Theoretical blue limits of the instability strip for delta Scuti-type stars

The stability of models of delta Scuti-type stars relative to radial pulsations in the four lowest modes is studied in the linear nonadiabatic approximation. It is shown that the positions of the theoretical blue limit (BL) of the instability strip on the H--R diagram depend strongly (δlogT/sub e/approx. =0.03) on the means of calculating the coefficient of opacity and on the variations of helium content in the envelopes. The influence of other factors is less important (δlogT/sub e/< or approx. =0.01): variations in the content of heavy elements, the use of different tables of the coefficient of opacity, the treatment of convection, the number of layers of envelopes. A comparison is made with the results of other authors and with observations. The positions of the observed and theoretical (third harmonic) BL are in satisfactory agreement. A nonlinear dependence of the positions of theoretical BL on the helium content Y is obtained. A star with Yapprox. =0.2 could pulsate in the instability strip if not located too close to the BL