Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708

Abstract

We have discovered an extraordinarily large mean longitudinal magnetic field of 7.5 kG in the ultra-cool low mass Ap star HD 154708 using FORS 1 in spectropolarimetric mode. From UVES spectra, we have measured a mean magnetic field modulus of 24.5 kG. This is the second-largest mean magnetic field modulus ever measured in an Ap star. Furthermore, it is very likely that this star is one of the coolest and least massive among the Ap stars and is located in the H-R diagram in the same region in which rapidly oscillating Ap stars have been detected. We note that all known roAp stars have much smaller magnetic fields, by at least a factor of three. © ESO 2005.