Searching for links between magnetic fields and stellar evolution: II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations

Context: The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known field Ap stars.
Aims: We are carrying out a large survey of magnetic fields in cluster Ap stars with the goal of obtaining a sample of these stars with well-determined ages. In this paper we analyse the information available from the survey as it currently stands.
Methods: We select from the available observational sample the stars that are probably (1) cluster or association members and (2) magnetic Ap stars. For the stars in this subsample we determine the fundamental parameters T_{eff}, L/L_⊙, and M/M_⊙. With these data and the cluster ages we assign both absolute age and fractional age (the fraction of the main sequence lifetime completed). For this purpose we have derived new bolometric corrections for Ap stars.
Results: Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M_⊙ the fields decline with advancing age approximately as expected from flux conservation together with increased stellar radius, or perhaps even faster than this rate, on a time scale of about 3×10⁷ yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10⁸ yr.
Conclusions: Study of magnetic cluster stars is now a powerful tool for obtaining constraints on evolution of Ap stars through the main sequence. Enlarging the sample of known cluster magnetic stars, and obtaining more precise rms fields, will help to clarify the results obtained so far. Further field observations are in progress.

Tables 2 and 3 are only available in electronic form at http://www.aanda.org