Magnetic Ap Stars in the Hertzsprung‐Russell Diagram

Abstract

The evolutionary state of magnetic Ap stars is rediscussed using the recently released Hipparcos data. The distribution of the magnetic Ap stars of mass below 3 in the H-R diagram di †ers from that of M _ the normal stars in the same temperature range at a high level of signiÐcance. Magnetic stars are concentrated toward the center of the main-sequence band. This is shown in two forms of the H-R diagram : one where log L is plotted against and a version more directly tied to the observed quantities, log T eff showing the astrometry-based luminosity (Arenou & Luri) against the index of Geneva photom-(B2ÈG) 0 etry. In particular, it is found that magnetic Ðelds appear only in stars that have already completed at least approximately 30% of their main-sequence lifetime. No clear picture emerges as to the possible evolution of the magnetic Ðeld across the main sequence. Hints of some (loose) relations between magnetic Ðeld strength and other stellar parameters are found : stars with shorter periods tend to have stronger Ðelds, as do higher temperature and higher mass stars. A marginal trend of the magnetic Ñux to be lower in more slowly rotating stars may possibly be seen as suggesting a dynamo origin for the Ðeld. No correlation between the rotation period and the fraction of the main-sequence lifetime completed is observed, indicating that the slow rotation in these stars must already have been achieved before they became observably magnetic.