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Stronger Magnetic Fields (stronger + magnetic_field)
Selected AbstractsEvolution of magnetic fields in stars across the upper main sequence: II.ASTRONOMISCHE NACHRICHTEN, Issue 6 2007Observed distribution of the magnetic field geometry Abstract We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M, are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M, seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M, and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities , is detected in both higher and lower mass stars. It is quite possible that the angle , becomes close to 0. in slower rotating stars of mass M > 3 M, too, analog to the behaviour of angles , in slowly rotating stars of M < 3 M,. The obliquity angle distribution as inferred from the distribution of r -values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle , tends to reach values close to either 90. or 0. for M < 3 M,. The evolution of the obliquity angle , seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of , with the elapsed time on the main sequence for stars with M > 3 M,, no similar trend is found for stars with M < 3 M,. However, the predominance of high values of , at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Equilibrium states of magnetized toroid,central compact object systemsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009Jun Otani ABSTRACT Equilibrium configurations of self-gravitating magnetized toroid,central compact object systems have been constructed in the framework of the Newtonian gravity. We have succeeded in including not only poloidal but also toroidal magnetic fields under the ideal magnetohydrodynamic approximation. We find two new and interesting results about the critical equilibrium states of such systems beyond which no equilibrium states are allowed to exist. First, there appear critical distances from the central compact objects to the inner surfaces of the magnetized toroids. Furthermore, these critical distances are much larger than the distances of the innermost stable circular orbits. It implies that even if these systems would be treated in the framework of general relativity, there would appear cusp structures of the effective total potential of the gravitational and magnetic forces for strongly magnetized toroids which are different from the general relativistic cusp structures. Secondly, since the strength of the magnetic field for the critical equilibrium configurations is roughly 1015 G if the mass of the central object is 1.4 M, and the maximum density of the toroid is 1011 g cm,3, the existence of equilibrium states of toroids around compact objects seems to set limit to the maximum value of the magnetic field of the system to be ,1015 G, i.e. no stronger magnetic fields can be realized for the systems consisting of magnetized toroids and central compact objects with the masses around the typical neutron star mass. The value of the maximum density of the toroid, 1011 g cm,3, is taken from the theoretical computational results of binary neutron star merging simulations in full general relativity. [source] Exciton Aharonov,Bohm effect and emission kinetics in nanoringsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2006M. Grochol Abstract The Aharonov,Bohm effect of excitons and their relaxation kinetics are investigated within the model of semiconductor nanorings of zero width. The kinetic equations where calculated deformation potential matrix elements for an acoustic phonon scattering are used as an input are solved for the steady state. Photoluminescence quenching is observed for stronger magnetic fields when electron and hole are spatially separated. The non-radiative decay plays a decisive role. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star,ASTRONOMISCHE NACHRICHTEN, Issue 4 2010S. Hubrig Abstract In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow-up studies of magnetism among young pre-main-sequence stars. We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb-Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period Prot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] New magnetic field measurements of , Cephei stars and slowly pulsating B stars,ASTRONOMISCHE NACHRICHTEN, Issue 4 2009S. Hubrig Abstract We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate , Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B-type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three , Cephei stars and two stars suspected to be , Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3, has been diagnosed in two normal B-type stars, the nitrogen-rich early B-type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of , Cephei stars have detected magnetic fields: Out of 13 , Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected , Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non-magnetic pulsating stars in the H-R diagram. We find that their domains in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i -values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||