Mode identification using equivalent width variation of Balmer lines

We investigate mode identification in delta Scuti stars using the ratio, R, between the relative equivalent width (EW) amplitude of the Balmer lines and the light amplitude. We use a nonadiabatic pulsation code applied to stellar models to calculate the temperature and pressure variations at the photosphere and static model atmospheres to compute the EW of the HP line and light amplitude in different wavebands. We find that R is not very sensitive to the spherical harmonic degree, l, of the pulsation mode, contrary to previous results. The dependence of EW amplitude and photometric amplitude on the angle of inclination is different owing to the additional term that is required to describe the EW variation. This leads to an increased spread of R with l and a dependence of R on m, the azimuthal spherical harmonic order. It appears that the temperature effect dominates the geometric term in which most of the sensitivity to spherical harmonic degree is to be found. The large change in limb darkening across the Balmer line profile, previously thought to contribute significantly to mode identification, does not seem to play a major role. Our results show that the close agreement in mode identification using the EW method and using relative photometric amplitudes and phases is not surprising, since in both methods the roles of the geometric, pressure and temperature terms are about the same.