In search of the Ap instability strip

Abstract

The lack of a definite excitation mechanism for the rapidly oscillating Ap (roAp) stars has hampered both the understanding of these p-mode pulsators and the photometric search for new members of the class. In an effort to identify an empirical ‘instability strip’ for the class, Strömgren colours (b-y, u-b, [m ₁], [c₁], and β) of the roAp stars have been compared to colour-colour distributions of large samples of Ap stars (Vogt and Faundez 1979) and spectroscopically normal δ Scuti stars (Breger 1979). The loci of roAp stars are best defined in the [c ₁]-[u-b], [c ₁]-(b-y) and [c ₁-[m ₁] planes. The [c ₁-(b-y) diagram suggests that roAp stars cover a range in luminosity comparable to the δ Scuti stars and share a common spectroscopic peculiarity more specific than the SrCrEu subclass to which they belong.