On the Dipolar F Mode of Stellar Oscillation

The classiication of adiabatic modes of nonradial stellar oscillation was established by Cowling (1941). In addition to acoustic and gravity modes he identiied an intermediate mode, which he labelled the f mode, and which in simple stellar models has no radial node. The motion of a dipolar f mode (of spherical-harmonic degree l = 1) shifts the centre of mass, and must have zero frequency. On the other hand, if the perturbation to the gravitational potential is neglected (the case considered by Cowling) the f mode has a frequency intermediate between those of the gravity and acoustic modes; this is true of modes of any degree (l 1). Here we consider the properties of the dipolar f mode, elucidating the origin of these diierences through continuous transformations between the various relevant cases; in addition, we discuss the broader issues of the classiication of modes of nonradial oscillation.