EVOLUTIONARY ESTIMATES FOR 10 MAGNETIC AP STARS CALCULATED FROM THEIRRIGID ROTATOR GEOMETRIES

I present estimates of the evolutionary states (effective temperatures , masses, radii, luminosities and ages) of 10 magnetic Ap stars, and s ubsequent constraints on the evolution of magnetic fields in these obj ects. Using rotational axis inclinations (sin i) reported by Leroy et al. (1996), combined with apparent rotational velocities (v sin i) and rotational periods (P-rot) obtained from a variety of sources, the ra dii of these stars have been calculated assuming rigid rotation. From the positions of these objects in the radius-effective temperature (lo g(R/R.) - log T-eff) plane I obtain their evolutionary states using th e model evolutionary calculations by Schaller et al. (1992). The stars in this study span the entire width of the main sequence, showing no tendancy to cluster near the ZAMS or the TAMS. In this respect these r esults are consistent with the conclusion of North (1993) (who reports that the Ap (CP2) stars appear to be distributed uniformly along the width of the main sequence) and inconsistent with that of Hubrig & Mat hys (1994) (who suggest that the magnetic Ap stars may be near the end of their main sequence life). When the magnetic field strengths of th ese stars are graphed versus the fraction of main sequence evolution c ompleted, no correlation is evident. However, it is of interest to not e that strong magnetic fields do exist in Ap stars at all evolutionary states (from the ZAMS to the TAMS), and that more than 70% of the sta rs discussed in this paper have polar magnetic field strengths between 3 and 6 kG. A similar graph of the magnetic axis obliquity angle beta of each star versus age shows that intermediate values of beta exist for stars as old as 10(9) y. This indicates that, if beta does evolve toward asymptotic values as suggested by Mestel et al. (1981), the tim escale for this evolution is quite long, at least for stars with simil ar to 5 kG surface magnetic fields and rotational periods near 10 days .