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VbvAstE-001

Book Boris V. Vasiliev Astrophysics

Chapter 7 Magnetic

Chapter 7 Magnetic fields and magnetic moments of stars 7.1 Magnetic moments of celestial bodies A thin spherical surface with radius r carrying an electric charge q at the rotation around its axis with frequency Ω obtains the magnetic moment m = r2 qΩ. (7.1) 3c The rotation of a ball charged at density ϱ(r) will induce the magnetic moment µ = Ω 3c ∫ R 0 r 2 ϱ(r) 4πr 2 dr. (7.2) Thus the positively charged core of a star induces the magnetic moment √ GM⋆R 2 ⋆ m + = Ω. (7.3) 5c A negative charge will be concentrated in the star atmosphere. The absolute value of atmospheric charge is equal to the positive charge of a core. As the atmospheric charge is placed near the surface of a star, its magnetic moment will be more than the core magnetic moment. The calculation shows that as a result, the total magnetic moment of the star will have the same order of magnitude as the core but it will be negative: √ G m Σ ≈ − c M⋆R2 ⋆Ω. (7.4) 55

Simultaneously, the torque of a ball with mass M and radius R is L ≈ M ⋆R 2 ⋆Ω. (7.5) As a result, for celestial bodies where the force of their gravity induces the electric polarization according to Eq.(4.2), the giromagnetic ratio will depend on world constants only: m Σ L ≈ − √ G c . (7.6) This relation was obtained for the first time by P.M.S.Blackett [6]. He shows that giromagnetic ratios of the Earth, the Sun and the star 78 Vir are really near to √ G/c. By now the magnetic fields, masses, radii and velocities of rotation are known for all planets of the Solar system and for a some stars [18]. These measuring data are shown in Fig.(7.1), which is taken from [18]. It is possible to see that these data are in satisfactory agreement with Blackett’s ratio. At some assumption, the same parameters can be calculated for pulsars. All measured masses of pulsars are equal by the order of magnitude [21]. It is in satisfactory agreement with the condition of equilibrium of relativistic matter (see ). It gives a possibility to consider that masses and radii of pulsars are determined. According to generally accepted point of view, pulsar radiation is related with its rotation, and it gives their rotation velocity. These assumptions permit to calculate the giromagnetic ratios for three pulsars with known magnetic fields on their poles [5]. It is possible to see from Fig.(7.1), the giromagnetic ratios of these pulsars are in agreement with Blackett’s ratio. 7.2 Magnetic fields of hot stars At the estimation of the magnetic field on the star pole, it is necessary to find the field which is induced by stellar atmosphere. The field which is induced by stellar core is small because R ⋆ ≪ R 0. The field of atmosphere m − = Ω 3c ∫ R0 R ⋆ 4π divP 3 r4 dr. (7.7) can be calculated numerically. But, for our purpose it is enough to estimate this field in order of value: H ≈ 2m− . (7.8) R0 3 As the field on the star pole √ G2M⋆R0 2 m − ≈ Ω (7.9) c H ≈ −4 √ GM⋆ cR 0 Ω. (7.10) 56

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