelope and the dissipation of these waves in the outer envelope. As the binary orbit decays, the WDs are synchronized from outside in (with the envelope synchronized first, followed by the core). We examine the deposition of tidal heat in the envelope of a Carbon-Oxygen WD and study how such tidal heating affects the structure and evolution of the WD. We show that significant tidal heating can occur in the star's degenerate hydrogen layer. This layer heats up faster than it cools, triggering runaway nuclear fusion. Such "tidal novae" may occur in all WD binaries containing a CO WD, at orbital periods between 5 min and 20 min, and precede the final merger by 10^5-10^6 years.

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arXiv2012

Tidal Excitation of Oscillation Modes in Compact White Dwarf Binaries: I. Linear Theory

We study the tidal excitation of gravity modes (g-modes) in compact white dwarf binary systems with periods ranging from minutes to hours. As the orbit of the system decays via gravitational radiation, the orbital frequency increases and sweeps through a series of resonances with the g-modes of the white dwarf. At each resonance, the tidal force excites the g-mode to a relatively large amplitude, transferring the orbital energy to the stellar oscillation. We calculate the eigenfrequencies of g-modes and their coupling coefficients with the tidal field for realistic non-rotating white dwarf models. Using these mode properties, we numerically compute the excited mode amplitude in the linear approximation as the orbit passes though the resonance, including the backreaction of the mode on the orbit. We also derive analytical estimates for the mode amplitude and the duration of the resonance, which accurately reproduce our numerical results for most binary parameters. We find that the g-modes can be excited to a dimensionless (mass-weighted) amplitude up to 0.1, with the mode energy approaching $10^{-3}$ of the gravitational binding energy of the star. This suggests that thousands of years prior to the binary merger, the white dwarf may be heated up significantly by tidal interactions. However, more study is needed since the physical amplitudes of the excited oscillation modes become highly nonlinear in the outer layer of the star, which can reduce the mode amplitude attained by tidal excitation.

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arXiv2012

Dynamical Tides in Compact White Dwarf Binaries: Tidal Synchronization and Dissipation

In compact white dwarf (WD) binary systems (with periods ranging from minutes to hours), dynamical tides involving the excitation and dissipation of gravity waves play a dominant role in determining the physical conditions of the WDs prior to mass transfer or binary merger. We calculate the amplitude of the tidally excited gravity waves as a function of the tidal forcing frequency ω=2(Ω-Ω_s) (where Ωis the orbital frequency and Ω_s is the spin frequency) for several realistic carbon-oxygen WD models, assuming that the waves are efficiently dissipated in the outer layer of the star by nonlinear effects or radiative damping. The mechanism of wave excitation in WDs is complex due to the sharp features associated with composition changes inside the WD, and in our WD models gravity waves are launched just below the helium-carbon boundary. We find that the tidal torque on the WD and the related tidal energy transfer rate, \dot E_{\rm tide}, depend on ωin an erratic way. On average, \dot E_{\rm tide} scales approximately as Ω^5ω^5 for a large range of tidal frequencies. We also study the effects of dynamical tides on the long-term evolution of WD binaries. Above a critical orbital frequency Ω_c, corresponding to an orbital period of order one hour (depending on WD models), dynamical tides efficiently drive Ω_s toward Ω, although a small, almost constant degree of asynchronization (Ω-Ω_s\sim {\rm constant}) is maintained even at the smallest binary periods. While the orbital decay is always dominated by gravitational radiation, the tidal energy transfer can induce significant phase error in the low-frequency gravitational waveforms, detectable by the planned LISA project. Tidal dissipation may also lead to significant heating of the WD envelope and brightening of the system long before binary merger.

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arXiv2012

Tidal Dissipation in Planet-Hosting Stars: Damping of Spin-Orbit Misalignment and Survival of Hot Jupiters

Observations of hot Jupiters around solar-type stars with very short orbital periods (~day) suggest that tidal dissipation in such stars is not too efficient so that these planets can survive against rapid orbital decay. This is consistent with recent theoretical works, which indicate that the tidal Q of planet-hosting stars can indeed be much larger than the values inferred from stellar binaries. On the other hand, recent measurements of Rossiter-McLaughlin effects in transiting hot Jupiter systems not only reveal that many such systems have misaligned stellar spin with respect to the orbital axis, but also show that systems with cooler host stars tend to have aligned spin and orbital axes. Winn et al. suggested that this obliquity - temperature correlation may be explained by efficient damping of stellar obliquity due to tidal dissipation in the star. This explanation, however, is in apparent contradiction with the survival of these short-period hot Jupiters. We show that in the solar-type parent stars of close-in exoplanetary systems, the effective tidal Q governing the damping of stellar obliquity can be much smaller than that governing orbital decay. This is because for misaligned systems, the tidal potential contains a Fourier component with frequency equal to the stellar spin frequency (in the rotating frame of the star). This component can excite inertial waves in the convective envelope of the star, and the dissipation of inertial waves then leads to a spin-orbit alignment torque, but not orbital decay. By contrast, for aligned systems, such inertial wave excitation is forbidden since the tidal forcing frequency is much larger than the stellar spin frequency. We derive a general effective tidal evolution theory for misaligned binaries, taking account of different tidal responses and dissipation rates for different tidal forcing components.

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arXiv2012

DC Circuit Powered by Orbital Motion: Magnetic Interactions in Compact Object Binaries and Exoplanetary Systems

The unipolar induction DC circuit model, originally developed by Goldreich & Lynden-Bell for the Jupiter-Io system, has been applied to different types of binary systems in recent years. We show that there exists an upper limit to the magnetic interaction torque and energy dissipation rate in such model. This arises because when the resistance of the circuit is too small, the large current flow severely twists the magnetic flux tube connecting the two binary components, leading to breakdown of the circuit. Applying this limit, we find that in coalescing neutron star binaries, magnetic interactions produce negligible correction to the phase evolution of the gravitational waveform, even for magnetar-like field strengths. However, energy dissipation in the binary magnetosphere may still give rise to electromagnetic radiation prior to the final merger. For ultra-compact white dwarf binaries, we find that DC circuit does not provide adequate energy dissipation to explain the observed X-ray luminosities of several sources. For exoplanetary systems containing close-in Jupiters or super-Earths, magnetic torque and dissipation are negligible, except possibly during the early T Tauri phase, when the stellar magnetic field is stronger than 10^3G.

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arXiv2012

The Hidden Magnetic Field of The Young Neutron Star in Kesteven 79

Recent observations of the central compact object in the Kesteven 79 supernova remnant show that this neutron star (NS) has a weak dipole magnetic field (a few x 10^{10} G) but an anomalously large (~ 64%) pulse fraction in its surface X-ray emission. We explore the idea that a substantial sub-surface magnetic field exists in the NS crust, which produces diffuse hot spots on the stellar surface due to anisotropic heat conduction, and gives rise to the observed X-ray pulsation. We develop a general-purpose method, termed "Temperature Template with Full Transport" (TTFT), that computes the synthetic pulse profile of surface X-ray emission from NSs with arbitrary magnetic field and surface temperature distributions, taking into account magnetic atmosphere opacities, beam pattern, vacuum polarization, and gravitational light bending. We show that a crustal toroidal magnetic field of order a few x 10^{14} G or higher, varying smoothly across the crust, can produce sufficiently distinct surface hot spots to generate the observed pulse fraction in the Kes 79 NS. This result suggests that substantial sub-surface magnetic fields, much stronger than the "visible" dipole fields, may be buried in the crusts of some young NSs, and such hidden magnetic fields can play an important role in their observational manifestations. The general TTFT tool we have developed can also be used for studying radiation from other magnetic NSs.

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arXiv2012

Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities

We study global non-axisymmetric oscillation modes and instabilities in magnetosphere- disc systems, as expected in neutron star X-ray binaries and possibly also in accreting black hole systems. Our two-dimensional magnetosphere-disc model consists of a Keplerian disc in contact with an uniformly rotating magnetosphere with low plasma density. Two types of global overstable modes exist in such systems, the interface modes and the disc inertial-acoustic modes. We examine various physical effects and parameters that influence the properties of these oscillation modes, particularly their growth rates, including the magnetosphere field configuration, the velocity and density contrasts across the magnetosphere-disc interface, the rotation profile (with Newtonian or General Relativistic potential), the sound speed and magnetic field of the disc. The interface modes are driven unstable by Rayleigh-Taylor and Kelvin-Helmholtz in- stabilities, but can be stabilized by the toroidal field (through magnetic tension) and disc differential rotation (through finite vorticity). General relativity increases their growth rates by modifying the disc vorticity outside the magnetosphere boundary. The interface modes may also be affected by wave absorption associated with corotation resonance in the disc. In the presence of a magnetosphere, the inertial-acoustic modes are effectively trapped at the innermost region of the relativistic disc just outside the interface. They are driven unstable by wave absorption at the corotation resonance, but can be stabilized by modest disc magnetic fields. The overstable oscillation modes studied in this paper have characteristic properties that make them possible candidates for the quasi-periodic oscillations observed in X-ray binaries.

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arXiv2011

Dynamical Tides in Eccentric Binaries and Tidally-Excited Stellar Pulsations in KEPLER KOI-54

Recent observation of the tidally-excited stellar oscillations in the main-sequence binary KOI-54 by the KEPLER satellite provides a unique opportunity for studying dynamical tides in eccentric binary systems. We develop a general theory of tidal excitation of oscillation modes of rotating binary stars, and apply our theory to tidally excited gravity modes (g-modes) in KOI-54. The strongest observed oscillations, which occur at 90 and 91 times the orbital frequency, are likely due to prograde m=2 modes (relative to the stellar spin axis) locked in resonance with the orbit. The remaining flux oscillations with frequencies that are integer multiples of the orbital frequency are likely due to nearly resonant m=0 g-modes; such axisymmetric modes generate larger flux variations compared to the m=2 modes, assuming that the spin inclination angle of the star is comparable to the orbital inclination angle. We examine the process of resonance mode locking under the combined effects of dynamical tides on the stellar spin and orbit and the intrinsic stellar spindown. We show that KOI-54 can naturally evolve into a state in which at least one m=2 mode is locked in resonance with the orbital frequency. Our analysis provides an explanation for the fact that only oscillations with frequencies less than 90-100 times the orbital frequency are observed. We have also found evidence from the published KEPLER result that three-mode nonlinear coupling occurs in the KOI-54 system. We suggest that such nonlinear mode coupling may explain the observed oscillations that are not harmonics of the orbital frequency.

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arXiv2011

Distorted, non-spherical transiting planets: impact on the transit depth and on the radius determination

We quantify the systematic impact of the non-spherical shape of transiting planets and brown dwarfs, due to tidal forces and rotation, on the observed transit depth. Such a departure from sphericity leads to a bias in the derivation of the transit radius from the light curve and affects the comparison with planet structure and evolution models which assume spherical symmetry. As the tidally deformed planet projects its smallest cross section area during the transit, the measured effective radius is smaller than the one of the unperturbed spherical planet. This effect can be corrected by calculating the theoretical shape of the observed planet. We derive simple analytical expressions for the ellipsoidal shape of a fluid object (star or planet) accounting for both tidal and rotational deformations and calibratre it with fully numerical evolution models in the 0.3Mjup-75Mjup mass range. Our calculations yield a 20% effect on the transit depth, i.e. a 10% decrease of the measured radius, for the extreme case of a 1Mjup planet orbiting a Sun-like star at 0.01AU. For the closest planets detected so far (< 0.05 AU), the effect on the radius is of the order of 1 to 10%, by no means a negligible effect, enhancing the puzzling problem of the anomalously large bloated planets. These corrections must thus be taken into account for a correct determination of the radius from the transit light curve. Our analytical expressions can be easily used to calculate these corrections, due to the non-spherical shape of the planet, on the observed transit depth and thus to derive the planet's real equilibrium radius. They can also be used to model ellipsoidal variations of the stellar flux now detected in the CoRoT and Kepler light curves. We also derive directly usable analytical expressions for the moment of inertia, oblateness and Love number (k_2) of a fluid planet as a function of its mass.

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arXiv2011

Faraday Rotation in Pulsar Magnetosphere

The magnetosphere of a pulsar is composed of relativistic plasmas streaming along the magnetic field lines and corotating with the pulsar. We study the intrinsic Faraday rotation in the pulsar magnetosphere by critically examining the wave modes and the variations of polarization properties for the circularly polarized natural modes under various assumptions about the magnetosphere plasma properties. Since it is difficult to describe analytically the Faraday rotation effect in such a plasma, we use numerical integrations to study the wave propagation effects in the corotating magnetosphere. Faraday rotation effect is identified among other propagation effects, such as wave mode coupling and the cyclotron absorption. In a highly symmetrical electron-positron pair plasma, the Faraday rotation effect is found to be negligible. Only for asymmetrical plasmas, such as the electron-ion streaming plasma, can the Faraday rotation effect become significant, and the Faraday rotation angle is found to be approximately proportional to $λ^{0.5}$ instead of the usual $λ^2$-law. For such electrons-ion plasma of pulsar magnetosphere, the induced rotation measure becomes larger at higher frequencies, and should have opposite signs for the emissions from opposite magnetic poles.

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arXiv2010

Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems: II. Warped Discs

Magnetic interactions between a protostar and its accretion disc tend to induce warping in the disc and produce secular changes in the stellar spin direction, so that the spin axis may not always be perpendicular to the disc. This may help explain the recently observed spin-orbit misalignment in a number of exoplanetary systems. We study the dynamics of warped protoplanetary discs under the combined effects of magnetic warping/precession torques and internal stresses in the disc, including viscous damping of warps and propagation of bending waves. We show that when the outer disc axis is misaligned with the stellar spin axis, the disc evolves towards a warped steady-state on a timescale that depends on the disc viscosity or the bending wave propagation speed, but in all cases is much shorter than the timescale for the spin evolution (of order of a million years). Moreover, for the most likely physical parameters characterizing magnetic protostars, circumstellar discs and their interactions, the steady-state disc has a rather small warp, such that the whole disc lies approximately in a single plane determined by the outer disc boundary conditions, although more extreme parameters may give rise to larger disc warps. In agreement with our recent analysis (Lai et al. 2010) based on flat discs, we find that the back-reaction magnetic torques of the slightly warped disc on the star can either align the stellar spin axis with the disc axis or push it towards misalignment, depending on the parameters of the star-disc system. This implies that newly formed planetary systems may have a range of inclination angles between the stellar spin axis and the symmetry axis of the planetary orbits.

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arXiv2010

Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems

Recent observations have shown that in many exoplanetary systems the spin axis of the parent star is misaligned with the planet's orbital axis. These have been used to argue against the scenario that short-period planets migrated to their present-day locations due to tidal interactions with their natal discs. However, this interpretation is based on the assumption that the spins of young stars are parallel to the rotation axes of protostellar discs around them. We show that the interaction between a magnetic star and its circumstellar disc can (but not always) have the effect of pushing the stellar spin axis away from the disc angular momentum axis toward the perpendicular state and even the retrograde state. Planets formed in the disc may therefore have their orbital axes misaligned with the stellar spin axis, even before any additional planet-planet scatterings or Kozai interactions take place. In general, magnetosphere--disc interactions lead to a broad distribution of the spin--orbit angles, with some systems aligned and other systems misaligned.

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arXiv2010

Low-$T/|W|$ instabilities in differentially rotating proto-neutron stars with magnetic fields

Recent hydrodynamical simulations have shown that differentially rotating neutron stars formed in core-collapse supernovae may develop global non-axisymmetric instabilities even when $T/|W|$ (the ratio of the rotational kinetic energy $T$ to the gravitational potential energy $|W|$) is relatively small (less than 0.1). Such low-$T/|W|$ instability can give rise to efficient gravitational wave emission from the proto-neutron star. We investigate how this instability is affected by magnetic fields using a cylindrical stellar model. Wave absorption at the corotation resonance plays an important role in facilitating the hydrodynamic low-$T/|W|$ instability. In the presence of a toroidal magnetic field, the corotation resonance is split into two magnetic resonances where wave absorptions take place. We show that the toroidal magnetic field suppresses the low-$T/|W|$ instability when the total magnetic energy $W_{\rm B}$ is of order $0.2\,T$ or larger, corresponding to toroidal fields of a few $\times 10^{16}$ G or stronger. Although poloidal magnetic fields do not influence the instability directly, they can affect the instability by generating toroidal fields through linear winding of the initial poloidal field and magneto-rotational instability. We show that an initial poloidal field with strength as small as $10^{14}$ G may suppress the low-$T/|W|$ instability.

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arXiv2010

Papaloizou-Pringle Instability of Magnetized Accretion Tori

Hot accretion tori around a compact object are known to be susceptible to a global hydrodynamical instability, the so-called Papaloizou-Pringle (PP) instability, arising from the interaction of non-axisymmetric waves across the corotation radius, where the wave pattern speed matches the fluid rotation rate. However, accretion tori produced in various astrophysical situations (e.g., collapsars and neutron star binary mergers) are likely to be highly magnetized. We study the effect of magnetic fields on the PP instability in incompressible tori with various magnetic strengths and structures. In general, toroidal magnetic fields have significant effects on the PP instability: For thin tori (with the fractional width relative to the outer torus radius much less than unity), the instability is suppressed at large field strengths with the corresponding toroidal Alfven speed $v_{Aϕ}\go 0.2rΩ$ (where $Ω$ is the flow rotation rate). For thicker tori (with the fractional width of order 0.4 or larger), which are hydrodynamically stable, the instability sets in for sufficiently strong magnetic fields (with $v_{Aϕ}\go 0.2 rΩ$). Our results suggest that highly magnetized accretion tori may be subjected to global instability even when it is stable against the usual magneto-rotational instability.

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