Evolution of magnetic fields in stars across the upper main sequence:: II.: Observed distribution of the magnetic field geometry

We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M-circle dot are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M-circle dot seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M-circle dot and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities beta is detected in both higher and lower mass stars. It is quite possible that the angle)3 becomes close to 0 degrees in slower rotating stars of mass M > 3 M(D too, analog to the behaviour of angles in slowly rotating stars of M < 3 M-circle dot The obliquity angle distribution as inferred from the distribution of r-values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle beta tends to reach values close to either 90 degrees or 0 degrees for M < 3 M-circle dot. The evolution of the obliquity angle beta seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of beta with the elapsed time on the main sequence for stars with M > 3 M-circle dot, no similar trend is found for stars with M < 3 M-circle dot However, the predominance of high values of beta at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.