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Astron. Astrophys. 334, 181-187 (1998)

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1. Introduction

It is well known that chemically peculiar stars of the Ap and Am types are rotating more slowly than their normal counterparts (e.g. North 1994). The question then arises, whether slow rotation is acquired during the main sequence life of the star, or before its arrival on the ZAMS, i.e. during the proto-stellar phase. Havnes & Conti (1971) had suggested that magnetic stars undergo magnetic braking during their main sequence lifetime, due to mass accretion from the interstellar medium, while Strittmatter & Norris (1971) proposed the same, but due to mass loss. These theoretical considerations seemed to get support from observational evidence when Wolff (1975, 1981), Stift (1976) and Abt (1979) found some correlation between the radii or ages of Ap stars and their rotational periods obtained from their photometric or spectroscopic variations. On the contrary, Hartoog (1977) concluded that magnetic Ap stars in young clusters do not rotate faster than those in older clusters, and this conclusion was also reached by North (1984a, b, 1985, 1986, 1987), Borra et al. (1985) and Klochkova & Kopylov (1985). The apparent correlation between radius and rotational period has been commented by Hensberge et al. (1991), who conclude that this correlation is real but possibly due to a detection bias depending on the inclination angle, and by Stepien (1994), who concluded on the contrary that this correlation does simply not exist, if spurious rotational periods are duly excluded.

Using the [FORMULA] vs. [FORMULA] diagram for field stars, North (1985, 1986, 1992) showed that for Si stars, there is indeed a trend towards longer periods for low-gravity stars, but which can be entirely explained by conservation of angular momentum as the star evolves with increasing radius within the main sequence.

In this note, we revisit the [FORMULA] vs. [FORMULA] diagram for field stars having both a rotational period in the literature and a reliable surface gravity, the latter being either spectroscopic or obtained from Hipparcos data.

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998