Searcharxiv⌕ Search

arXiv subjects

R. V. E. Lovelace

Publications and source records attributed to R. V. E. Lovelace.

At least 19 recordsLinked to original sources
arXiv2025

3D MHD simulations of planet migration in cavities and inner discs of magnetized stars

We investigate the Type I migration of planets in low-density cavities and inner discs of strongly magnetized young stars using global three-dimensional (3D) magnetohydrodynamic (MHD) simulations, where the strong magnetic field carves the low-density cavity. Simulations show that a planet in the cavity migrates inwards up to the radius at which the outer Lindblad resonances are inside the cavity. At smaller radii, the migration stalls. The migration is faster if a star accretes in the unstable regime where the temporary tongues penetrate the magnetosphere. If a planet is in a highly inclined orbit, it interacts with the disc, and the eccentricity increases due to the Kozai-Lydov mechanism. A planet may stop or reverse its migration in the inner disc before entering the cavity. The magnetosphere interacts with the inner disc, changing its density distribution such that migration slows down or is even reversed. A tilted magnetosphere also excites density and bending waves in the disc, which may slow down migration and also increase the inclination and eccentricity of the planet. When a planet reaches the disc-cavity boundary, it is typically trapped at the boundary by asymmetric corotation torque. A planet moves together with the boundary when the cavity expands. Overall, a magnetized star provides an environment for slow or reverse migration.

astro-ph.EP↗
arXiv2025

Unstable accretion in TW Hya: 3D simulations and comparisons with observations

We investigate the origin of photometric variability in the classical T Tauri star TW Hya by comparing light curves obtained by TESS and ground-based telescopes with light curves created using three-dimensional (3D) magnetohydrodynamic (MHD) simulations. TW Hya is modeled as a rotating star with a dipole magnetic moment, slightly tilted about the rotational axis. We observed that for various model parameters, matter accretes in the unstable regime and produces multiple hot spots on the star's surface, which leads to stochastic-looking light curves similar to the observed ones. Wavelet and Fourier spectra of observed and modeled light curves show multiple quasiperiodic oscillations (QPOs) with quasiperiods from less than 0.1 to 9 days. Models show that variation in the strength and tilt of the dipole magnetosphere leads to different periodograms, where the period of the star may dominate or be hidden. The amplitude of QPOs associated with the stellar period can be smaller than that of other QPOs if the tilt of the dipole magnetosphere is small and when the unstable regime is stronger. In models with small magnetospheres, the short-period QPOs associated with rotation of the inner disc dominate and can be mistaken for a stellar period. We show that longer-period (5-9 days) QPOs can be caused by waves forming beyond the corotation radius.

astro-ph.SR↗
arXiv2024

Eccentricity and Inclination of Massive Planets Inside Low-density Cavities: Results of 3D Simulations

We study the evolution of eccentricity and inclination of massive planets in low-density cavities of protoplanetary discs using three-dimensional (3D) simulations. When the planet's orbit is aligned with the equatorial plane of the disc, the eccentricity increases to high values of 0.7-0.9 due to the resonant interaction with the inner parts of the disc. For planets on inclined orbits, the eccentricity increases due to the Kozai-Lidov mechanism, where the disc acts as an external massive body that perturbs the planet's orbit. At small inclination angles, < 30 degrees, the resonant interaction with the inner disc strongly contributes to the eccentricity growth, while at larger angles, eccentricity growth is mainly due to the Kozai-Lidov mechanism. We conclude that planets inside low-density cavities tend to acquire high eccentricity if favorable conditions give sufficient time for growth. The final value of the planet's eccentricity, after the disc dispersal depends on the planet's mass and properties of the cavity and protoplanetary disc.

astro-ph.EP↗
arXiv2021

3D MHD Simulations of Accretion onto Stars with Tilted Magnetic and Rotational Axes

We present results of global three-dimensional (3D) magnetohydrodynamic (MHD) simulations of accretion onto magnetized stars where both the magnetic and rotational axes of the star are tilted about the rotational axis of the disc. We observed that initially the inner parts of the disc are warped, tilted, and process due to the magnetic interaction between the magnetosphere and the disc. Later, larger tilted discs form with the size increasing with the magnetic moment of the star. The normal vector to the discs are tilted at different angles, from 5-10 degrees up to 30-40 degrees. Small tilts may result from the winding of the magnetic field lines about the rotational axis of the star and the action of the magnetic force which tends to align the disc. Another possible explanation is the magnetic Bardeen-Petterson effect in which the disc settles in the equatorial plane of the star due to precessional and viscous torques in the disc. Tilted discs slowly precess with the time scale of the order of 50 Keplerian periods at the reference radius (approx. 3 stellar radii). Our results can be applied to different types of stars where signs of tilted discs and/or slow precession have been observed.

astro-ph.SR↗
arXiv2019

Comparisons of MHD Propeller Model with Observations of Cataclysmic Variable AE Aqr

We have developed a numerical MHD model of the propeller candidate star AE Aqr using axisymmetric magneto-hydrodynamic (MHD) simulations. We suggest that AE Aqr is an intermediate polar-type star, where the magnetic field is relatively weak and an accretion disc may form around the white dwarf. The star is in the propeller regime, and many of its observational properties are determined by the disc-magnetosphere interaction. Comparisons of the characteristics of the observed versus modelled AE Aqr star show that the model can explain many observational properties of AE Aqr. In a representative model, the magnetic field of the star is B\approx 3.3x10^5 G and the time averaged accretion rate in the disc is 5.5\times 10^{16} g/s. Most of this matter is ejected into conically-shaped winds. The numerical model explains the rapid spin-down of AE Aqr through the outflow of angular momentum from the surface of the star to the wind, corona and disc. The energy budget in the outflows, 9x10^{33} erg/s, is sufficient for explaining the observed flaring radiation in different wavebands. The time scale of ejections into the wind matches the short time scale variability in the light curves of AE Aqr.

astro-ph.SR↗
arXiv2019

3D Simulations of Planet Trapping at Disc-Cavity Boundaries

Inward migration of low-mass planets and embryos of giant planets can be stopped at the disc-cavity boundaries due to co-orbital corotation torque. We performed the first global three-dimensional (3D) simulations of planet migration at the disc-cavity boundary, and have shown that the boundary is a robust trap for low-mass planets and embryos. A protoplanetary disc may have several such trapping regions at various distances from the star, such as at the edge of the stellar magnetosphere, the inner edge of the dead zone, the dust-sublimation radius and the snow lines. Corotation traps located at different distances from a star, and moving outward during the disc dispersal phase, may possibly explain the observed homogeneous distribution of low-mass planets with distance from their host stars.

astro-ph.EP↗
arXiv2018

Modelling the bow shock Pulsar Wind Nebulae propagating through a non-uniform ISM

Many pulsars propagate through the interstellar medium (ISM) with supersonic velocities, and their pulsar winds interact with the interstellar medium (ISM), forming bow shocks and magnetotails (PWN). We model the propagation of pulsars through the inhomogeneous ISM using non-relativistic axisymmetric magneto-hydrodynamic (MHD) simulations. We take into account the wind from the star, and the azimuthal and poloidal components of the magnetic field, and investigate the PWN at different levels of magnetization (the ratio of magnetic to matter energy-densities) in the wind. We consider the interaction of PWN with small-scale and large-scale imhomogeneities in the ISM at different values of magnetization. We conclude that the inhomogeneities in the ISM can change the shapes of the bow shocks and magnetotails at different values of the magnetization.We compare the results of our simulations with the images of the Guitar Nebula and other PWN that show irregularities in the shapes of their bow shocks and magnetotails. We conclude that these irregularities may be caused by the interaction of PWN with the inhomogeneities in the ISM.

astro-ph.HE↗
arXiv2016

Boundary between Stable and Unstable Regimes of Accretion. Ordered and Chaotic Unstable Regimes

We present a study of the Rayleigh-Taylor unstable regime of accretion onto rotating magnetized stars in a set of high grid resolution three-dimensional (3D) magnetohydrodynamic (MHD) simulations performed in low-viscosity discs. We find that the boundary between the stable and unstable regimes is determined almost entirely by the fastness parameter omega_s=Omega_star/Omega_K(r_m), where Omega_star is the angular velocity of the star and Omega_K(r_m) is the angular velocity of the Keplerian disc at the disc-magnetosphere boundary r=r_m. We found that accretion is unstable if omega_s < 0.6. Accretion through instabilities is present in stars with different magnetospheric sizes. However, only in stars with relatively small magnetospheres, r_m/R_star < 7, do the unstable tongues produce chaotic hot spots on the stellar surface and irregular light-curves. At even smaller values of the fastness parameter, omega_s < 0.45, multiple irregular tongues merge, forming one or two ordered unstable tongues that rotate with the angular frequency of the inner disc. This transition occurs in stars with even smaller magnetospheres, r_m/R_star < 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, Theta=5 degrees, and a small viscosity parameter alpha=0.02. Test simulations at higher alpha values show that many more cases become unstable, and the light-curves become even more irregular. Test simulations at larger tilts of the dipole Theta show that instability is present, however, accretion in two funnel streams dominates if Theta > 15 degrees. The results of these simulations can be applied to accreting magnetized stars with relatively small magnetospheres: Classical T Tauri stars, accreting millisecond X-ray pulsars, and cataclysmics variables.

astro-ph.SR↗
arXiv2015

The effects of a magnetic field on planetary migration in laminar and turbulent discs

We investigate the migration of low-mass planets ($5 M_{\oplus}$ and $20 M_{\oplus}$) in accretion discs threaded with a magnetic field using 2D MHD code in polar coordinates. We observed that, in the case of a strong azimuthal magnetic field where the plasma parameter is $β\sim 1-2$, density waves at the magnetic resonances exert a positive torque on the planet and may slow down or reverse its migration. However, when the magnetic field is weaker (i.e., the plasma parameter $β$ is relatively large), then non-axisymmetric density waves excited by the planet lead to growth of the radial component of the field and, subsequently, to development of the magneto-rotational instability, such that the disc becomes turbulent. Migration in a turbulent disc is stochastic, and the migration direction may change as such. To understand migration in a turbulent disc, both the interaction between a planet and individual turbulent cells, as well as the interaction between a planet and ordered density waves, have been investigated.

astro-ph.EP↗
arXiv2015

Numerical MHD Codes for Modeling Astrophysical Flows

We describe a Godunov-type magnetohydrodynamic (MHD) code based on the Miyoshi and Kusano (2005) solver which can be used to solve various astrophysical hydrodynamic and MHD problems. The energy equation is in the form of entropy conservation. The code has been implemented on several different coordinate systems: 2.5D axisymmetric cylindrical coordinates, 2D Cartesian coordinates, 2D plane polar coordinates, and fully 3D cylindrical coordinates. Viscosity and diffusivity are implemented in the code to control the accretion rate in the disk and the rate of penetration of the disk matter through the magnetic field lines. The code has been utilized for the numerical investigations of a number of different astrophysical problems, several examples of which are shown.

astro-ph.GA↗
arXiv2014

Rossby Wave Instability in Astrophysical Discs

A brief review is given of the Rossby wave instability (RWI) in astrophysical discs. In non-self-gravitating discs, around for example a newly forming stars, the instability can be triggered by an axisymmetric bump at some radius $r_0$ in the disc surface mass-density. It gives rise to exponentially growing non-axisymmetric perturbation [$\propto \exp({\rm i}mϕ)$, $m=1,2..$] in the vicinity of $r_0$ consisting of {\it anticyclonic} vortices. These vortices are regions of high pressure and consequently act to trap dust particles which in turn can facilitate planetesimal growth in proto-planetary discs. The Rossby vortices in the discs around stars and black holes may cause the observed quasi-periodic modulations of the disc's thermal emission.

astro-ph.SR↗
arXiv2013

The effect of an ordered azimuthal magnetic field on a migrating planet in a non-turbulent disc

In this work, we consider the physics of the interaction between a planet and a magnetized gaseous protoplanetary disc. We investigate the migration of a planet in a disc that is threaded with an azimuthal magnetic field. We find that, for a larger magnetic field amplitude, there is an increasingly large positive torque on the planet from the disc, resulting in slowed and even outward migration. Our results indicate that magnetic resonances due to a purely azimuthal, ordered magnetic field can slow or stop the inward migration of Jupiter-mass, Saturn-mass, and $5 M_{\oplus}$ planets.

astro-ph.EP↗
arXiv2013

Boundary Between Stable and Unstable Regimes of Accretion

We investigated the boundary between stable and unstable regimes of accretion and its dependence on different parameters. Simulations were performed using a "cubed sphere" code with high grid resolution (244 grid points in the azimuthal direction), which is twice as high as that used in our earlier studies. We chose a very low viscosity value, with alpha-parameter alpha=0.02. We observed from the simulations that the boundary strongly depends on the ratio between magnetospheric radius r_m (where the magnetic stress in the magnetosphere matches the matter stress in the disk) and corotation radius r_cor (where the Keplerian velocity in the disk is equal to the angular velocity of the star). For a small misalignment angle of the dipole field, Theta=5 degrees, accretion is unstable if r_cor/r_m>1.35, and is stable otherwise. In cases of a larger misalignment angle of the dipole, Theta=20 degrees, instability occurs at slightly larger values, r_cor/r_m>1.41.

astro-ph.SR↗
arXiv2013

Magnetically Driven and Collimated Jets from the Disc-Magnetosphere Boundary of Rotating Stars

We discuss recent progress in understanding the launching of outflows/jets from the disc-magnetosphere boundary of slowly and rapidly rotating magnetized stars. In most of the discussed models the interior of the disc is assumed to have a turbulent viscosity and magnetic diffusivity (as described by two ``alpha'' parameters), whereas the coronal region outside of the disc is treated using ideal magnetohydrodynamics (MHD). Extensive MHD simulations have established the occurrence of long-lasting outflows in both the cases of slowly and rapidly rotating stars. In the case of {\it slowly rotating stars}, a new type of outflow, {\it a conical wind}, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the inward motion of the accretion disc. Near their region of origin, the winds have the shape of a thin conical shell with a half-opening angle of $ \sim 30^\circ$. At large distances these outflows become magnetically collimated by their toroidal magnetic field and form matter dominated jets. That is, the jets are current carrying. The predominant driving force for the conical winds is the magnetic force proportional to the negative gradient of the square of the toroidal magnetic field and not the centrifugal force. In the case of {\it rapidly rotating stars} in the ``propeller regime,'' a two-component outflow is observed. The first component is similar to the matter dominated conical winds. A large fraction of the disc matter may be ejected into the winds in this regime. The second component is a high-velocity, low-density magnetically dominated {\it axial jet} where matter flows along the opened polar field lines of the star.

astro-ph.SR↗
arXiv2013

Advection of Matter and B-Fields in Alpha-Discs

We have carried out and analyzed a set of axisymmetric MHD simulations of the evolution of a turbulent/diffusive accretion disc around an initially unmagnetized star. The disc is initially threaded by a weak magnetic field where the magnetic pressure is significantly less than the kinetic pressure in the disc. The viscosity and magnetic diffusivity are modelled by two "alpha" parameters, while the coronal region above the disc is treated using ideal MHD. The initial magnetic field is taken to consist of three poloidal field loops threading the disc. The motivation for this study is to understand the advection of disc matter and magnetic field by the turbulent/diffusive disc. At early times the innermost field loop twists and its field lines become open. The twisting of the opened field lines leads to the formation of both an inner collimated, magnetically-dominated jet, and at larger distances from the axis a matter dominated uncollimated wind. For later times, the strength of the magnetic field decreases owing to field reconnection and annihilation in the disc. For the early times, we have derived from the simulations both the matter accretion speed in the disc and the accretion speed of the magnetic field. We show that the derived matter accretion speed agrees approximately with the predictions of a model where the accretion speed is the sum of two terms, one due to the disc's viscosity (which gives a radial outflow of angular momentum in the disc), and a second due to the twisted magnetic field at the disc's surface (which gives a vertical outflow of angular momentum). For early times we find that the magnetic contribution is roughly twice the viscous contribution for the case where the alpha parameters are both equal to 0.1. At later times the magnetic contribution to the matter speed becomes small compared to the viscous contribution.

astro-ph.HE↗
arXiv2013

Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations

We report results of the first global three-dimensional magnetohydrodynamic simulations of the waves excited in an accretion disc by a rotating star with a dipole magnetic field misaligned from the star's rotation axis (which is aligned with the disc axis). The main results are the following: (1) If the magnetosphere of the star corotates approximately with the inner disc, then we observe a strong one-armed bending wave (a warp). This warp corotates with the star and has a maximum amplitude between corotation radius and the radius of the vertical resonance. The disc's center of mass can deviate from the equatorial plane up to the distance of z_w\approx 0.1 r. However, the effective height of the warp can be larger, h_w \approx 0.3 r due to the finite thickness of the disc. Stars with a range of misalignment angles excite warps. However, the amplitude of the warps is larger for misalignment angles between 15 and 60 degrees. (2) If the magnetosphere rotates slower, than the inner disc, then a bending wave is excited at the disc-magnetosphere boundary, but does not form a large-scale warp. Instead, high-frequency oscillations become strong at the inner region of the disc. These are (a) trapped density waves which form inside the radius where the disc angular velocity has a maximum, and (b) inner bending waves which appear in the case of accretion through magnetic Raleigh-Taylor instability. These two types of waves are connected with the inner disc and their frequencies will vary with accretion rate. Bending oscillations at lower frequencies are also excited including global oscillations of the disc. In cases where the simulation region is small, slowly-precessing warp forms. Simulations are applicable to young stars, cataclysmic variables, and accreting millisecond pulsars.

astro-ph.SR↗
arXiv2013

On the Dispersion Measure of High-Redshift Synchrotron Sources

A method is proposed for deriving the dispersion measure (related to the line-of-sight integrated free electron density) to high-redshift, powerful synchrotron radio sources by analysis of the temporal/spectral properties of the recorded signals. This would allow direct measurements of the distributed density of ionized baryons along the line-of-sights to the sources.

astro-ph.CO↗