The magnetic fields of forming solar-like stars

Gregory, S. G.; Jardine, M.; Gray, C.G.; Donati, J. F.

doi:10.1088/0034-4885/73/12/126901

Cited by 41 publications

(67 citation statements)

References 219 publications

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“…Physically, this simplifying assumption corresponds to a non-accreting, unmagnetised young stellar object that rotates at a negligibly slow rate. This picture is somewhat contrary to real pre-main-sequence stars, which typically accrete ∼10 −8 M /year from the disk (Hartmann 2008;Hillenbrand 2008), have ∼1 kGauss magnetic fields at the stellar surface (Shu et al 1994;Gregory et al 2010), and rotate with characteristic periods in the range P rot 1−10 days (Herbst et al 2007). Accordingly, Batygin & Adams (2013) examined magnetically and gravitationally facilitated disk-star angular momentum transfer with an eye towards constraining the conditions needed for the acquisition of spin-orbit misalignment.…”

Section: Monte Carlo Simulationsmentioning

confidence: 87%

Spin-orbit angle distribution and the origin of (mis)aligned hot Jupiters

Crida

Batygin

2014

A&A

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Context. For 61 transiting hot Jupiters, the projection of the angle between the orbital plane and the stellar equator (called the spinorbit angle) has been measured. For about half of them, a significant misalignment is detected, and retrograde planets have been observed. This challenges scenarios of the formation of hot Jupiters. Aims. In order to better constrain formation models, we relate the distribution of the real spin-orbit angle Ψ to the projected one β. Then, a comparison with the observations is relevant. Methods. We analyse the geometry of the problem to link analytically the projected angle β to the real spin-orbit angle Ψ. The distribution of Ψ expected in various models is taken from the literature, or derived with a simplified model and Monte Carlo simulations in the case of the disk-torquing mechanism.Results. An easy formula to compute the probability density function (PDF) of β knowing the PDF of Ψ is provided. All models tested here look compatible with the observed distribution beyond 40 degrees, which is so far poorly constrained by only 18 observations. But only the disk-torquing mechanism can account for the excess of aligned hot Jupiters, provided that the torquing is not always efficient. This is the case if the exciting binaries have semi-major axes as large as ∼10 4 AU. Conclusions. Based on comparison with the set of observations available today, scattering models and the Kozai cycle with tidal friction models can not be solely responsible for the production of all hot Jupiters. Conversely, the presently observed distribution of the spin-orbit angles is compatible with most hot Jupiters having been transported by smooth migration inside a proto-planetary disk, itself possibly torqued by a companion.

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Section: Monte Carlo Simulationsmentioning

confidence: 87%

Spin-orbit angle distribution and the origin of (mis)aligned hot Jupiters

Crida

Batygin

2014

A&A

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“…For both the dipole and octupole terms, the leading factors of 1/2 are included so that B dip and B oct are the polar strengths of the dipole and octupole components (see also Gregory et al 2010;Adams & Gregory 2012). It is convenient to scale out the dipole field strength so that the relative size of the octupole contribution 7 Currents exist within the star and most likely within the disk, and act as source terms for the magnetic field.…”

Section: Magnetically Controlled Accretionmentioning

confidence: 99%

“…For T Tauri star-disk systems, observations of magnetic accretion signatures indicate that this parameter typically falls within the range of 0 Γ 10 (e.g., see Donati et al 2007Donati et al , 2008Donati et al , 2010Donati et al , 2012, and also Gregory et al 2010).…”

Section: Magnetically Controlled Accretionmentioning

confidence: 99%

Magnetic and Gravitational Disk-Star Interactions: An Interdependence of PMS Stellar Rotation Rates and Spin-Orbit Misalignments

Batygin

Adams

2013

ApJ

120

126

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The presence of giant gaseous planets that reside in close proximity to their host stars, i.e., hot Jupiters, may be a consequence of large-scale radial migration through the protoplanetary nebulae. Within the framework of this picture, significant orbital obliquities characteristic of a substantial fraction of such planets can be attributed to external torques that perturb the natal disks out of alignment with the spin axes of their host stars. Therefore, the acquisition of orbital obliquity likely exhibits sensitive dependence on the physics of disk-star interactions. Here, we analyze the primordial excitation of spin-orbit misalignment of Sun-like stars in light of disk-star angular momentum transfer. We begin by calculating the stellar pre-main-sequence rotational evolution, accounting for spin-up due to gravitational contraction and accretion as well as spin-down due to magnetic star-disk coupling. We devote particular attention to angular momentum transfer by accretion, and show that while generally subdominant to gravitational contraction, this process is largely controlled by the morphology of the stellar magnetic field (that is, specific angular momentum accreted by stars with octupole-dominated surface fields is smaller than that accreted by dipole-dominated stars by an order of magnitude). Subsequently, we examine the secular spin-axis dynamics of diskbearing stars, accounting for the time-evolution of stellar and disk properties, and demonstrate that misalignments are preferentially excited in systems where stellar rotation is not overwhelmingly rapid. Moreover, we show that the excitation of spin-orbit misalignment occurs impulsively through an encounter with a resonance between the stellar precession frequency and the disk-torquing frequency. Cumulatively, the model developed herein opens up a previously unexplored avenue toward understanding star-disk evolution and its consequences in a unified manner.

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“…The potential approximation is usually used to extrapolate the surface magnetic field to larger distances. That is, it is suggested that the field outside of the star is not disturbed by the surrounding plasma (e.g., Gregory et al 2010). Gregory et al (2006) calculated possible paths of gas flow around stars with magnetic fields constructed from measurements using the potential approximation.…”

Section: Introductionmentioning

confidence: 99%

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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The magnetic fields of forming solar-like stars

Cited by 41 publications

References 219 publications

Spin-orbit angle distribution and the origin of (mis)aligned hot Jupiters

Spin-orbit angle distribution and the origin of (mis)aligned hot Jupiters

Magnetic and Gravitational Disk-Star Interactions: An Interdependence of PMS Stellar Rotation Rates and Spin-Orbit Misalignments

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

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