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Astron. Astrophys. 325, 1063-1069 (1997)

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Evolutionary estimates for 10 magnetic Ap stars calculated from their rigid rotator geometries

G.A. Wade

Physics & Astronomy Department, University of Western Ontario, London, Ontario, N6A 3K7, Canada (gwade@phobos.astro.uwo.ca, http://corona.astro.uwo.ca/~gwade)

Received 20 January 1997 / Accepted 20 March 1997

Abstract

I present estimates of the evolutionary states (effective temperatures, masses, radii, luminosities and ages) of 10 magnetic Ap stars, and subsequent constraints on the evolution of magnetic fields in these objects.

Using rotational axis inclinations ([FORMULA]) reported by Leroy et al. (1996), combined with apparent rotational velocities ([FORMULA]) and rotational periods ([FORMULA]) obtained from a variety of sources, the radii of these stars have been calculated assuming rigid rotation. From the positions of these objects in the radius-effective temperature ([FORMULA]) plane I obtain their evolutionary states using the model evolutionary calculations by Schaller et al. (1992).

The stars in this study span the entire width of the main sequence, showing no tendancy to cluster near the ZAMS or the TAMS. In this respect these results are consistent with the conclusion of North (1993) (who reports that the Ap (CP2) stars appear to be distributed uniformly along the width of the main sequence) and inconsistent with that of Hubrig & Mathys (1994) (who suggest that the magnetic Ap stars may be near the end of their main sequence life).

When the magnetic field strengths of these stars are graphed versus the fraction of main sequence evolution completed, no correlation is evident. However, it is of interest to note that strong magnetic fields do exist in Ap stars at all evolutionary states (from the ZAMS to the TAMS), and that more than 70% of the stars discussed in this paper have polar magnetic field strengths between 3 and 6 kG. A similar graph of the magnetic axis obliquity angle [FORMULA] of each star versus age shows that intermediate values of [FORMULA] exist for stars as old as [FORMULA]  y. This indicates that, if [FORMULA] does evolve toward asymptotic values as suggested by Mestel et al. (1981), the timescale for this evolution is quite long, at least for stars with [FORMULA]  kG surface magnetic fields and rotational periods near 10 days.

Key words: stars: chemically peculiar – stars: evolution – stars: magnetic field – stars: temperatures – stars: rotation

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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