Magnetic Spin-Up of Line-Driven Stellar Winds (original) (raw)
Related papers
Monthly Notices of the Royal Astronomical Society, 2008
Building upon our previous MHD simulation study of magnetic channeling in radiatively driven stellar winds, we examine here the additional dynamical effects of stellar rotation in the (still) 2-D axisymmetric case of an aligned dipole surface field. In addition to the magnetic confinement parameter η * introduced in Paper I, we characterize the stellar rotation in terms of a parameter W ≡ V rot /V orb (the ratio of the equatorial surface rotation speed to orbital speed), examining specifically models with moderately strong rotation W = 0.25 and 0.5, and comparing these to analogous non-rotating cases. Defining the associated Alfvén radius R A ≈ η 1/4 * R * and Kepler corotation radius R K ≈ W −2/3 R * , we find rotation effects are weak for models with R A < R K , but can be substantial and even dominant for models with R A ∼ > R K . In particular, by extending our simulations to magnetic confinement parameters (up to η * = 1000) that are well above those (η * = 10) considered in Paper I, we are able to study cases with R A R K ; we find that these do indeed show clear formation of the rigid-body disk predicted in previous analytic models, with however a rather complex, dynamic behavior characterized by both episodes of downward infall and outward breakout that limit the buildup of disk mass. Overall, the results provide an intriguing glimpse into the complex interplay between rotation and magnetic confinement, and form the basis for a full MHD description of the rigid-body disks expected in strongly magnetic Bp stars like σ Ori E.
Centrifugal Breakout of Magnetically Confined Line-driven Stellar Winds
The Astrophysical Journal, 2006
We present 2D MHD simulations of radiatively driven winds from a hot star having a dipole magnetic field aligned with the star's rotation axis. We focus in particular on a model with a moderately rapid rotation (about half the critical value), and also a strong magnetic confinement parameter, η * ≡ B 2 eq R * 2/Ṁ v ∞ = 600. The magnetic field channels and torques the wind outflow into an equatorial, rigidly rotating disk extending from near the Keplerian corotation radius outwards. The strong centrifugal force on material in the outer edge of this disk stretches the magnetic loops, leading to episodic breakout of mass when the field reconnects. The associated dissipation of magnetic energy heats material to temperatures of nearly 10 8 K, high enough to emit hard (several keV) X-rays. Such centrifugal mass ejection represents a novel mechanism for explaining X-ray flares recently observed in the magnetic Bp star σ Ori E.
Magnetic Spin-Up of Line-Driven Winds
2003
We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with a dipole magnetic field aligned to the star's rotation axis. For moderate to strong fields, much wind outflow is initially along closed magnetic loops that nearly corotate as a solid body with the underlying star, thus providing a torque that results in an effective angular momentum
Stellar Winds, Magnetic Fields and Disks
The Environments of the Sun and the Stars, 2013
All main sequence stars lose mass via stellar winds. The winds of cool stars like the sun are driven by gas pressure gradient. However, the winds of hot massive stars which tend to be luminous are driven by emitted by the star radiation pressure. Mass loss from such winds are significantly higher. In this article, I describe the nature of such radiatively driven winds and show how they interact with rotation and magnetic fields leading to stellar spindown and large-scale disk-like structures. In particular, I show that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, "wind magnetic confinement parameter", η * (=B 2 eq R 2 * /Ṁv ∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. A. ud-Doula ()
The effects of magnetic fields on line-driven hot-star winds
2003
This talk summarizes results from recent MHD simulations of the role of a dipole magnetic field in inducing large-scale structure in the line-driven stellar winds of hot, luminous stars. Unlike previous fixed-field analyses, the MHD simulations here take full account of the dynamical competition between the field and the flow. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless 'wind magnetic confinement parameter', η * (= B 2 eq R 2 * /Ṁ v ∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement, η * ≤ 1, the field is fully opened by wind outflow, but nonetheless, for confinement as small as η * = 1/10 it can have significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement, η * > 1, the magnetic field remains closed over limited range of latitude and height above the equatorial surface, but eventually is opened into nearly radial configuration at large radii. Within the closed loops, the flow is channeled toward loop tops into shock collisions that are strong enough to produce hard X-rays. Within the open field region, the equatorial channeling leads to oblique shocks that are again strong enough to produce X-rays and also lead to a thin, dense, slowly outflowing "disk" at the magnetic equator.
Spin Evolution of Accreting Young Stars. II. Effect of Accretion-Powered Stellar Winds
The Astrophysical Journal, 2012
We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind. For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the accretion-powered stellar wind scenario and the two types of disk-locking models (namely the X-wind and Ghosh & Lamb type models) and identify some remaining theoretical issues for understanding young star spins.
The nature of stellar winds in the star-disk interaction
Proceedings of the International Astronomical Union, 2007
Stellar winds may be important for angular momentum transport from accreting T Tauri stars, but the nature of these winds is still not well-constrained. We present some simulation results for hypothetical, hot (∼106K) coronal winds from T Tauri stars, and we calculate the expected emission properties. For the high mass loss rates required to solve the angular momentum problem, we find that the radiative losses will be much greater than can be powered by the accretion process. We place an upper limit to the mass loss rate from accretion-powered coronal winds of ∼ 10−11Myr−1. We conclude that accretion powered stellar winds are still a promising scenario for solving the stellar angular momentum problem, but the winds must be cool (e.g., 104K) and thus are not driven by thermal pressure.
Accretion‐powered Stellar Winds. III. Spin‐Equilibrium Solutions
The Astrophysical Journal, 2008
We compare the stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating (∼ 10% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that stellar winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a stellar wind. We derive a semi-analytic formula for predicting the equilibrium spin rate as a function only of the ratio ofṀ w /Ṁ a and a dimensionless magnetization parameter, Ψ ≡ B 2 * R 2 * (Ṁ a v esc) −1 , whereṀ w is the stellar wind mass outflow rate,Ṁ a the accretion rate, B * the stellar surface magnetic field strength, R * the stellar radius, and v esc the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of ∼ 10% of breakup speed forṀ w /Ṁ a ∼ 0.1. Finally, the assumption that the stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio ofṀ w /Ṁ a 0.6.
Two-component magnetohydrodynamical outflows around young stellar objects
Astronomy and Astrophysics, 2006
Context. We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar magnetospheric winds coupled with discdriven jets where the resistive and viscous accretion disc is self-consistently described. Aims. These innovative MHD simulations are devoted to the study of the interplay between a stellar wind (having different ejection mass rates) and an MHD disc-driven jet embedding the stellar wind. Methods. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. We first investigate the various angular momentum transports occurring in the magneto-viscous accretion disc. We then analyze the modifications induced by the interaction between the two components of the outflow. Results. Our simulations show that the inner outflow is accelerated from the central object's hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force. Conclusions. The simulations show that the MHD disc-driven outflow extracts angular momentum more efficiently than do viscous effects in near-equipartition, thin-magnetized discs where turbulence is fully developed. We also show that, when a dense inner stellar wind occurs, the resulting disc-driven jet has a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass-ejection rate in the disc-driven jet and a larger radial extension that is in better agreement with the observations, besides being more consistent.
Wreathes of Magnetism in Rapidly Rotating Suns
When our Sun was young it rotated much more rapidly than now. Observations of young, rapidly rotating stars indicate that many possess substantial magnetic activity and strong axisymmetric magnetic fields. We conduct simulations of dynamo action in rapidly rotating suns with the 3-D MHD anelastic spherical harmonic (ASH) code to explore the complex coupling between rotation, convection and magnetism. Here we study dynamo action realized in the bulk of the convection zone for two systems, rotating at three and five times the current solar rate. We find that substantial organized global-scale magnetic fields are achieved by dynamo action in these systems. Striking wreathes of magnetism are built in the midst of the convection zone, coexisting with the turbulent convection. This is a great surprise, for many solar dynamo theories have suggested that a tachocline of penetration and shear at the base of the convection zone is a crucial ingredient for organized dynamo action, whereas thes...