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Dong Lai
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Dong Lai
United States

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Earth and Planetary Astrophysics (29)
High Energy Astrophysical Phenomena (21)
Solar and Stellar Astrophysics (14)
General Relativity and Quantum Cosmology (4)
Cosmology and Nongalactic Astrophysics (2)
High Energy Physics - Phenomenology (1)
Instrumentation and Methods for Astrophysics (1)

Publications Authored By Dong Lai

Hierarchical quadruple systems arise naturally in stellar binaries and triples that harbor planets. Examples are hot Jupiters (HJs) in stellar triple systems, and planetary companions to HJs in stellar binaries. The secular dynamical evolution of these systems is generally complex, with secular chaotic motion possible in certain parameter regimes. Read More

It is well known that asteroids and comets fall into the Sun. Metal pollution of white dwarfs and transient spectroscopic signatures of young stars like $\beta$-Pic provide growing evidence that extra solar planetesimals can attain extreme orbital eccentricities and fall onto their parent stars. We aim to develop a general, practically implementable, semi-analytical theory of secular eccentricity excitation of small bodies in mean motion resonances with an eccentric planet valid for arbitrary values of the eccentricities and including the short-range force due to General Relativity. Read More

Many protostellar gapped and binary discs show misalignments between their inner and outer discs; in some cases, $\sim70$ degree misalignments have been observed. Here we show that these misalignments can be generated through a "secular precession resonance" between the nodal precession of the inner disc and the precession of the gap-opening (stellar or massive planetary) companion. An evolving protostellar system may naturally cross this resonance during its lifetime due to disc dissipation and/or companion migration. Read More

Rocky planets orbiting M-dwarf stars in the habitable zone tend to be driven to synchronous rotation by tidal dissipation, potentially causing difficulties for maintaining a habitable climate on the planet. However, the planet may be captured into asynchronous spin-orbit resonances, and this capture may be more likely if the planet has a sufficiently large intrinsic triaxial deformation. We derive the analytic expression for the maximum triaxiality of a rocky planet, with and without a liquid envelope, as a function of the planet's radius, density, rigidity and critical strain of fracture. Read More

A white dwarf (WD) captured into a high-eccentricity orbit around a massive black hole (MBH) may undergo many pericenter passages before tidal disruption. During these passages, the tidal potential of the MBH excites internal oscillations or waves in the WD, and the dissipation of these oscillations can significantly influence the physical properties of the WD prior to its disruption. We calculate the amplitude of the tidally excited gravity (buoyancy) waves in the WD as a function of the pericenter distance and eccentricity for realistic WD models, under the assumption that these outgoing gravity waves are efficiently dissipated in outer layers of the WD by non-linear effects or radiative damping. Read More

Recent SPH simulations by Martin et al. (2014) suggest a circumstellar gaseous disk may exhibit coherent eccentricity-inclination oscillations due to the tidal forcing of an inclined binary companion, in a manner that resembles Lidov-Kozai oscillations in hierarchical triple systems. We carry out linear stability analysis for the eccentricity growth of circumstellar disks in binaries, including the effects of gas pressure and viscosity and secular (orbital-averaged) tidal force from the inclined companion. Read More

Recent observations of Kepler multi-planet systems have revealed a number of systems with planets very close to second-order mean motion resonances (MMRs, with period ratio $1:3$, $3:5$, etc.) We present an analytic study of resonance capture and its stability for planets migrating in gaseous disks. Resonance capture requires slow convergent migration of the planets, with sufficiently large eccentricity damping timescale $T_e$ and small pre-resonance eccentricities. Read More

We carry out numerical simulations of circumbinary discs, solving the viscous hydrodynamics equations on a polar grid covering an extended disc outside the binary co-orbital region. We use carefully controlled outer boundary conditions and long-term integrations to ensure that the disc reaches a quasi-steady state, in which the time-averaged mass accretion rate onto the binary, $\langle\dot{M}\rangle$, matches the mass supply rate at the outer disc. We focus on binaries with comparable masses and a wide range of eccentricities ($e_\mathrm{B}$). Read More

Eclipsing binaries are observed to have a range of eccentricities and spin-orbit misalignments (stellar obliquities). Whether such properties are primordial, or arise from post-formation dynamical interactions remains uncertain. This paper considers the scenario in which the binary is the inner component of a hierarchical triple stellar system, and derives the requirements that the tertiary companion must satisfy in order to raise the eccentricity and obliquity of the inner binary. Read More

Bailey et al.~(2016) and Gomes et al.~(2016) recently suggested that the 6 degree misalignment between the Sun's rotational equator and the orbital plane of the major planets may be produced by the forcing from the hypothetical Planet Nine on an inclined orbit. Read More

Many exoplanetary systems containing hot Jupiters (HJs) exhibit significant misalignment between the spin axes of the host stars and the orbital angular momentum axes of the planets ("spin-orbit misalignment"). High-eccentricity migration involving Lidov-Kozai oscillations of the planet's orbit induced by a distant perturber is a possible channel for producing such misaligned HJ systems. Previous works have shown that the dynamical evolution of the stellar spin axis during the high-$e$ migration plays a dominant role in generating the observed spin-orbit misalignment. Read More

The {\it Kepler} mission has detected thousands of planetary systems with 1-7 transiting planets packed within 0.7~au from their host stars. There is an apparent excess of single-transit planet systems that cannot be explained by transit geometries alone, when a single planetary mutual inclination dispersion is assumed. Read More

Newly formed planets (or brown dwarfs) may possess disks or rings that occupy an appreciable fraction of the planet's Hill sphere and extend beyond the Laplace radius, where the tidal torque from the host star dominates over the torque from the oblate planet. Such a disk/ring can exhibit unique, detectable transit signatures, provided that the disk/ring is significantly misaligned with the orbital plane of the planet. There exists tentative evidence for an extended ring system around the young K5 star 1 SWASP J140747-354542. Read More

We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ~5 times the binary period P_b,accretion onto an eccentric binary is predominantly modulated at the period ~1P_b. Read More

Planets around binary stars and those in multiplanet systems may experience resonant eccentricity excitation and disruption due to perturbations from a distant stellar companion. This "evection resonance" occurs when the apsidal precession frequency of the planet, driven by the quadrupole associated with the inner binary or the other planets, matches the orbital frequency of the external companion. We develop an analytic theory to study the effects of evection resonance on circumbinary planets and multiplanet systems. Read More

Lidov-Kozai oscillations of planets in stellar binaries, combined with tidal dissipation, can lead to the formation of hot Jupiters (HJs) or tidal disruption of planets. Recent population synthesis studies have found that the fraction of systems resulting in HJs (F_HJ) depends strongly on the planet mass, host stellar type and tidal dissipation strength, while the total migration fraction F_mig = F_ HJ + F_dis (including both HJ formation and tidal disruption) exhibits much weaker dependence. We present an analytical method for calculating F_HJ and F_mig in the Lidov-Kozai migration scenario. Read More

The physical mechanism of angular momentum transport in poorly ionized regions of protoplanetary discs, the dead zones (DZs), is not understood. The presence of a DZ naturally leads to conditions susceptible to the Rossby wave instability (RWI), which produces vortices and spiral density waves that may revive the DZ and be responsible for observed large-scale disc structures. We present a series of two-dimensional hydrodynamic simulations to investigate the role of the RWI in DZs, including its impact on the long-term evolution of the disc and its morphology. Read More

Oscillations between photons and axion-like particles (ALP) travelling in intergalactic magnetic fields have been invoked to explain a number of astrophysical phenomena, or used to constrain ALP properties using observations. One example is the anomalous transparency of the universe to TeV gamma-rays. The intergalactic magnetic field is usually modeled as patches of coherent domains, each with a uniform magnetic field, but the field orientation changes randomly from one domain to the next ("discrete-$\varphi$ model"). Read More

Observed hot Jupiter (HJ) systems exhibit a wide range of stellar spin-orbit misalignment angles. The origin of these HJs remains unclear. This paper investigates the inward migration of giant planets due to Lidov-Kozai (LK) oscillations induced by a distant (100-1000 AU) stellar companion. Read More

We study the secular dynamics of compact binaries (consisting of white dwarfs, neutron stars or black holes) with tertiary companions in hierarchical triple systems. As the inner binary (with initially negligible eccentricity) undergoes orbital decay due to gravitational radiation, its eccentricity can be excited by gravitational forcing from the tertiary. This excitation occurs when the triple system passes through an "apsidal precession resonance", when the precession rate of the inner binary, driven by the gravitational perturbation of the external companion and general relativity, matches the precession rate of the outer binary. Read More

The discovery of transiting circumbinary planets by the Kepler mission suggests that planets can form efficiently around binary stars. None of the stellar binaries currently known to host planets has a period shorter than 7 days, despite the large number of eclipsing binaries found in the Kepler target list with periods shorter than a few days. These compact binaries are believed to have evolved from wider orbits into their current configurations via the so-called Lidov-Kozai migration mechanism, in which gravitational perturbations from a distant tertiary companion induce large-amplitude eccentricity oscillations in the binary, followed by orbital decay and circularization due to tidal dissipation in the stars. Read More

Recent observations have shown that circumstellar and circumbinary discs in young stellar binaries are often misaligned with respect to the binary orbital plane. We analyze the tidal truncation of such misaligned discs due to torques applied to the disc at the Lindblad resonances from the tidal forcings of the binary. We consider eccentric binaries with arbitrary binary-disc inclination angles. Read More

We present analytic calculations of the electromagnetic torques acting on a magnetic neutron star rotating in vacuum, including near-zone torques associated with the inertia of dipole and quadrupole magnetic fields. We incorporate these torques into the rotational dynamics of a rigid-body neutron star, and show that the effects of the inertial torque can be understood as a modification of the moment of inertia tensor of the star. We apply our rotational dynamics equation to the Crab pulsar, including intrinsic distortions of the star and various electromagnetic torques, to investigate the possibility that the counter-alignment of the magnetic inclination angle, as suggested by recent observations, could be explained by pulsar precession. Read More

We present analytical expressions for the tidal Love numbers of a giant planet with a solid core and a fluid envelope. We model the core as a uniform, incompressible, elastic solid, and the envelope as a non-viscous fluid satisfying the $n=1$ polytropic equation of state. We discuss how the Love numbers depend on the size, density, and shear modulus of the core. Read More

We study the effect of large-scale magnetic fields on the non-axisymmetric inertial-acoustic modes (also called p-modes) trapped in the innermost regions of accretion discs around black holes (BHs). These global modes could provide an explanation for the high-frequency quasi-periodic oscillations (HFQPOs) observed in BH X-ray binaries. There may be observational evidence for the presence of such large-scale magnetic fields in the disks since episodic jets are observed in the same spectral state when HFQPOs are detected. Read More

This paper provides an introduction to a number of astrophysics problems related to strong magnetic fields. The first part deals with issues related to atoms, condensed matter and high-energy processes in very strong magnetic fields, and how these issues influence various aspects of neutron star astrophysics. The second part deals with classical astrophysical effects of magnetic fields: Even relatively "weak" fields can play a strong role in various astrophysical problems, ranging from stars, accretion disks and outflows, to the formation and merger of compact objects. Read More

Many exoplanetary systems containing hot Jupiters are found to possess significant misalignment between the spin axis of the host star and the planet's orbital angular momentum axis. A possible channel for producing such misaligned hot Jupiters involves Lidov-Kozai oscillations of the planet's orbital eccentricity and inclination driven by a distant binary companion. In a recent work (Storch, Anderson & Lai 2014), we have shown that a proto-hot Jupiter undergoing Lidov-Kozai oscillations can induce complex, and often chaotic, evolution of the spin axis of its host star. Read More

The Lidov-Kozai (LK) mechanism plays an important role in the secular evolution of many hierarchical triple systems. The standard LK mechanism consists of large-amplitude oscillations in eccentricity and inclination of a binary subject to the quadrupole potential from an outer perturber. Recent work has shown that when the octupole terms are included in the potential, the inner binary can reach more extreme eccentricities as well as undergo orientation flips. Read More

Many exoplanetary systems containing hot Jupiters are observed to have highly misaligned orbital axes relative to the stellar spin axes. Kozai-Lidov oscillations of orbital eccentricity/inclination induced by a binary companion, in conjunction with tidal dissipation, is a major channel for the production of hot Jupiters. We demonstrate that gravitational interaction between the planet and its oblate host star can lead to chaotic evolution of the stellar spin axis during Kozai cycles. Read More

We examine the role played by viscosity in the excitation of global oscillation modes (both axisymmetric and non-axisymmetric) in accretion discs around black holes using two-dimensional hydrodynamic simulations. The turbulent viscosity is modeled by the $\alpha$-ansatz, with different equations of state. We consider both discs with transonic radial inflows across the innermost stable circular orbit, and stationary discs truncated by a reflecting wall at their inner edge, representing a magnetosphere. Read More

Warped accretion discs are expected in many protostellar binary systems. In this paper, we study the long-term evolution of disc warp and precession for discs with dimensionless thickness $H/r$ larger than their viscosity parameter $\alpha$, such that bending waves can propagate and dominate the warp evolution. For small warps, these discs undergo approximately rigid-body precession. Read More

Tidal interactions play an important role in the evolution and ultimate fate of compact white dwarf (WD) binaries. Not only do tides affect the pre-merger state (such as temperature and rotation rate) of the WDs, but they may also determine which systems merge and which undergo stable mass transfer. In this paper, we attempt to quantify the effects of rotation on tidal angular momentum transport in binary stars, with specific calculations applied to WD stellar models. Read More

A number of radio pulsars exhibit intriguing mode-switching behavior. Recent observations of PSR B0943+10 revealed correlated radio and X-ray mode switches, providing a new avenue for understanding this class of objects. The large X-ray pulse fraction observed during the radio quiet phase (Q mode) was previously interpreted as a result of changing obscuration of X-rays by dense magnetosphere plasma. Read More

Disk accretion onto magnetic stars occurs in a variety of systems, including accreting neutron stars (with both high and low magnetic fields), white dwarfs, and protostars. We review some of the key physical processes in magnetosphere-disk interaction, highlighting the theoretical uncertainties. We also discuss some applications to the observations of accreting neutron stars and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets. Read More

We study the interactions between a protostar and its circumstellar disc under the influence of an external binary companion to determine the evolution of the mutual stellar spin - disc misalignment angle. The gravitational torque on the disc from an inclined binary makes the disc precess around the binary axis, while the star-disc interaction torque due to the rotation-induced quadrupole makes the stellar spin and the disc angular momentum axes precess around each other. A significant star-disc misalignment angle can be generated from a small initial value as the star-disc system evolves in time such that the two precession frequencies cross each other. Read More

Authors: Paolo Soffitta, Xavier Barcons, Ronaldo Bellazzini, João Braga, Enrico Costa, George W. Fraser, Szymon Gburek, Juhani Huovelin, Giorgio Matt, Mark Pearce, Juri Poutanen, Victor Reglero, Andrea Santangelo, Rashid A. Sunyaev, Gianpiero Tagliaferri, Martin Weisskopf, Roberto Aloisio, Elena Amato, Primo Attiná, Magnus Axelsson, Luca Baldini, Stefano Basso, Stefano Bianchi, Pasquale Blasi, Johan Bregeon, Alessandro Brez, Niccoló Bucciantini, Luciano Burderi, Vadim Burwitz, Piergiorgio Casella, Eugene Churazov, Marta Civitani, Stefano Covino, Rui Miguel Curado da Silva, Giancarlo Cusumano, Mauro Dadina, Flavio D'Amico, Alessandra De Rosa, Sergio Di Cosimo, Giuseppe Di Persio, Tiziana Di Salvo, Michal Dovciak, Ronald Elsner, Chris J. Eyles, Andrew C. Fabian, Sergio Fabiani, Hua Feng, Salvatore Giarrusso, René W. Goosmann, Paola Grandi, Nicolas Grosso, Gianluca Israel, Miranda Jackson, Philip Kaaret, Vladimir Karas, Michael Kuss, Dong Lai, Giovanni La Rosa, Josefin Larsson, Stefan Larsson, Luca Latronico, Antonio Maggio, Jorge Maia, Frédéric Marin, Marco Maria Massai, Teresa Mineo, Massimo Minuti, Elena Moretti, Fabio Muleri, Stephen L. O'Dell, Giovanni Pareschi, Giovanni Peres, Melissa Pesce, Pierre-Olivier Petrucci, Michele Pinchera, Delphine Porquet, Brian Ramsey, Nanda Rea, Fabio Reale, Juana Maria Rodrigo, Agata Różańska, Alda Rubini, Pawel Rudawy, Felix Ryde, Marco Salvati, Valdivino Alexandre de Santiago Júnior, Sergey Sazonov, Carmelo Sgró, Eric Silver, Gloria Spandre, Daniele Spiga, Luigi Stella, Toru Tamagawa, Francesco Tamborra, Fabrizio Tavecchio, Teresa Teixeira Dias, Matthew van Adelsberg, Kinwah Wu, Silvia Zane

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. Read More

Recent observations have revealed evidence for the global oscillations of Jupiter and Saturn, which can potentially provide a new window into the interior structure of giant planets. Motivated by these observations, we study the non-radial oscillation modes of giant planets containing a solid core. Our calculations include the elastic response of the core and consider a wide range of possible values of the core shear modulus. Read More

We study the possibility of tidal dissipation in the solid cores of giant planets and its implication for the formation of hot Jupiters through high-eccentricity migration. We present a general framework by which the tidal evolution of planetary systems can be computed for any form of tidal dissipation, characterized by the imaginary part of the complex tidal Love number, ${\rm Im}[{\tilde k}_2(\omega)]$, as a function of the forcing frequency $\omega$. Using the simplest viscoelastic dissipation model (the Maxwell model) for the rocky core and including the effect of a nondissipative fluid envelope, we show that with reasonable (but uncertain) physical parameters for the core (size, viscosity and shear modulus), tidal dissipation in the core can accommodate the tidal-Q constraint of the Solar system gas giants and at the same time allows exoplanetary hot Jupiters to form via tidal circularization in the high-e migration scenario. Read More

We study the dynamics of spiral waves and oscillation modes in relativistic rotating discs around black holes. Generalizing the Newtonian theory, we show that wave absorption can take place at the corotation resonance, where the pattern frequency of the wave matches the background disc rotation rate. We derive the general relativistic expression for the disc vortensity (vorticity divided by surface density), which governs the behaviour of density perturbation near corotation. Read More

Affiliations: 1Wash. Univ. in St. Louis, 2NASA/GSFC, 3Rice, 4NASA/GSFC, 5NASA/GSFC, 6NASA/ARC, 7TIFR, India, 8NASA/GSFC, 9NASA/GSFC, 10NASA/GSFC, 11U. Iowa, 12NASA/GSFC, 13JHU, 14Cornell, 15NASA/GSFC, 16MIT, 17Temple, 18NASA/ARC, 19NASA/GSFC, 20NASA/GSFC, 21U. Oula, Finland, 22NCSU, 23NASA/ARC, 24NASA/GSFC, 25NASA/GSFC, 26NASA/GSFC, 27NASA/GSFC, 28NASA/GSFC, 29Nagoya U., Japan, 30Riken U., Japan

In this document, we describe the scientific potential of blazar observations with a X-ray polarimetry mission like GEMS (Gravity and Extreme Magnetism SMEX). We describe five blazar science investigations that such a mission would enable: (i) the structure and the role of magnetic fields in AGN jets, (ii) analysis of the polarization of the synchrotron X-ray emission from AGN jets, (iii) discrimination between synchrotron self-Compton and external Compton models for blazars with inverse Compton emission in the X-ray band, (iv) a precision study of the polarization properties of the X-ray emission from Cen-A, (v) tests of Lorentz Invariance based on X-ray polarimetric observations of blazars. We conclude with a discussion of a straw man observation program and recommended accompanying multiwavelength observations. Read More

Affiliations: 1NASA/GSFC, 2NASA/GSFC, 3Rice, 4NASA/GSFC, 5NASA/GSFC, 6NASA/ARC, 7TIFR, India, 8NASA/GSFC, 9NASA/GSFC, 10NASA/GSFC, 11U. Iowa, 12NASA/GSFC, 13Washington U., 14JHU, 15Cornell, 16NASA/GSFC, 17MIT, 18Temple, 19NASA/ARC, 20NASA/GSFC, 21NASA/GSFC, 22U. Oula, Finland, 23NCSU, 24NASA/ARC, 25NASA/GSFC, 26NASA/GSFC, 27NASA/GSFC, 28NASA/GSFC, 29Nagoya U., Japan, 30Riken U., Japan

We present here a summary of the scientific goals behind the Gravity and Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH) observing program. The primary targets can be divided into two classes: stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be divided into various X-ray spectral states: thermal-dominant (disk), hard (radio jet), and steep power-law (hot corona). Read More

Merging neutron star (NS) binaries may be detected by ground-based gravitational wave (GW) interferometers (e.g. LIGO/VIRGO) within this decade and may also generate electromagnetic radiation detectable by wide-field, fast imaging telescopes that are coming online. Read More

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. Read More

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. Read More

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. Read More

The Kepler satellite has discovered a number of transiting planets around close binary stars. These circumbinary systems have highly aligned planetary and binary orbits. In this paper, we explore how the mutual inclination between the planetary and binary orbits may reflect the physical conditions of the assembly of protoplanetary disks and the interaction between protostellar binaries and circumbinary disks. Read More

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. Read More

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. Read More

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). Read More