An Attempt to Detect the Magnetic Field of a White Dwarf

Abstract

DR. A. D. THACKERAY has now published a description of his attempts to test Blackett's prediction on the magnetic field of massive rotating bodies (Mon. Not. Roy. Astro. Soc., 107, 5 and 6 ; 1947). Blackett pointed out that the ratio between the magnetic moment and the angular momentum of the earth and sun was approximately the square root of the gravitational constant divided by the velocity of light, and that this also applied to 78 Virginis (Nature, 159, 658 ; 1947). He also dealt with the application of this formula to the white dwarfs and showed that in the case of Sirius B the equatorial field might be as much as 3 X 106 gauss. Such a field should lead to Paschen–Back splitting of the hydrogen lines of the order of 50 A., and Thackeray gives a full description of his work with the 36-inch Common reflector at the Solar Physics Observatory, Cambridge, to detect this effect. Unfortunately, there is not a very wide choice of white dwarfs suitable for the experiment, and Wolf 1346, magnitude 11·3, was selected as it was attainable with a 1-prism slitless spectrograph attached to the 36-in. reflector. It was shown that Paschen–Back splitting of Hγ is unlikely to exceed 8 A. in this star, corresponding to an upper limit of 106 gauss. It is emphasized that there is need for greater spectral purity in a discussion of an apparent structure in the lines, which is independent of the polarizing analyser. When 40 Erid. B became accessible in September 1947 a 2-in. f/l aspherical singlet, figured and lent by Dr. C. B. Burch, was attached to the Newall 25-in. refractor for an attempt with the Wood aluminized grating spectrograph. An hour's exposure without 'Polaroid' was obtained on September 25, showing Hγ only, but exposures of one hour on September 25 and 3½ hours on September 27 through the analyser failed owing to the formation of thick haze during exposure.