Global 3-D solar-type star-disc dynamo systems: I. MHD modeling

Rekowski, B. von; Piskunov, Nikolai

doi:10.1002/asna.200510526

Cited by 7 publications

(7 citation statements)

References 37 publications

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“…Another new accretion model may offer insights into the connection between X‐ray emission and accretion in T Tauri stars. A magnetohydrodynamic model by von Rekowski & Piskunov (2006) suggests that rather than material being loaded on to field lines at the inner edge of the disc, it flows on to the star only at times when, and into regions where, the accretion flow is able to overcome the stellar wind. In such a way, this model predicts that accretion only occurs into regions of low field strength.…”

Section: Summary and Discussionmentioning

confidence: 99%

Rotationally modulated X-ray emission from T Tauri stars

Gregory

Jardine

Cameron

et al. 2006

Monthly Notices of the Royal Astronomical Society

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We have modelled the rotational modulation of X-ray emission from T Tauri stars assuming that they have isothermal, magnetically confined coronae. By extrapolating surface magnetograms, we find that T Tauri coronae are compact and clumpy, such that rotational modulation arises from X-ray emitting regions being eclipsed as the star rotates. Emitting regions are close to the stellar surface and inhomogeneously distributed about the star. However, some regions of the stellar surface, which contain wind bearing open field lines, are dark in X-rays. From simulated X-ray light curves, obtained using stellar parameters from the Chandra Orion Ultradeep Project, we calculate X-ray periods and make comparisons with optically determined rotation periods. We find that X-ray periods are typically equal to, or are half of, the optical periods. Further, we find that X-ray periods are dependent upon the stellar inclination, but that the ratio of X-ray to optical period is independent of stellar mass and radius

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Section: Summary and Discussionmentioning

confidence: 99%

Rotationally modulated X-ray emission from T Tauri stars

Gregory

Jardine

Cameron

et al. 2006

Monthly Notices of the Royal Astronomical Society

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“…In principle, we could set s int as close to zero as possible (by not using smoothing but retaining an inner boundary), in order to study the processes that happen in the immediate vicinity of the star, like winds, the magnetic cavity and surface accretion (von Rekowski & Piskunov 2006). However, due to the increasing Keplerian velocity in the advection and the decreasing resolution of the orbits, non-axisymmetric wave modes (particularly the m = 4 mode) build up in the inner disk as we try to push s int → 0.…”

Section: Boundary Conditionsmentioning

confidence: 99%

Global magnetohydrodynamical models of turbulence in protoplanetary disks

Lyra¹,

Johansen

Klahr

et al. 2008

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Aims. We present global 3D MHD simulations of disks of gas and solids, aiming at developing models that can be used to study various scenarios of planet formation and planet-disk interaction in turbulent accretion disks. A second goal is to demonstrate that Cartesian codes are comparable to cylindrical and spherical ones in handling the magnetohydrodynamics of the disk simulations while offering advantages, such as the absence of a grid singularity, for certain applications, e.g., circumbinary disks and disk-jet simulations. Methods. We employ the Pencil Code, a 3D high-order finite-difference MHD code using Cartesian coordinates. We solve the equations of ideal MHD with a local isothermal equation of state. Planets and stars are treated as particles evolved with an N-body scheme. Solid boulders are treated as individual superparticles that couple to the gas through a drag force that is linear in the local relative velocity between gas and particle. Results. We find that Cartesian grids are well-suited for accretion disk problems. The disk-in-a-box models based on Cartesian grids presented here develop and sustain MHD turbulence, in good agreement with published results achieved with cylindrical codes. Models without an inner boundary do not show the spurious build-up of magnetic pressure and Reynolds stress seen in the models with boundaries, but the global stresses and alpha viscosities are similar in the two cases. We investigate the dependence of the magnetorotational instability on disk scale height, finding evidence that the turbulence generated by the magnetorotational instability grows with thermal pressure. The turbulent stresses depend on the thermal pressure obeying a power law of 0.24 ± 0.03, compatible with the value of 0.25 found in shearing box calculations. The ratio of Maxwell to Reynolds stresses decreases with increasing temperature, dropping from 5 to 1 when the sound speed was raised by a factor 4, maintaing the same field strength. We also study the dynamics of solid boulders in the hydromagnetic turbulence, by making use of 10 6 Lagrangian particles embedded in the Eulerian grid. The effective diffusion provided by the turbulence prevents settling of the solids in a infinitesimally thin layer, forming instead a layer of solids of finite vertical thickness. The measured scale height of this diffusion-supported layer of solids implies turbulent vertical diffusion coefficients with globally averaged Schmidt numbers of 1.0 ± 0.2 for a model with α ≈ 10 −3 and 0.78 ± 0.06 for a model with α ≈ 10 −1 . That is, the vertical turbulent diffusion acting on the solids phase is comparable to the turbulent viscosity acting on the gas phase. The average bulk density of solids in the turbulent flow is quite low (ρ p = 6.0 × 10 −11 kg m −3 ), but in the high pressure regions, significant overdensities are observed, where the solid-to-gas ratio reached values as great as 85, corresponding to 4 orders of magnitude higher than the initial interstellar value of 0.01.

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“…Considerable progress has already been made in studying the interaction of simulated stellar magnetic fields with accretion discs (von Rekowski & Piskunov 2006; Long et al 2007; Matt & Pudritz 2008a,b). Recent models of multipolar fields in particular show that accreting material may take the most energetically favourable path (i.e.…”

Section: The Accreting Fieldmentioning

confidence: 99%

Coronal structure of the classical T Tauri star V2129 Oph

Jardine

Gregory

Donati

2008

Monthly Notices RAS

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The nature of the magnetic coupling between T Tauri stars and their discs determines not only the mass accretion process but possibly the spin evolution of the central star. We have taken a recently published surface magnetogram of one moderately accreting T Tauri star (V2129 Oph) and used it to extrapolate the geometry of its large-scale field. We determine the structure of the open (wind-bearing) field lines, the closed (X-ray bright) field lines and the relatively small subset of field lines that pass through the equatorial plane inside the Keplerian corotation radius and which are therefore available to accrete.We consider a series of models in which the stellar magnetic field is opened up by the outward pressure of the hot coronal gas at a range of radii or source surfaces. As the source surface is increased, accretion takes place along progressively simpler field structures and impacts on progressively fewer sites at the stellar surface. This is consistent with the observed variation in the Ca II IRT and He I lines which suggests that accretion in the visible hemisphere is confined to a single high-latitude spot. By determining the density and velocity of the accretion flows, we find that in order to have most of the total mass accretion rate impacting on a single highlatitude region we need disc material to accrete from approximately 7R , close to the Keplerian corotation radius at 6.8R .We also calculate the coronal density and X-ray emission measure. We find that both the magnitude and rotational modulation of the emission measure increase as the source surface is increased. For the field structure of V2129 Oph which is dominantly octupolar, the emission forms a bright, high-latitude ring that is always in view as the star rotates. Since the accretion funnels are not dense enough to cause significant scattering of coronal X-ray photons, they provide only a low rotational modulation of around 10 per cent at most.

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Global 3-D solar-type star-disc dynamo systems: I. MHD modeling

Cited by 7 publications

References 37 publications

Rotationally modulated X-ray emission from T Tauri stars

Rotationally modulated X-ray emission from T Tauri stars

Global magnetohydrodynamical models of turbulence in protoplanetary disks

Coronal structure of the classical T Tauri star V2129 Oph

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