Simulations of Winds of Weak-Lined T Tauri Stars: The Magnetic Field Geometry and the Influence of the Wind on Giant Planet Migration (original) (raw)
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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.
2011
We carry out the first time-dependent numerical MagnetoHydroDynamic modeling of an extrasolar planetary system to study the interaction of the stellar magnetic field and wind with the planetary magnetosphere and outflow. We base our model on the parameters of the HD 189733 system, which harbors a close-in giant planet. Our simulation reveals a highly structured stellar corona characterized by sectors with different plasma properties. The star-planet interaction varies in magnitude and complexity, depending on the planetary phase, planetary magnetic field strength, and the relative orientation of the stellar and planetary fields. It also reveals a long, comet-like tail which is a result of the wrapping of the planetary magnetospheric tail by its fast orbital motion. A reconnection event occurs at a specific orbital phase, causing mass loss from the planetary magnetosphere that can generate a hot spot on the stellar surface. The simulation also shows that the system has sufficient energy to produce hot-spots observed in Ca II lines in giant planet hosting stars. However, the short duration of the reconnection event suggests that such SPI cannot be observed persistently.
INTERACTIONS OF THE MAGNETOSPHERES OF STARS AND CLOSE-IN GIANT PLANETS
The Astrophysical Journal, 2009
Since the first discovery of an extrasolar planetary system more than a decade ago, hundreds more have been discovered. Surprisingly, many of these systems harbor Jupiter-class gas giants located close to the central star, at distances of 0.1 AU or less. Observations of chromospheric "hot spots" that rotate in phase with the planetary orbit, and elevated stellar X-ray luminosities, suggest that these close-in planets significantly affect the structure of the outer atmosphere of the star through interactions between the stellar magnetic field and the planetary magnetosphere. Here, we carry out the first detailed three-dimensional magnetohydrodynamics simulation containing the two magnetic bodies and explore the consequences of such interactions on the steady-state coronal structure. The simulations reproduce the observable features of (1) increase in the total X-ray luminosity, (2) appearance of coronal hot spots, and (3) phase shift of these spots with respect to the direction of the planet. The proximate cause of these is an increase in the density of coronal plasma in the direction of the planet, which prevents the corona from expanding and leaking away this plasma via a stellar wind. The simulations produce significant low temperature heating. By including dynamical effects, such as the planetary orbital motion, the simulation should better reproduce the observed coronal heating.
Stellar coronal magnetic fields and star-planet interaction
Astronomy and Astrophysics, 2009
Context. Evidence of magnetic interaction between late-type stars and close-in giant planets is provided by the observations of stellar hot spots rotating synchronously with the planets and showing an enhancement of chromospheric and X-ray fluxes. Possible photospheric signatures of such an interaction have also been reported. Aims. We investigate star-planet interaction in the framework of a magnetic field model of a stellar corona, considering the interaction between the coronal field and that of a planetary magnetosphere moving through the corona. This is motivated, among other reasons, by the difficulty of accounting for the energy budgets of the interaction phenomena with previous models. Methods. A linear force-free model is applied to describe the coronal field and study the evolution of its total magnetic energy and relative helicity according to the boundary conditions at the stellar surface and the effects related to the planetary motion through the corona. Results. The energy budget of the star-planet interaction is discussed, assuming that the planet may trigger a release of the energy of the coronal field by decreasing its relative helicity. The observed intermittent character of the star-planet interaction is explained by a topological change in the stellar coronal field, induced by a variation in its relative helicity. The model predicts the formation of many prominence-like structures in the case of highly active stars owing to the accumulation of matter evaporated from the planet inside an azimuthal flux rope in the outer corona. Moreover, the model can explain why stars accompanied by close-in planets have a higher X-ray luminosity than those with distant planets. It predicts that the best conditions for detecting radio emission from the exoplanets and their host stars are achieved when the field topology is characterized by field lines connected to the surface of the star, leading to a chromospheric hot spot rotating synchronously with the planet. Conclusions. The main predictions of the model can be verified with current observational techniques, by a simultaneous monitoring of the chromospheric flux and X-ray (or radio) emission, and spectropolarimetric observations of the photospheric magnetic fields.
Magnetic Fields In Stars And Disks
Journal of the Korean Astronomical Society
Magnetic fields are thought to play a role in a wide variety of important astrophysical processes, from angular momentum transport and jet formation in accretion disks to corona formation in stars. Unfortunately, the dynamics of magnetic fields in astrophysical plasmas are extremely complicated, and the success of current theoretical models and computer simulations seems to be inversely correlated with the amount of observational detail available to us. Here I will discuss some of the more striking conflicts between numerical simulations and observations, and present an explanation for them based on an important dynamical process which is not adequately modeled in current numerical simulations. These processes will lead to the formation of flux tubes in stars and accretion disks, in accordance with observations. I will discuss some of the implications of flux tube formation for stellar and accretion disk dynamos. Key Words : magnetic fields, turbulence, accretion I. INTRODUCTION Mag...
The Astrophysical Journal, 2018
We present results from a set of numerical simulations aimed at exploring the mechanism of coronal mass ejection (CME) suppression in active stars by an overlying large-scale magnetic field. We use a state-of-the-art 3D magnetohydrodynamic code that considers a self-consistent coupling between an Alfvén wave-driven stellar wind solution, and a first-principles CME model based on the eruption of a flux rope anchored to a mixed-polarity region. By replicating the driving conditions used in simulations of strong solar CMEs, we show that a large-scale dipolar magnetic field of 75G is able to fully confine eruptions within the stellar corona. Our simulations also consider CMEs exceeding the magnetic energy used in solar studies, which are able to escape the large-scale magnetic field confinement. The analysis includes a qualitative and quantitative description of the simulated CMEs and their dynamics, which reveals a drastic reduction of the radial speed caused by the overlying magnetic field. With the aid of recent observational studies, we place our numerical results in the context of solar and stellar flaring events. In this way, we find that this particular large-scale magnetic field configuration establishes a suppression threshold around ∼3×10 32 erg in the CME kinetic energy. Extending the solar flare-CME relations to other stars, such CME kinetic energies could be typically achieved during erupting flaring events with total energies larger than 6×10 32 erg (GOES class ∼X70).
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.
Influence of surface stressing on stellar coronae and winds
Monthly Notices of the Royal Astronomical Society, 2013
The large-scale field of the Sun is well represented by its lowest energy (or potential) state. Recent observations, by comparison, reveal that many solar-type stars show large-scale surface magnetic fields that are highly non-potential-that is, they have been stressed above their lowest energy state. This non-potential component of the surface field is neglected by current stellar wind models. The aim of this paper is to determine its effect on the coronal structure and wind. We use Zeeman-Doppler surface magnetograms of two stars-one with an almost potential, one with a non-potential surface field-to extrapolate a static model of the coronal structure for each star. We find that the stresses are carried almost exclusively in a band of unidirectional azimuthal field that is confined to mid-latitudes. Using this static solution as an initial state for a magnetohydrodynamic (MHD) wind model, we then find that the final state is determined primarily by the potential component of the surface magnetic field. The band of azimuthal field must be confined close to the stellar surface, as it is not compatible with a steady-state wind. By artificially increasing the stellar rotation rate, we demonstrate that the observed azimuthal fields cannot be produced by the action of the wind but must be due to processes at or below the stellar surface. We conclude that the background winds of solar-like stars are largely unaffected by these highly stressed surface fields. Nonetheless, the increased flare activity and associated coronal mass ejections that may be expected to accompany such highly stressed fields may have a significant impact on any surrounding planets.
The Astrophysical Journal, 2005
This paper explores the effects of post-AGB winds driven solely by magnetic pressure from the stellar surface. It is found that winds can reach high speeds under this assumption, and lead to the formation of highly collimated proto-planetary nebulae. Bipolar knotty jets with periodic features and constant velocity are well reproduced by the models. Several wind models with terminal velocities from a few tens of km s −1 up to 10 3 km s −1 are calculated, yielding outflows with linear momenta in the range 10 36 − 10 40 g cm s −1 , and kinetic energies in the range 10 42 − 10 47 erg. These results are in accord with recent observations of proto-planetary nebulae that have pointed out serious energy and momentum deficits if radiation pressure is considered as the only driver for these outflows. Our models strengthen the notion that the large mass-loss rates of post-AGB stars, together with the short transition times from the late AGB to the planetary nebula stage, could be directly linked with the generation of strong magnetic fields during this transition stage.