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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., Comment: 19 pages, 17 figures, submitted to MNRAS

Spectral and photometric variability of the Classical T Tauri stars RY Tau and SU Aur from 2013 to 2022 is analyzed. We find that in SU Aur the H-alpha line's flux at radial velocity RV = -50 +- 7 km/s varies with a period P = 255 +- 5 days. A similar effect previously discovered in RY Tau is confirmed with these new data: P = 21.6 days at RV = -95 +- 5 km/s. In both stars, the radial velocity of these variations, the period, and the mass of the star turn out to be related by Kepler's law, suggesting structural features on the disc plane orbiting at radii of 0.2 AU in RY Tau and 0.9 AU in SU Aur, respectively. Both stars have a large inclination of the accretion disc to the line of sight - so that the line of sight passes through the region of the disc wind. We propose there is an azimuthal asymmetry in the disc wind, presumably in the form of 'density streams', caused by substructures of the accretion disc surface. These streams cannot dissipate until they go beyond the Alfven surface in the disc's magnetic field. These findings open up the possibility to learn about the structure of the inner accretion disc of CTTS on scales less than 1 AU and to reveal the orbital distances related to the planet's formation., Comment: 9 pages, 10 figures

Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto the star via funnel flows which follow the stellar field lines and shock on the stellar surface leaving a hot spot with a density gradient. Previous work has inferred different densities in the hot spot, but has not been sensitive to the radial density distribution. Attempts have been made to measure this with X-ray observations, but X-ray emission only traces a fraction of the hot spot and also coronal emission. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aur that display a time lag of about 1 day between their peaks. The periodicity arises as the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time-lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of the magnetospheric accretion model, this indicates a radial density gradient in a hot spot on the stellar surface since different density parts of the hot spot are expected to emit radiation at different wavelengths. These results are the first observational confirmation of the magnetospheric accretion model's prediction of a density gradient in the hot spot and demonstrate the insights gained from focusing on the wavelengths where the bulk of the accretion energy can be observed., Comment: published in Nature; open access link: https://www.nature.com/articles/s41586-021-03751-5

Planets are thought to form at the early stage of stellar evolution when the mass accretion is still ongoing. RY Tau is a T Tauri type star at the age of a few Myr, with accretion disc seen at high inclination, so that line of sight crosses both the wind and the accretion gas flows. In a long series of spectroscopic monitoring of the star in 2013-2020, we detected variations in H-alpha and NaI D absorptions at radial velocities of infall (accretion) and outflow (wind) with a period of about 22 days. The absorptions in the infalling and the outflowing gas streams vary in anti-phase: an increase of infall is accompanied by a decrease of outflow, and vice versa. These flip-flop oscillations retain phase over several years of observations. We suggest that this may result from the MHD processes at the disk-magnetosphere boundary in the propeller mode. Another possibility is that a massive planet modulates some processes in the disc and provides the observed effects. The period, if Keplerian, corresponds to a distance of 0.2 AU, which is close to the dust sublimation radius in this star. The presence of the putative planet may be confirmed by radial velocity measurements: expected amplitude is > 90 m/s if a planet mass is > 2 Mj., Comment: 7 pages, 12 figures, accepted for publication in MNRAS

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., Comment: 14 pages, 13 figures, MNRAS, accepted. The new version of the paper is more complete compared with the earlier version

The properties of the compactness of interpolation sets of algebras of generalized analytic functions are investigated and convenient sufficient conditions for interpolation are given., Comment: in Russian

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., Comment: 17 pages, 14 figures, accepted by MNRAS

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., Comment: 11 pages, 4 figures, accepted by MNRAS

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., Comment: 11 pages, 12 figures, submitted to MNRAS

Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars are radiatively driven. In all of these cases, the magnetic field influences matter flow from the stars and determines many observational properties. In this chapter we review recent studies of accretion, outflows, and winds of magnetized stars with a focus on three main topics: (1) accretion onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and (3) winds from isolated massive magnetized stars. We show results obtained from global magnetohydrodynamic simulations and, in a number of cases compare global simulations with observations., Comment: 60 pages, 44 figures

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 $\beta\sim 1-2$, 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 $\beta$ 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., Comment: 16 pages, 12 figures

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., Comment: 23 pages, 17 figures, submitted to New Astronomy

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., Comment: 17 pages, 14 figures, published in MNRAS

The mass outflows in T Tauri stars (TTS) are thought to be an effective mechanism to remove angular momentum during the pre-main-sequence contraction of a low-mass star. The most powerful winds are observed at the FUor stage of stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007 and 20-d series of spectra taken in 2012. For the first time the photospheric spectrum of the star is detected and stellar parameters are derived: spectral type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric spectrum is highly veiled, but the amount of veiling is not the same in different spectral lines, being lower in weak transitions and much higher in strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of metals are accompanied by a `shell' absorption at radial velocity of about -240 km/s. We show that these absorptions form in the post-shock gas in the jet, i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta indicate the terminal wind velocity of about 500 km/s, which vary on time-scales from several days to years. A model of the stellar wind is developed to interpret the observations. The model is based on calculation of hydrogen spectral lines using the radiative transfer code TORUS. The observed H-alpha and H-beta line profiles and their variability can be well reproduced with a stellar wind model, where the mass-loss rate and collimation (opening angle) of the wind are variable. The changes of the opening angle may be induced by small variability in magetization of the inner disc wind. The mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the accretion rate of 2.0x10^{-6} Msun/yr., Comment: 11 pages, 12 figures; accepted for publication in MNRAS. Typographical errors have been corrected after the proof stage

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 [$\propto \exp({\rm i}m\phi)$, $m=1,2..$] 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., Comment: 11 pages, 4 figures

We review recent axisymmetric and three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetospheric accretion, plasma-field interaction and outflows from the disk-magnetosphere boundary., Comment: 11 pages, 8 figures, conference proceedings: "Physics at the Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 2013

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., Comment: 14 pages, 11 figures, submitted to MNRAS

Recent spectropolarimetric observations suggest that young low-mass stars such as classical T Tauri stars (CTTSs) possess relatively strong (~kG) magnetic field. This supports a scenario in which the final accretion onto the stellar surface proceeds through a magnetosphere, and the winds are formed in magnetohydrodynamics (MHD) processes. We examine recent numerical simulations of magnetospheric accretions via an inclined dipole and a complex magnetic fields. The difference between a stable accretion regime, in which accretion occurs in ordered funnel streams, and an unstable regime, in which gas penetrates through the magnetosphere in several unstable streams due to the magnetic Rayleigh-Taylor instability, will be discussed. We describe how MHD simulation results can be used in separate radiative transfer (RT) models to predict observable quantiles such as line profiles and light curves. The plausibility of the accretion flows and outflows predicted by MHD simulations (via RT models) can be tested against observations. We also address the issue of outflows/winds that arise from the innermost part of CTTSs. First, we discuss the line formations in a simple disk wind and a stellar wind models. We then discuss the formation of the conically shaped magnetically driven outflow that arises from the disk-magnetosphere boundary when the magnetosphere is compressed into an X-type configuration., Comment: 10 pages, 4 figures, to appear in the proceedings of IAUS 302: "Magnetic Fields Throughout Stellar Evolution", Biarritz, France, Aug. 26-30, 2013

We observed persistent high-frequency oscillations of the boundary layer near an accreting, weakly-magnetized star in global 3D MHD simulations. The tilted dipole magnetic field is not strong enough to open a gap between the star and the disk. Instead, it forms a highly-wrapped azimuthal field near the surface of the star which slows down rotation of the disk matter, while a small tilt of the field excites oscillations of the boundary layer with a frequency below the Keplerian frequency. This mechanism may be responsible for the high-frequency oscillations in accreting neutron stars, white dwarfs and classical T Tauri stars., Comment: 4 pages, 3 figures, conference proceedings: "Physics at the Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 2013

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., Comment: 4 pages, 4 figures, conference proceedings: "Physics at the Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 2013

We discuss key observational signatures of Classical T Tauri stars (CTTSs) accreting through Rayleigh-Taylor instability, which occurs at the interface between an accretion disk and a stellar magnetosphere. In this study, the results of global 3-D MHD simulations of accretion flows, in both stable and unstable regimes, are used to predict the variability of hydrogen emission lines and light curves associated with those two distinctive accretion flow patterns. In the stable regime, a redshifted absorption component (RAC) periodically appears in some hydrogen lines, but only during a fraction of a stellar rotation period. In the unstable regime, the RAC is present rather persistently during a whole stellar rotation period, and its strength varies non-periodically. The latter is caused by multiple accreting streams, formed randomly due to the instability, passing across the line of sight to an observer during one stellar rotation. This results in the quasi-stationarity appearance of the RAC because at least one of the accretion stream is almost always in the line of sight to an observer. In the stable regime, two stable hot spots produce a smooth and periodic light curve that shows only one or two peaks per stellar rotation. In the unstable regime, multiple hot spots formed on the surface of the star, produce the stochastic light curve with several peaks per rotation period., Comment: 6 pages, 3 figures, to appear in the proceedings of the conference: "Physics at the Magnetspheric Boundary", Geneva, Switzerland, 25-28 June, 2013

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., Comment: 14 pages, 8 figures. arXiv admin note: substantial text overlap with arXiv:0907.3394

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., Comment: 14 pages, 17 figures

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., Comment: 26 pages, 25 figures

We report here results of spectropolarimetric observations of the classical T Tauri star (cTTS) GQ Lup carried out with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT) in the framework of the "Magnetic Protostars and Planets" (MaPP) programme, and obtained at 2 different epochs (2009 July & 2011 June). From these observations, we first infer that GQ Lup has a photospheric temperature of 4,300+-50\^A K and a rotation period of 8.4+-0.3 d; it implies that it is a 1.05+-0.07 Msun star viewed at an inclination of ~30deg, with an age of 2-5 Myr, a radius of 1.7+-0.2 Rsun, and has just started to develop a radiative core. Large Zeeman signatures are clearly detected at all times, both in photospheric lines & in accretion-powered emission lines, probing longitudinal fields of up to 6 kG and hence making GQ Lup the cTTS with the strongest large-scale fields known as of today. Rotational modulation of Zeeman signatures is clearly different between our 2 runs, demonstrating that large-scale fields of cTTSs are evolving with time and are likely produced by non-stationary dynamo processes. Using tomographic imaging, we reconstruct maps of the large-scale field, of the photospheric brightness & of the accretion-powered emission of GQ Lup. We find that the magnetic topology is mostly poloidal & axisymmetric; moreover, the octupolar component of the large-scale field (of strength 2.4 & 1.6 kG in 2009 & 2011) dominates the dipolar component (of strength ~1 kG) by a factor of ~2, consistent with the fact that GQ Lup is no longer fully-convective. GQ Lup also features dominantly poleward magnetospheric accretion at both epochs. The large-scale dipole of GQ Lup is however not strong enough to disrupt the surrounding accretion disc further than about half-way to the corotation radius, suggesting that GQ Lup should rapidly spin up like other similar partly-convective cTTSs (abridged)., Comment: MNRAS, in press (17 pages, 10 figures, 1 table)

We analyze the photometric and spectroscopic variability of the classical T Tauri star V2129 Oph over several rotational cycles to test the dynamical predictions of magnetospheric accretion models. The photometric variability and the radial velocity variations in the photospheric lines can be explained by rotational modulation due to cold spots, while the radial velocity variations of the He I (5876 \AA) line and the veiling variability are due to hot spot rotational modulation. The hot and cold spots are located at high latitudes and about the same phase, but the hot spot is expected to sit at the chromospheric level, while the cold spot is at the photospheric level. Using the dipole+octupole magnetic-field configuration previously proposed in the literature for the system, we compute 3D MHD magnetospheric simulations of the star-disk system. We use the simulation's density, velocity and scaled temperature structures as input to a radiative transfer code, from which we calculate theoretical line profiles at all rotational phases. The theoretical profiles tend to be narrower than the observed ones, but the qualitative behavior and the observed rotational modulation of the H\alpha and H\beta emission lines are well reproduced by the theoretical profiles. The spectroscopic and photometric variability observed in V2129 Oph support the general predictions of complex magnetospheric accretion models with non-axisymmetric, multipolar fields., Comment: Accepted by Astronomy and Astrophysics

We discuss results of global 3D MHD simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magneto-rotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards due primarily to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally-stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. In the magnetospheric regime, the accretion disc is truncated by the star's magnetic field within a few stellar radii from the star, and matter flows to the star in funnel streams. The funnel streams flowing towards the south and north magnetic poles but are not equal due to the inhomogeneity of the flow. In the BL regime, matter accretes to the surface of the star through the boundary layer. The magnetic field in the inner disc is strongly amplified by the shear of the accretion flow, and the matter and magnetic stresses become comparable. Accreting matter forms a belt-shaped region on the surface of the star. The belt has inhomogeneous density distribution which varies in time due to variable accretion rate. Results of simulations can be applied to classical T Tauri stars, accreting brown dwarfs, millisecond pulsars, dwarf novae cataclysmic variables, and other stars with magnetospheres smaller than several stellar radii., Comment: 15 pages, 13 figures, accepted by MNRAS

Axisymmetric MHD simulations are used to investigate the Bondi-Hoyle accretion onto an isolated magnetized neutron star moving supersonically (with Mach number of 3) through the interstellar medium. The star is assumed to have a dipole magnetic field aligned with its motion and a magnetospheric radius R_m less then the accretion radius R_BH, so that the gravitational focusing is important. We find that the accretion rate to a magnetized star is smaller than that to a non-magnetized star for the parameters considered. Close to the star the accreting matter falls to the star's surface along the magnetic poles with a larger mass flow to the leeward pole of the star. In the case of a relatively large stellar magnetic field, the star's magnetic field is stretched in the direction of the matter flow outside of R_m (towards the windward side of the star). For weaker magnetic fields we observed oscillations of the closed magnetosphere frontward and backward. These are accompanied by strong oscillations of the mass accretion rate which varies by factors ~ 3. Old slowly rotating neutron stars with no radio emission may be visible in the X-ray band due to accretion of interstellar matter. In general, the star's velocity, magnetic moment, and angular velocity vectors may all be in different directions so that the accretion luminosity will be modulated at the star's rotation rate., Comment: 9 pages, 9 figures, accepted for publication in MNRAS

The magnetic field of the classical T Tauri star BP Tau can be approximated as a superposition of dipole and octupole moments with respective strengths of the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt the measured properties of BP Tau and model the disc accretion onto the star. We observed in simulations that the disc is disrupted by the dipole component and matter flows towards the star in two funnel streams which form two accretion spots below the dipole magnetic poles. The octupolar component becomes dynamically important very close to the star and it redirects the matter flow to higher latitudes. The spots are meridionally elongated and are located at higher latitudes, compared with the pure dipole case, where crescent-shaped, latitudinally elongated spots form at lower latitudes. The position and shape of the spots are in good agreement with observations. The disk-magnetosphere interaction leads to the inflation of the field lines and to the formation of magnetic towers above and below the disk. The magnetic field of BP Tau is close to the potential only near the star, inside the magnetospheric surface, where magnetic stress dominates over the matter stress. A series of simulation runs were performed for different accretion rates. They show that an accretion rate is lower than obtained in many observations, unless the disc is truncated close to the star. The torque acting on the star is about an order of magnitude lower than that which is required for the rotational equilibrium. We suggest that a star could lose most of its angular momentum at earlier stages of its evolution., Comment: 11 pages, 13 figures, submitted to MNRAS

We investigate the generation of intrinsically asymmetric or {\it one-sided} outflows or jets from disk accretion onto rotating stars with complex magnetic fields using axisymmetric (2.5D) magnetohydrodynamic simulations. The intrinsic magnetic field of the star is assumed to consist of a superposition of an aligned dipole and an aligned quadrupole in different proportions. The star is assumed to be rapidly rotating in the sense that the star's magnetosphere is in the propeller regime where strong outflows occur. Our simulations show that for conditions where there is a significant quadrupole component in addition to the dipole component, then a dominantly {\it one-sided} conical wind tends to form on the side of the equatorial plane with the larger value of the intrinsic axial magnetic field at a given distance. For cases where the quadrupole component is absent or very small, we find that dominantly one-sided outflows also form, but the direction of the flow "flip-flops" between upward and downward on a time-scale of $\sim 30$ days for a protostar. The average outflow will thus be symmetrical. In the case of a pure quadrupole field we find symmetric outflows in the upward and downward directions., Comment: 10 pages, 11 figures

This work investigates the short wavelength stability of the magnetopause between a rapidly-rotating, supersonic, dense accretion disc and a slowly-rotating low-density magnetosphere of a magnetized star. The magnetopause is a strong shear layer with rapid changes in the azimuthal velocity, the density, and the magnetic field over a short radial distance and thus the Kelvin-Helmholtz (KH) instability may be important. The plasma dynamics is treated using non-relativistic, compressible (isentropic) magnetohydrodynamics. It is necessary to include the displacement current in order that plasma wave velocities remain less than the speed of light. We focus mainly on the case of a star with an aligned dipole magnetic field so that the magnetic field is axial in the disc midplane and perpendicular to the disc flow velocity. However, we also give results for cases where the magnetic field is at an arbitrary angle to the flow velocity. For the aligned dipole case the magnetopause is most unstable for KH waves propagating in the azimuthal direction perpendicular to the magnetic field which tends to stabilize waves propagating parallel to it. The wave phase velocity is that of the disc matter. A quasi-linear theory of the saturation of the instability leads to a wavenumber ($k$) power spectrum $\propto k^{-1}$ of the density and temperature fluctuations of the magnetopause, and it gives the mass accretion and angular momentum inflow rates across the magnetopause. For self-consistent conditions this mass accretion rate will be equal to the disc accretion rate at large distances from the magnetopause., Comment: 8 pages, 7 figures

We investigate the launching of outflows from the disk-magnetosphere boundary of slowly and rapidly rotating magnetized stars using axisymmetric and exploratory 3D magnetohydrodynamic (MHD) simulations. We find long-lasting outflows in both cases. (1) In the case of slowly rotating stars, a new type of outflow, 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 disk into an X-type configuration. The winds have the shape of a thin conical shell with a half-opening angle 30-40 degrees. The conical winds may be responsible for episodic as well as long-lasting outflows in different types of stars. (2) In the case of rapidly rotating stars (the "propeller regime"), a two-component outflow is observed. One component is similar to the conical winds. A significant fraction of the disk matter may be ejected into the winds. A second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the opened polar field lines of the star. The jet has a mass flux about 10% that of the conical wind, but its energy flux (dominantly magnetic) can be larger than the energy flux of the conical wind. The jet's angular momentum flux (also dominantly magnetic) causes the star to spin-down rapidly. Propeller-driven outflows may be responsible for the jets in protostars and for their rapid spin-down. The jet is collimated by the magnetic force while the conical winds are only weakly collimated in the simulation region., Comment: 29 pages and 29 figures. This version has a major expansion after comments by a referee. The 1-st version is correct but mainly describes the conical wind. This version describes in greater detail both the conical winds and the propeller regime. Accepted to the MNRAS

We discuss three modes of oscillation of accretion disks around rotating magnetized neutron stars which may explain the separations of the kilo-Hertz quasi periodic oscillations (QPO) seen in low mass X-ray binaries. The existence of these compressible, non-barotropic magnetohydrodynamic (MHD) modes requires that there be a maximum in the angular velocity $\Omega_\phi(r)$ of the accreting material larger than the angular velocity of the star $\Omega_*$, and that the fluid is in approximately circular motion near this maximum rather than moving rapidly towards the star or out of the disk plane into funnel flows. Our MHD simulations show this type of flow and $\Omega_\phi(r)$ profile. The first mode is a Rossby wave instability (RWI) mode which is radially trapped in the vicinity of the maximum of a key function $g(r){\cal F}(r)$ at $r_{R}$. The real part of the angular frequency of the mode is $\omega_r=m\Omega_\phi(r_{R})$, where $m=1,2...$ is the azimuthal mode number. The second mode, is a mode driven by the rotating, non-axisymmetric component of the star's magnetic field. It has an angular frequency equal to the star's angular rotation rate $\Omega_*$. This mode is strongly excited near the radius of the Lindblad resonance which is slightly outside of $r_R$. The third mode arises naturally from the interaction of flow perturbation with the rotating non-axisymmetric component of the star's magnetic field. It has an angular frequency $\Omega_*/2$. We suggest that the first mode with $m=1$ is associated with the upper QPO frequency, $\nu_u$; that the nonlinear interaction of the first and second modes gives the lower QPO frequency, $\nu_\ell =\nu_u-\nu_*$; and that the nonlinear interaction of the first and third modes gives the lower QPO frequency $\nu_\ell=\nu_u-\nu_*/2$, where $\nu_*=\Omega_*/2\pi$., Comment: 10 pages, 7 figures

The jets observed to emanate from many compact accreting objects may arise from the twisting of the magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic outflows, which have a significant mass flux and have energy and angular momentum carried by both matter and electromagnetic field and, Poynting outflows, where the mass flux is negligible and energy and angular momentum are carried predominantly by the electromagnetic field. We describe recent theoretical work on the formation of relativistic Poynting jets from magnetized accretion disks and new relativistic, fully-electromagnetic, particle-in-cell simulations of the formation of jets from accretion disks., Comment: 6 pages, 5 figures. Published in: Triggering Relativistic Jets (Eds. William H. Lee & Enrico Ramirez-Ruiz) Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias) Vol. 27, Contents of Supplementary CD, p.240; 2007RMxAC..27..240L

We carried out axisymmetric simulations of disk accretion to a rapidly rotating magnetized star in the "propeller" regime. Simulations show that propellers may be "weak" (with no outflows), and "strong" (with outflows). Investigation of the difference between these two regimes have shown that outflows appear only in the case where the "friction" between the disk and magnetosphere is sufficiently large, and when accreting matter flux is not very small. Matter outflows in a wide cone and is magneto-centrifugally ejected from the inner regions of the disk. Closer to the axis there is a strong, collimated, magnetically dominated outflow of energy and angular momentum carried by the open magnetic field lines from the star. The "efficiency" of the propeller may be very high in the respect that most of the incoming disk matter is expelled from the system in winds. The star spins-down rapidly due to the magnetic interaction with the disk through closed field lines and with corona through open field lines. This mechanism may act in a variety of situations where magnetized star rotates with super-Keplerian velocity at the magnetospheric boundary. We speculate that in general any object rotating with super-Keplerian velocity may drive outflows from accreting disk, if the friction between them is sufficiently large., Comment: 7 pages, 9 figures; Published in: Triggering Relativistic Jets (Eds. William H. Lee & Enrico Ramirez-Ruiz) Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias) Vol. 27, Contents of Supplementary CD, p.245

A new type of wind - a conical wind - has been discovered in axisymmetric magnetohydrodynamic simulations of the disk-magnetosphere interaction in cases where the magnetic field of the star is bunched into an X-type configuration. Such a configuration arises if the effective viscosity of the disk is larger than the effective diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the disk into a narrow shell with half-opening angle of 30-45 degrees. The outflow carries about 0.1-0.3 of the disk mass accretion rate and part of the disk's angular momentum. The conical winds are driven by the gradient of the magnetic pressure which exists above the disk due to the winding of the stellar magnetic field. Exploratory 3D simulations show that conical winds are symmetric about rotation axis of the disk even if the magnetic dipole is significantly misaligned with the disk's rotation axis. Conical winds appear around stars of different periods. However, in the case of a star in the "propeller" regime, an additional - much faster component appears: an axial jet, where matter is accelerated up to very high velocities at small distances from the star by magnetic pressure force above the surface of the star. The simulations are done in dimensionless units and are applicable to a variety of the disk-accreting magnetized stars: young stars, white dwarfs, neutron stars, and possibly black holes. For the case of young stars, conical winds and axial jets may appear in different cases, including Class I young stars, classical T Tauri stars, and EXors. In EXors periods of enhanced accretion may lead to the formation of conical winds which correspond to the outflows observed from these stars., Comment: 14 pages, 14 figures, submitted to the MNRAS. See animations at http://www.astro.cornell.edu/~romanova/conical.htm ; http://www.astro.cornell.edu/~romanova/propeller.htm ; http://www.astro.cornell.edu/us-rus/disk_prop.htm

We investigate outflows from the disk-magnetosphere boundary of rotating magnetized stars in cases where the magnetic field of a star is bunched into an X-type configuration using axisymmetric and full 3D MHD simulations. Such configuration appears if viscosity in the disk is larger than diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the disk to a narrow shell with an opening angle 30-45 degrees. Outflows carry 0.1-0.3 of the disk mass and part of the disk's angular momentum outward. Conical outflows appear around stars of different periods, however in case of stars in the "propeller" regime, an additional - much faster component appears: an axial jet, where matter is accelerated up to very high velocities at small distances from the star by magnetic pressure force above the surface of the star. Exploratory 3D simulations show that conical outflows are symmetric about rotational axis of the disk even if magnetic dipole is significantly misaligned. Conical outflows and axial jets may appear in different types of young stars including Class I young stars, classical T Tauri stars, and EXors., Comment: Invited review, conference proceedings of the meeting "Protostellar Jets in Context", 7-12 July 2008, island of Rhodes, Greece; editors: profs. Tom Ray and Kanaris Tsinganos; 10 pages, 10 figures, see animations at http://www.astro.cornell.edu/~romanova/conical.htm and http://www.astro.cornell.edu/~romanova/propeller.htm

We report on the numerical discovery of quasi-periodic oscillations (QPOs) associated with accretion through a non-axisymmetric magnetic boundary layer in the unstable regime, when two ordered equatorial streams form and rotate synchronously at approximately the angular velocity of the inner disk The streams hit the star's surface producing hot spots. Rotation of the spots leads to high-frequency QPOs. We performed a number of simulation runs for different magnetospheric sizes from small to tiny, and observed a definite correlation between the inner disk radius and the QPO frequency: the frequency is higher when the magnetosphere is smaller. In the stable regime a small magnetosphere forms and accretion through the usual funnel streams is observed, and the frequency of the star is expected to dominate the lightcurve. We performed exploratory investigations of the case in which the magnetosphere becomes negligibly small and the disk interacts with the star through an equatorial belt. We also performed investigation of somewhat larger magnetospheres where one or two ordered tongues may dominate over other chaotic tongues. In application to millisecond pulsars we obtain QPO frequencies in the range of 350 Hz to 990 Hz for one spot. The frequency associated with rotation of one spot may dominate if spots are not identical or antipodal. If the spots are similar and antipodal then the frequencies are twice as high. We show that variation of the accretion rate leads to drift of the QPO peak., Comment: 13 pages, 8 figures, accepted by MNRAS, v4: final version after significant revision, added new figures, Table 2. See sample animations as: http://www.astro.cornell.edu/~romanova/qpo.htm

This paper investigates the influence of magneto-centrifugally driven or simply magnetic winds of rapidly-rotating, strongly-magnetized T Tauri stars in causing the inward or outward migration of close-in giant planets. The azimuthal ram pressure of the magnetized wind acting on the planet tends to increase the planet's angular momentum and cause outward migration if the star's rotation period $P_*$ is less than the planet's orbital period $P_p$. In the opposite case, $P_* > P_p$, the planet migrates inward. Thus, planets orbiting at distances larger (smaller) than $0.06 {\rm AU}(P_*/5{\rm d})^{2/3}$ tend to be pushed outward (inward), where $P_*$ is the rotation period of the star assumed to have the mass of the sun. The magnetic winds are likely to occur in T Tauri stars where the thermal speed of the gas close to the star is small, where the star's magnetic field is strong, and where the star rotates rapidly. The time-scale for appreciable radial motion of the planet is estimated as $\sim 2 - 20$ Myr. A sufficiently massive close-in planet may cause tidal locking and once this happens the radial migration due to the magnetic wind ceases. The magnetic winds are expected to be important for planet migration for the case of a multipolar magnetic field rather than a dipole field where the wind is directed away from the equatorial plane and where a magnetospheric cavity forms. The influence of the magnetic wind in eroding and eventually destroying the accretion disk is analyzed. A momentum integral is derived for the turbulent wind/disk boundary layer and this is used to estimate the disk erosion time-scale as $\sim 1-10^2$ Myr, with the lower value favored., Comment: 8 pages, 6 figures