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Magnetic Pressure (magnetic + pressure)
Selected AbstractsImpact of tangled magnetic fields on fossil radio bubblesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007M. Ruszkowski ABSTRACT There is growing consensus that feedback from active galactic nuclei (AGN) is the main mechanism responsible for stopping cooling flows in clusters of galaxies. AGN are known to inflate buoyant bubbles that supply mechanical power to the intracluster gas [intracluster medium (ICM)]. High Reynolds number hydrodynamical simulations show that such bubbles get entirely disrupted within 100 Myr, as they rise in cluster atmospheres, which is contrary to observations. This artificial mixing has consequences for models trying to quantify the amount of heating and star formation in cool core clusters of galaxies. It has been suggested that magnetic fields can stabilize bubbles against disruption. We perform magnetohydrodynamical simulations of fossil bubbles in the presence of tangled magnetic fields using the high-order pencil code. We focus on the physically motivated case where thermal pressure dominates over magnetic pressure and consider randomly oriented fields with and without maximum helicity and a case where large-scale external fields drape the bubble. We find that helicity has some stabilizing effect. However, unless the coherence length of magnetic fields exceeds the bubble size, the bubbles are quickly shredded. As observations of Hydra A suggest that length-scale of magnetic fields may be smaller than typical bubble size, this may suggest that other mechanisms, such as viscosity, may be responsible for stabilizing the bubbles. However, since Faraday rotation observations of radio lobes do not constrain large-scale ICM fields well if they are aligned with the bubble surface, the draping case may be a viable alternative solution to the problem. A generic feature found in our simulations is the formation of magnetic wakes where fields are ordered and amplified. We suggest that this effect could prevent evaporation by thermal conduction of cold H, filaments observed in the Perseus cluster. [source] Inferring X-ray coronal structures from Zeeman,Doppler imagesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2002M. Jardine Abstract We have modelled the X-ray emission from the young rapid rotator AB Doradus (Prot= 0.514 d) using as a basis Zeeman,Doppler maps of the surface magnetic field. This allows us to reconcile the apparently conflicting observations of a high X-ray emission measure and coronal density with a low rotational modulation in the X-ray band. The technique is to extrapolate the coronal field from the surface maps by assuming the field to be potential. We then determine the coronal density for an isothermal corona by solving hydrostatic equilibrium along each field line and scaling the surface plasma pressure with the surface magnetic pressure. We set the density to zero along those field lines that are open and those where at any point along their length the plasma pressure exceeds the magnetic pressure. We then calculate the optically thin X-ray emission measure and rotational modulation for models with a range of coronal densities. Although the corona can be very extended, much of the emission comes from high-latitude regions close to the stellar surface. Since these are always in view as the star rotates, there is little rotational modulation. We find that emission measures in the observed range 1052.8,1053.3 cm,3 can be reproduced with densities in the range 109,1010.7 cm,3 for coronae at temperatures of 106,107 K. [source] Helium pre-enrichment in the first starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006Leonid Chuzhoy ABSTRACT We show that element diffusion can produce large fluctuations in the initial helium abundance of stars. Diffusion time-scale, which in stellar cores is much larger than the Hubble time, can fall below 108 yr in the neutral gas clouds of stellar mass, dominated by collisionless dark matter or with dynamically important radiation or magnetic pressure. Helium diffusion may therefore explain the recent observations of globular clusters, which are inconsistent with initially homogeneous helium distribution. [source] Formation of large-scale magnetic-towers in quasarsASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006Y. Kato Abstract The evolution of accretion disks and the formation of jets around super-massive black holes are studied by 3-D magnetohydrodynamic simulations. The previous studies of astrophysical jets have been concentrated on the effects of large-scale magnetic fields permeating accretion disks. However, the existence of such global magnetic fields is not evident in astrophysical objects, and their origin is not well understood. Instead, we concentrate on the effect of the magnetic fields amplified as a result of the magneto-rotational instability in the differentially rotating disk. In our simulations, we found the emergence of accumulated toroidal magnetic fields from the inner region of the disk (a so-called magnetic-tower) and also the formation of jets accelerated by the magnetic pressure of the magnetic-tower. The magnetic energy, ejected by the magnetic-tower jet during a lifetime of an active galactic nuclei (AGNs), is about 1046 erg. Our results indicate that the magnetic-tower jet can be one of the most powerful process for the formation and the evolution of the large-scale structured magnetic .elds in the cluster of galaxies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||