Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph*

Donati, J. F.; Jardine, M.; Gregory, S. G.; Petit, P.; Bouvier, J.; Dougados, C.; Ménard, F.; Cameron, A. Collier; Harries, Tim J.; Jeffers, S. V.; Paletou, F.

doi:10.1111/j.1365-2966.2007.12194.x

Cited by 190 publications

(320 citation statements)

References 37 publications

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“…Recent measurements by Donati et al (2007Donati et al ( , 2008 of the two CTTSs V2129 Oph and BP Tau have shown that their fields have a significant octupolar component. In our next papers (Romanova et al 2011a;Long et al 2011) we compare our numerical model with observations of these stars.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…The potential approximation is often used in extrapolation of the observed surface fields to larger distances (e.g., Jardine et al 2002;Gregory et al 2006;Donati et al 2007). Panels (b)-(d) show that the potential approximation is sufficiently good inside the Alfvén surface, where β 1 < 1.…”

Section: Pure Octupolementioning

confidence: 99%

“…The observed surface field is often approximated with a set of multipoles whose magnetic moments have different misalignment angles relative to the rotational axis (i.e., differ-ent tilts) and different phases in the longitudinal direction (e.g., Jardine et al 2002). Recent observations of two CTTSs have shown that in one star (V2129 Oph) the surface magnetic field can be approximated with the superposition of a slightly (but differently) tilted octupole, which dominates, and a dipole, where the magnetic moments are oriented at different phases (Donati et al 2007. In the other star, BP Tau, the magnetic field can be approximated with slightly (but differently) tilted dipole and octupole moments of equal strengths, where the magnetic moments are in anti-phase .…”

Section: Introductionmentioning

confidence: 99%

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Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

2012

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Recent measurements of the surface magnetic fields of classical T Tauri stars (CTTSs) and magnetic cataclysmic variables show that their magnetic fields have a complex structure. Investigation of accretion onto such stars requires global three-dimensional (3D) magnetohydrodynamic (MHD) simulations, where the complexity of simulations strongly increases with each higher-order multipole. Previously, we were able to model disc accretion onto stars with magnetic fields described by a superposition of dipole and quadrupole moments. However, in some stars, like CTTS V2129 Oph and BP Tau, the octupolar component is significant and it was necessary to include the next octupolar component. Here, we show results of global 3D MHD simulations of accretion onto stars with superposition of the dipole and octupole fields, where we vary the ratio between components. Simulations show that if octupolar field strongly dominates at the disc-magnetosphere boundary, then matter flows into the ring-like octupolar poles, forming ring-shape spots at the surface of the star above and below equator. The light-curves are complex and may have two peaks per period. In case where the dipole field dominates, matter accretes in two ordered funnel streams towards poles, however the polar spots are meridionally-elongated due to the action of the octupolar component. In the case when the fields are of similar strengths, both, polar and belt-like spots are present. In many cases the light-curves show the evidence of complex fields, excluding the cases of small inclinations angles, where sinusoidal light-curve is observed and 'hides' the information about the field complexity.We also propose new mechanisms of phase shift in stars with complex magnetic fields. We suggest that the phase shifts can be connected with: (1) temporal variation of the star's intrinsic magnetic field and subsequent redistribution of main magnetic poles; (2) variation of the accretion rate, which causes the disc to interact with the magnetic fields associated with different magnetic moments. We use our model to demonstrate these phase shift mechanisms, and we discuss possible applications of these mechanisms to accreting millisecond pulsars and young stars.

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Section: Conclusion and Discussionmentioning

confidence: 99%

Section: Pure Octupolementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

2012

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“…Since that time ZDI was a success story of its own and has enormously contributed to our current understanding of stellar magnetism (e.g. Donati 1999;Hussain et al 2002;Kochukhov et al 2004;Donati et al 2006Donati et al , 2007Petit et al 2008). However, there are a number of fundamental problems which has to be considered when dealing with ZDI as an inverse problem.…”

Section: Introductionmentioning

confidence: 99%

Zeeman-Doppler imaging: old problems and new methods

et al. 2008

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Abstract. Zeeman-Doppler Imaging (ZDI) is a powerful inversion method to reconstruct stellar magnetic surface fields. The reconstruction process is usually solved by translating the inverse problem into a regularized least-square or optimization problem. In this contribution we will emphasize that ZDI is an inherent non-linear problem and the corresponding regularized optimization is, like many non-linear problems, potentially prone to local minima. We show how this problem will be exacerbated by using an inadequate forward model. To facilitate a more consistent full radiative transfer driven approach to ZDI we describe a two-stage strategy that consist of a principal component analysis (PCA) based line profile reconstruction and a fast approximate polarized radiative transfer method to synthesize local Stokes profiles. Moreover, we introduce a novel statistical inversion method based on artificial neural networks (ANN) which provide a fast calculation of a first guess model and allows to incorporate better physical constraints into the inversion process.

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“…The intensity and topology of the stellar surface magnetic field can be obtained through the broadening of Zeeman sensitive lines [29,43] and Zeeman Doppler Imaging (ZDI) [16,17]. Ultraviolet (UV) and optical excess, that veils the photospheric lines, are produced at the hot spot [5,26].…”

Section: Introductionmentioning

confidence: 99%

Observational clues to the physics at the magnetosphere in young stellar objects

Alencar

2014

EPJ Web of Conferences

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Abstract. Classical T Tauri stars offer an excellent laboratory to study the accretion process and star-disk interaction, which define the transfer of mass and angular momentum in the system. The interaction takes place very close to the star, in a region of about a few stellar radii, and it is therefore challenging to study in detail the physics of the magnetosphere in young stellar objects. We will discuss, through observational examples, how our description of a static and axisymmetric stellar magnetosphere from the early 90s has evolved to a very dynamical multipolar region that takes into account non-steady accretion and time-dependent star-disk interactions.

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Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph*

Cited by 190 publications

References 37 publications

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Zeeman-Doppler imaging: old problems and new methods

Observational clues to the physics at the magnetosphere in young stellar objects

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