Evolution of the Magnetic Fields of Magnetic Ap Stars During the Main Sequence Phase

Abstract. This paper discusses the results of our survey of magnetic eldsamong Ap stars that are members of open clusters. Such stars are unlike eldAp stars in that they have relatively well{determined ages, both absolutely (inyears) and as a fraction of the main sequence lifetime elapsed. By measuring the elds of such stars once or a few times per star, we can estimate the RMS lon-gitudinal eld strength of each star, and study how this quantity varies with agethrough the 10 8 to 10 9 yr of main sequence life. Dividing our sample of some 80stars into mass bins of 2{3, 3{4, and 4{5M , we nd that both the typical eldstrength and the total magnetic ux, emerging from the surface of stars declineon time scales that are a modest fraction of the main sequence lifetime in each ofthe three mass bins.Key words: stellar magnetic elds { upper main sequence stars { open clusters 1 Introduction About 10% of A and B main sequence stars show peculiar atmospheric chemistry, suciently peculiarthat the anomalies relative to stars of approximately solar chemical composition are readily seenin the classi cation spectra. The chemically anomalous stars form several roughly homogeneousclassi cation groups are: Am stars, which range from about F0 to A0 and show a modest excessof iron peak elements; Ap and Bp stars, which range between about F0 and B3, and show verysubstantial excesses of Cr and rare earths in cooler stars, changing gradually to excess Si andde cient He in hotter stars; HgMn stars, in the range between A0 and B6, which show large excessesof a small number of elements such as P, Mn, Ga, and Hg; He{weak stars, in the range betweenabout B8 and B3, with clearly de cient He for their e ective temperatures; and He{strong stars,near B2, whose principal anomaly is strongly overabundant He.Magnetic elds in such stars may be directly detected by observation of the Zeeman e ect in thestellar spectrum. This e ect leads to readily detectable circular polarisation in spectral line pro les,and in the special case of a very small projected rotation velocity v