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

TL;DR

This study investigates how magnetic fields in Ap stars evolve during the main sequence by analyzing a carefully selected sample of cluster stars with well-determined ages, revealing mass-dependent magnetic field decay patterns.

Contribution

It provides new observational evidence on the evolution of magnetic fields in Ap stars, especially highlighting differences between stars above and below 3 solar masses.

Findings

Magnetic fields are present from ZAMS to TAMS in Ap stars.

Fields decline with age in stars > 3 M_o, consistent with flux conservation.

No clear evidence of field decay in stars < 3 M_o over several hundred million years.

Abstract

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. 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. 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, log(L/L_o), and M/M_o. 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. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M_o 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 10^7 yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10^8 yr. 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.