Absorption-line polarization in magnetic stars.

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view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Absorption-line polarization in magnetic stars. Warwick, James W. Abstract Magnetic fields of the strength observed in sunspots and stars are frequently strong enough to separate completely the components of normal Zeeman triplets lying on the 450 or flat parts of the curve of growth. Each component then acts independently of the others on its two associated states of polarization, passing one completely, and absorbing only the other. Two ~components and one ~ component make up the Zeeman effect of a triplet seen at right angles to the lines of force of a magnetic field. Each ~ component absorbs light from the polarqzed continuum of light characterized by an electric vector vibrating in the direction perpendicular to the magnetic field. The ~ component, on the other hand, absorbs from the orthogonal continuum, characterized by an electric vector vibrating in the direction parallel to the magnetic field. If, by means of a Nicol prism or other device for analyzing plane-polarized light, we block out the continuum of light whose electric vector vibrates parallel to the field, we would, in the absorption Zeeman effect, expect to observe an equivalent width equal to twice the equivalent width of a single component, because of the presence of two ~ components which can absorb from the continuum of polarized light vibrating with electric vector perpendicular to the magnetic field. If we block out the light with electric vector vibrating in the direction perpendicular to the magnetic field, we would expect to observe only the equivalent width absorbed by a single component. The equivalent width of a spectral line will therefore, under idealized conditions of separation and saturation, and when observed transversally, show a plane-polarization effect, with maximum equivalent width observed in the direction at right angles to the magnetic field. The amount of plane-polarization expected in a magnetic star has been predicted on the basis of a number of simplifying assumptions, the most important of which are: (I) an idealized curve of growth of the type used by H. W. Babcock in his studies of the Zeeman intensification of spectral lines in the magnetic stars; (2) the presence of a magnetic field of the form of a magnetic dipole situated at the center of the star. The results of the calculations are that four or five per cent plane-polarization may be present in the absorption lines of a magnetic star, when the star is viewed so that its magnetic dipole is transverse to the line of sight. The maximum equivalent width would be observed in the direction at right angles to the direction of the dipole axis. From symmetry, it is clear that a magnetic star which presents the end of its dipole axis towards the observer will show no plane-polarization of the type envisaged here. Intermediate directions will show polarization up to the equatorial maximum of five per cent, but the direction of maximum equivalent width will always be at right angles to the projected stellar dipole axis. One may reasonably suppose that the magnetic stars are rotating rapidly about an axis identical with the dipole axis of the magnetic field. If this is so, it is likely that when observed from a direction in the equatorial plane, such a star will show absorption lines washed out to the extent that fine detail will not be seen on spectrograms. On the other hand, rotation will not affect the equivalent widths of the absorption lines, and so one might still expect to observe the polarization effect, if sufficiently accurate tracings of the spectrum are available in two states of plane- polarization. The possibility that atoms of a given species are distributed non-uniformly over the surface of the star has not been considered here. Nonuniformity of itself does not preclude the possibility of observing such a polarization effect, and might in fact emphasize it. Measurement of the degree of plane-polarization in the absorption lines of stars and sunspots might contribute to an understanding of the geometry of the stars and to an understanding of the extent to which absorption lines are strengthened through the action of a magnetic field. Whitin Observatory, Wetlesley College, Wellestey, Mass. Publication: The Astronomical Journal Pub Date: April 1951 DOI: 10.1086/106504 Bibcode: 1951AJ.....56...50W full text sources ADS |