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Stellar Magnetic Field (stellar + magnetic_field)

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Physics and Astronomy100%

Selected Abstracts

Measuring stellar magnetic fields with the low-resolution spectropolarimeter of the William Herschel Telescope

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
F. Leone
ABSTRACT Although the influence of magnetic fields on the structure and evolution of stars has, to a great extent, been demonstrated theoretically, observational evidence for this in non-degenerated stars is still rather scarce and is mainly restricted to bright objects (V < 10). Stellar magnetic fields are commonly measured on the basis of circular spectropolarimetry at high/middle resolution across the profile of metal lines. The level of sensitivity of telescopes and spectrographs at present makes this still an almost impossible method for faint stars. In principle, stellar magnetic fields can also be measured on the basis of low-resolution spectropolarimetry, and very important results have been obtained at the 8-m European Southern Observatory telescopes with the Focal Reducer and Low-Dispersion Spectrograph (FORS1). The trade-off between signal-to-noise ratio (S/N) and spectral resolution in measuring stellar magnetic fields justifies an attempt, presented here, to perform these measurements at the 4.5-m William Herschel Telescope. One of the stars with the weakest known magnetic field, HD 3360, and the magnetic chemically peculiar stars, HD 10783, HD 74521 and HD 201601, have been observed with the Intermediate Dispersion Spectrograph and Imaging System (ISIS) in the 3785,4480 Å range. The measured stellar magnetic fields, from Stokes I and V spectra with S/N > 600, show an internal error of ,50 G when selecting the whole interval and ,200 G within a Balmer line. Ripples in the Stokes V spectra of HD 3360 result in an instrumental positive magnetic field certainly no larger than 80 G. [source]

Non-ideal evolution of non-axisymmetric, force-free magnetic fields in a magnetar

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008
A. Mastrano
ABSTRACT Recent numerical magnetohydrodynamic calculations by Braithwaite and collaborators support the ,fossil field' hypothesis regarding the origin of magnetic fields in compact stars and suggest that the resistive evolution of the fossil field can explain the reorganization and decay of magnetar magnetic fields. Here, these findings are modelled analytically by allowing the stellar magnetic field to relax through a quasi-static sequence of non-axisymmetric, force-free states, by analogy with spheromak relaxation experiments, starting from a random field. Under the hypothesis that the force-free modes approach energy equipartition in the absence of resistivity, the output of the numerical calculations is semiquantitatively recovered: the field settles down to a linked poloidal,toroidal configuration, which inflates and becomes more toroidal as time passes. A qualitatively similar (but not identical) end state is reached if the magnetic field evolves by exchanging helicity between small and large scales according to an ,-dynamo-like, mean-field mechanism, arising from the fluctuating electromotive force produced by the initial random field. The impossibility of matching a force-free internal field to a potential exterior field is discussed in the magnetar context. [source]

Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
J.-F. Donati
ABSTRACT From observations collected with the ESPaDOnS spectropolarimeter, we report the discovery of magnetic fields at the surface of the mildly accreting classical T Tauri star (cTTS) V2129 Oph. Zeeman signatures are detected, both in photospheric lines and in the emission lines formed at the base of the accretion funnels linking the disc to the protostar, and monitored over the whole rotation cycle of V2129 Oph. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized line profiles. We reconstruct the large-scale magnetic topology at the surface of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to be rather complex, with a dominant octupolar component and a weak dipole of strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar radial field spots located at high latitudes and coinciding with cool dark polar spots at photospheric level. This large-scale field geometry is unusually complex compared to those of non-accreting cool active subgiants with moderate rotation rates. As an illustration, we provide a first attempt at modelling the magnetospheric topology and accretion funnels of V2129 Oph using field extrapolation. We find that the magnetosphere of V2129 Oph must extend to about 7R, to ensure that the footpoints of accretion funnels coincide with the high-latitude accretion spots on the stellar surface. It suggests that the stellar magnetic field succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of V2129 Oph. The magnetospheric geometry we derive qualitatively reproduces the modulation of Balmer lines and produces X-ray coronal fluxes typical of those observed in cTTSs. [source]

The pulsar synchrotron: coherent radio emission

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009
I. Contopoulos
ABSTRACT We propose a simple physical picture for the generation of coherent radio emission in the axisymmetric pulsar magnetosphere that is quite different from the canonical paradigm of radio emission coming from the magnetic polar caps. In this first paper, we consider only the axisymmetric case of an aligned rotator. Our picture capitalizes on an important element of the magnetohydrodynamic (MHD) representation of the magnetosphere, namely the separatrix between the corotating closed-line region (the ,dead zone') and the open-field lines that originate in the polar caps. Along the separatrix flows the return current that corresponds to the main magnetospheric electric current emanating from the polar caps. Across the separatrix, both the toroidal and poloidal components of the magnetic field change discontinuously. The poloidal component discontinuity requires the presence of a significant annular electric current which has up to now been unaccounted for. We estimate the position and thickness of this annular current at the tip of the closed line region, and show that it consists of electrons (positrons) corotating with Lorentz factors on the order of 105, emitting incoherent synchrotron radiation that peaks in the hard X-rays. These particles stay in the region of highest annular current close to the equator for a path-length of the order of 1 m. We propose that, at wavelengths comparable to that path-length, the particles emit coherent radiation, with radiated power proportional to N2, where N is the population of particles in the above path-length. We calculate the total radio power in this wavelength regime and its scaling with pulsar period and stellar magnetic field and show that it is consistent with estimates of radio luminosity based on observations. [source]

Measuring stellar magnetic fields with the low-resolution spectropolarimeter of the William Herschel Telescope

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
F. Leone
ABSTRACT Although the influence of magnetic fields on the structure and evolution of stars has, to a great extent, been demonstrated theoretically, observational evidence for this in non-degenerated stars is still rather scarce and is mainly restricted to bright objects (V < 10). Stellar magnetic fields are commonly measured on the basis of circular spectropolarimetry at high/middle resolution across the profile of metal lines. The level of sensitivity of telescopes and spectrographs at present makes this still an almost impossible method for faint stars. In principle, stellar magnetic fields can also be measured on the basis of low-resolution spectropolarimetry, and very important results have been obtained at the 8-m European Southern Observatory telescopes with the Focal Reducer and Low-Dispersion Spectrograph (FORS1). The trade-off between signal-to-noise ratio (S/N) and spectral resolution in measuring stellar magnetic fields justifies an attempt, presented here, to perform these measurements at the 4.5-m William Herschel Telescope. One of the stars with the weakest known magnetic field, HD 3360, and the magnetic chemically peculiar stars, HD 10783, HD 74521 and HD 201601, have been observed with the Intermediate Dispersion Spectrograph and Imaging System (ISIS) in the 3785,4480 Å range. The measured stellar magnetic fields, from Stokes I and V spectra with S/N > 600, show an internal error of ,50 G when selecting the whole interval and ,200 G within a Balmer line. Ripples in the Stokes V spectra of HD 3360 result in an instrumental positive magnetic field certainly no larger than 80 G. [source]

Magnetohydrodynamics of stellar interiors

ASTRONOMY & GEOPHYSICS, Issue 4 2005
David Hughes
David Hughes, Robert Rosner and Nigel Weiss describe what was achieved during a programme on stellar magnetic fields at the Isaac Newton Institute in Cambridge. Over a four-month period more than 90 participants visited the Institute for a mixture of structured workshops and informal collaboration. [source]