[Paper Review] 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

J. D. Landstreet, S. Bagnulo+5 more|arXiv (Cornell University)|Jun 3, 2007

Stellar, planetary, and galactic studiesPhysics and Astronomy56 references97 citations

TL;DR

This study investigates the evolution of magnetic fields in Ap stars by analyzing magnetic field strengths in cluster and association members with well-determined ages. It finds that magnetic fields in massive Ap stars ($M > 3M_\odot$) decline over ~30 million years, consistent with flux conservation and stellar expansion, while lower-mass Ap stars show no significant field decrease over several hundred million years, suggesting distinct field evolution mechanisms by mass.

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.

Motivation & Objective

• To constrain the evolution of magnetic fields in Ap stars during the main sequence phase, a period poorly understood due to uncertain ages of field Ap stars.
• To overcome age uncertainty by studying Ap stars in open clusters and associations with well-determined cluster ages.
• To determine how magnetic field strength changes with age and stellar mass using observed field strengths and fundamental parameters.
• To derive new bolometric corrections specific to Ap stars to improve luminosity and temperature estimates.
• To test whether magnetic field evolution follows flux conservation or faster decay, particularly in relation to stellar radius growth.

Proposed method

• Selecting Ap stars from a survey that are confirmed cluster or association members based on kinematic and photometric membership criteria.
• Measuring magnetic field strengths via circular polarization observations, primarily using the ESPaDOnS spectrograph on the Canada-France-Hawaii Telescope.
• Determining stellar parameters ($T_{\mathrm{eff}}$, $L/L_\odot$, $M/M_\odot$) from spectral energy distributions and cluster membership constraints.
• Calculating absolute age and fractional age (fraction of main sequence lifetime completed) using cluster age estimates and evolutionary models.
• Deriving new bolometric corrections tailored for Ap stars to improve luminosity determination from observed magnitudes.
• Analyzing the correlation between magnetic field strength and stellar age, separately for high- and low-mass Ap stars.

Experimental results

Research questions

• RQ1 Do magnetic fields in Ap stars decay over the main sequence lifetime, and if so, at what rate?
• RQ2 How does the rate of magnetic field decay depend on stellar mass, particularly for $M > 3M_\odot$ versus lower-mass stars?
• RQ3 Is the observed field decay consistent with flux conservation as the star expands during evolution?
• RQ4 Are there significant differences in field evolution between Ap stars in young clusters and older associations?
• RQ5 Do Ap stars in lower-mass systems show evidence of field strength evolution over timescales exceeding $10^8$ years?

Key findings

• Magnetic fields are detectable at the surface of Ap stars from the zero-age main sequence (ZAMS) to the terminal-age main sequence (TAMS).
• For Ap stars with $M > 3M_\odot$, the magnetic field strength declines approximately as expected from flux conservation combined with stellar radius expansion, or possibly faster, on a timescale of about $3 \times 10^7$ years.
• No compelling evidence for magnetic field decrease is observed in lower-mass Ap stars over timescales of several times $10^8$ years.
• The observed field evolution suggests that field decay mechanisms may be mass-dependent, with faster decay in more massive stars.
• The study confirms that cluster Ap stars provide a powerful observational tool for constraining magnetic field evolution during the main sequence.
• Further observations, especially with higher signal-to-noise RMS field measurements, are needed to clarify the field evolution trends and reduce uncertainties.