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abstract
Among the field stars of spectral class A in the neighborhood of the sun, nearly 10\% exhibit chemical peculiarities in their spectra. Based upon an examination of 177 type A stars in 6 galactic clusters, and many more from the published literature, we demonstrate that the relative frequency of occurrence of chemically peculiar (Ap) stars in such clusters is significantly less than their occurrence among the field-star population. Using Poisson counting statistics we demonstrate to 95\% confidence that the distribution of such stars within gravitationally bound clusters is different from that of the field stars. We argue that the Ap stars in the field probably did not originate in the gravitationally bound clusters, subsequently released by evaporation; rather that they originated in loose unbound associations that disrupted on free-fall time scales in the galactic tidal field. We argue further that it is the initial conditions in the environment of star formation regions that determines whether (and which) stars of type A will become chemically peculiar. Our findings imply that the magnetic conditions differed considerably between those star formation regions that formed the gravitationally bound clusters, and those that produced the unbound associations from which much of the field population is drawn.
