Ga. Wade, EVOLUTIONARY ESTIMATES FOR 10 MAGNETIC AP STARS CALCULATED FROM THEIRRIGID ROTATOR GEOMETRIES, Astronomy and astrophysics, 325(3), 1997, pp. 1063-1069
I present estimates of the evolutionary states (effective temperatures
, masses, radii, luminosities and ages) of 10 magnetic Ap stars, and s
ubsequent constraints on the evolution of magnetic fields in these obj
ects. Using rotational axis inclinations (sin i) reported by Leroy et
al. (1996), combined with apparent rotational velocities (v sin i) and
rotational periods (P-rot) obtained from a variety of sources, the ra
dii of these stars have been calculated assuming rigid rotation. From
the positions of these objects in the radius-effective temperature (lo
g(R/R.) - log T-eff) plane I obtain their evolutionary states using th
e 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 r
esults 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 & Mat
hys (1994) (who suggest that the magnetic Ap stars may be near the end
of their main sequence life). When the magnetic field strengths of th
ese stars are graphed versus the fraction of main sequence evolution c
ompleted, no correlation is evident. However, it is of interest to not
e 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 sta
rs discussed in this paper have polar magnetic field strengths between
3 and 6 kG. A similar graph of the magnetic axis obliquity angle beta
of each star versus age shows that intermediate values of beta exist
for stars as old as 10(9) y. This indicates that, if beta does evolve
toward asymptotic values as suggested by Mestel et al. (1981), the tim
escale for this evolution is quite long, at least for stars with simil
ar to 5 kG surface magnetic fields and rotational periods near 10 days
.