The δ Scuti star θ Tucanae -: III.: Observational guidelines for mode identification

On the basis of almost 2000 b, y photoelectric observations of theta Tucanae collected in an international campaign at two sites and of almost 1500 nu, u observations collected at one site, observational guidelines for mode identification have been searched. Not only the generally used plane (A(b-y)/A(y) versus phi (b-y) - phi (y)) but each combination of the nonadiabatic observables including amplitude ratio vs. amplitude ratio and phase difference vs. phase difference planes were checked for guidelines. Especially A(b)/A(y) amplitude ratio and phi (b-y) - phi (b) phase difference proved to be useful criteria for mode separation in the case of theta Tucanae. The 10 pulsational frequency values published previously by Paparo et al. (1996) are distributed in three groups according to A(b)/A(y) and in two distinct levels according to phi (b-y) - phi (b). The groups consist of closely spaced frequencies. Group III(15.86, 15.94 and 17.06 cycles/day) displays a higher, distinct level than the others according to the A(b-y)/A(y) amplitude ratio and unusual behaviour in u colour. Although the discriminative plane does not give the exact value of quantum numbers but predicts which theoretical modes must share the same behaviour. Calibration of the pure observational guidelines by new, delicate theoretical calculation for planes of nonadiabatic observables concerning the location of theoretical modes is urged.