A search for magnetic stars in late stages of stellar evolution

Two well-known types of stars with strong dipolar magnetic fields have been extensively studied. These are the magnetic CP stars and the magnetic white dwarfs. In a number of papers, it is suggested that these types of stars may be related by evolution: CP stars could be progenitors of magnetic white dwarfs (Angel et al. 1981). Investigation showed that magnetic fields in CP stars do not change significantly during their lifetime on the main sequence (Glagolevskij et al. 1986). What happened with the magnetic field later on, after the star has left the main sequence? Stȩpień (1993) proposed that some very slowly rotating yellow giants or subgiants with strong chromospheric and spot activity, could be former Ap stars with strong magnetic fields. When the stars become red giants, they undergo a deep convective mixing (first dredge-up) at the red giant branch (RGB), before helium ignition in the core. At the asymptotic giant branch (AGB), they have a second dredge-up (Iben 1991). The internal structure of the star significantly changes in this case. After the RGB and AGB, the star looses a large part of its own mass. If the field is not completely destroyed at these episodes, it must also change much. This can be traced if the magnetic fields are investigated in stars of intermediate evolution stages, between the main sequence and white dwarfs. In this context, it is our opinion that there are two types of stars which should be examined for the presence of strong dipolar magnetic fields: the horizontal branch A and B stars, and the hot subdwarfs. These stars are appropriate for direct magnetic measurements using Zeeman spectroscopy, since they show strong and narrow absorption lines in their spectra. In addition, some of them may possess chemical abundance anomalies similar to main sequence CP stars (Bashek and Sargent 1976, Kodaira and Philip 1984, Heber 1992), suggesting an intriguing link between the CP stars and the evolved A and B stars. Here, we present the results of a continuing search for strong magnetic fields in horizontal branch stars and in hot subdwarfs, using Zeeman spectra obtained at the 6-metre telescope.