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

摘要

The evolution of magnetic fields in Ap stars during the main sequence phaseis presently mostly unconstrained by observation because of the difficulty ofassigning accurate ages to known field Ap stars. We are carrying out a large survey of magnetic fields in cluster Ap starswith 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 itcurrently stands. 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 inthis subsample we determine the fundamental parameters T_eff, log(L/L_o), andM/M_o. With these data and the cluster ages we assign both absolute age andfractional age (the fraction of the main sequence lifetime completed). For thispurpose we have derived new bolometric corrections for Ap stars. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to theTAMS. Statistically for the stars with M > 3 M_o the fields decline withadvancing age approximately as expected from flux conservation together withincreased stellar radius, or perhaps even faster than this rate, on a timescale of about 3 10^7 yr. In contrast, lower mass stars show no compellingevidence for field decrease even on a timescale of several times 10^8 yr. Study of magnetic cluster stars is now a powerful tool for obtainingconstraints on evolution of Ap stars through the main sequence. Enlarging thesample of known cluster magnetic stars, and obtaining more precise RMS fields,will help to clarify the results obtained so far. Further field observationsare in progress.