Stellar Magnetic Fields

This comparison suggests that there will be a transition region between materially-dominated and magnetically-dominated flow. The region inside of which the stellar magnetic field controls the flow is called the magnetosphere. To within a factor of 2-3, the radius at which the transition occurs is (e.g., Ghosh Lamb 1979)... 

We assume that the stellar magnetic field is dipolar ( m d), and has axial symmetry everywhere. We use cylindrical coordinates (w,, z) centered on the neutron star and aligned with the stellar rotation axis. This configuration is sketched in Figure 1. We obtain the nondimensionalized equations which construct a complete set for the dynamics of reservoir ring, as following,... 

Stellar evolution Stellar magnetic fields Stellar magnitudes Stellar populations Stellar structure Stellar wind Stem cells Ster eochem istry Sticklebacks Stilts and avocets Stimulus... 

We have now to go one step further and to build stellar evolution models where the transport of angular momentum will be followed self-consistently under the action of meridional circulation, shear turbulence, and internal gravity waves. In this path some important aspects still need to be clarified Can we better describe the excitation mechanisms and evaluate in a more reliable way the quantitative properties of the wave spectra What is the direct contribution of 1GW to the transport of chemicals, especially in the dynamical shear layer produced just below the convective envelope by the wave-mean flow interaction What is the influence of the Coriolis force on IGW How do 1GW interact with a magnetic field Work is in progress in this direction. 

We see that the models which best reproduce the location of all the six data points are the tracks which do not fit the solar location. The models whose convection is calibrated on the 2D simulation make a poor job, as the FST models and other models with efficient convection do therefore this result can not be inputed to the fact that we employ local convection models. A possibility is that we are in front of an opacity problem, more that in front of a convection problem. Actually we would be inclined to say that opacities are not a problem (we have shown this in Montalban et al. (2004), by comparing models computed with Heiter et al (2002) or with AH97 model atmospheres), but something can still be badly wrong, as implied by the recent redetermination of solar metallicity (Asplund et al., 2004). A further possibility is that the inefficient convection in PMS requires the introduction of a second parameter -linked to the stellar rotation and magnetic field, as we have suggested in the past (Ventura et al., 1998 D Antona et al., 2000), but this remains to be worked out. 

Rapid rotation and mass-loss connection in Be stars was suggested for the first time by Struve(1931), which necessarily leads to the requirement of break-up velocity in Be stars. However, Vsin i statistics suggests almost all Be stars are well below the break-up velocity. The additional forces have been searched for so far i.e., stellar wind, magnetic field, mass accretion, and so on. At the moment, none of them can succeed in explaining the episodic mass-loss in Be stars. 

In 1954 Lust and Schluter [14] introduced force-free magnetic fields (FFMFs) into a theoretical model for stellar media in order to allow intense magnetic fields to coexist with large currents in stellar matter with vanishing Lorentz force. Notice should be taken that the Lorentz force is the electrodynamic analogue of the Magnus force alluded to above (see Fig. 6 and compare with Fig. 2). 

The stellarators are similar to Tokamaks, both employing a closed toroidal magnetic field for basic plasma confinement. The stabilization in stellarators is, however, provided by special helical windings16. They possess some potential advantages as compared to the Tokamaks with respect to confinement properties and the possibility of steady state operation. Technical and technological limitations such as the small size of the presently available stellarators and the lack of appropriate, sufficiently intense heating systems have inhibited progress comparable to that which Tokamaks have so far achieved. 

The coolest stars with just enough mass to fuse hydrogen are the M-dwarfs (see chapter 3). Two new classes of brown dwarfs have been added to the cool end of the stellar spectrum. The L-dwarfs (1,300 to 2,000 Kelvin) are slightly cooler and less massive than M-dwarfs. T-dwarfs are cooler and lighter than the L-dwarfs. Both of these new dwarfs cannot sustain hydrogen fusion. Researchers have recently discovered hundreds of T-dwarfs and tens of L-dwarfs. Even the cool T-dwarfs may have magnetic fields that create occasional stellar flares. [For more information, see Linda Rowan, Cooler dwarf stars, Science 289(5480) 697 (August 4, 2000).]... 

All young stellar objects show indications for stellar winds and outflows. These phenomena are always observed to occur in systems that undergo mass accretion that interacts with magnetic fields and rotation. They are not limited to star formation but are also observed in other cases, e.g. during accretion onto central black holes in galaxies. 

Observations of the Orion nebular cluster by the Chandra X-ray observatory reveal the presence of frequent (one every 6 days), large (up to 0.5 AU), and highly energetic stellar flares (with inferred magnetic fields as large as 3000 G) (Favata et al. 2005). The duration of the flares varies from less than an hour to almost three... 

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