The Magnetically Variable Star HD 125248.

Abstract

view Abstract Citations (90) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Magnetically Variable Star HD 125248. Babcock, Horace W. Abstract The reversing magnetic field of the peculiar A-type star HD 125248 = BD - has been investigated on twenty-seven high-dispersion spectrograms made with the double analyzer for circularly polarized light. The magnetic-field intensity at the pole, Hp, as determined from the Zeeman effect on numerous Fraunhofer lines, has been tabulated for several elements at different phases in the 9.295-day period found by Deutsch for the variations in line intensity. The lines of Fe I, Fe ii, and Ti ii, which vary only slightly in intensity, indicate a field represented by Hp = 2000 + 6600 cos 0-1600 cos 20 gauss. The positive extreme (+7000 gauss) is reached near zero phase, when the lines of the rare earths are at maximum intensity and lines of Cr I and Cr ii are at minimum. The negative extreme (-6200 gauss) is reached at mid-period when Eu ii is weak and Cr is at maximum intensity. Hp determined from the Cr lines is "more negative" than that indicated by Fe-Ti, while, especially at the "cross-over" phases (0.35 and 0.7, when Hp = 0), Eu ii gives a stronger positive field. If results from all elements are averaged, the extremes are close to +7600 and - 7600 gauss. Velocities in the line of sight are also tabulated for elements at various phases. Again, systematic differences are found between the Fe-Ti group, the rare-earth group, and Cr. Rather sudden decelerations occur just as 11p reaches maximum. A "cross-over effect" on the profiles of Fraunhofer lines was discovered on plates made near phases 0.35 and 0.7. This is interpreted as a combination of differential velocity and temporary opposite magnetic polarity on two zones of the star; apparently, the magnetic field reverses in a rapid, progressive manner over the surface. The rare earths and Cr seem to be concentrated in separate atmospheric zones, which vary in effective size or location; Fe and Ti are supposed to be rather uniformly distributed and to represent best the general magnetic field. Two fundamentally different physical models are considered: the magnetic oscillator and the oblique rotator. The latter does not satisfactorily represent the observations. Consequently, it is concluded that HD 125248 is observed pole-on and that it is oscillating, perhaps in a manner suggested by Schwarzschild, for only by large-scale motions of the material of the star can rapid changes of the magnetic field be explained. In his solution (odd mode), specialized pulsations of the star are coupled with an oscillating dipole component of the field. The constant component (+2000 gauss) indicated by Fe-Ti can perhaps be interpreted most plausibly according to the Elsasser-Bullard theory, although the observations yield no data as to rate of axial rotation (or as to differential rotation). For lack of these data the electromotive forces in the atmosphere cannot be specified completely; however, the oscillating component of the electric field induced by the changing magnetic field is specified in Schwarzschild's solution (for a nonconducting atmosphere). This has an an plltude at the equator of 36 volts/cm. To account for the varying equivalent widths of lines, it is assumed that the effective atmospheric Eu and Cr have become concentrated in zones, Eu near the poles, Cr in an equatorial belt, and that the extent of these zones varies as the star oscillates. As a tentative approach to this separation of elements, a mass-spectrograph effect is considered, depending upon the transient differential drift velocities of ions, as a function of charge/mass, in crossed electric and magnetic fields. Publication: The Astrophysical Journal Pub Date: July 1951 DOI: 10.1086/145448 Bibcode: 1951ApJ...114....1B full text sources ADS | data products SIMBAD (4)