A New Pulsation Spectrum and Asteroseismology of delta Scuti.

We present the results of a five-year Strömgren y photometric campaign on δ Scuti. Our data set consists of 6515 discrete differential magnitudes, and spans the period of 1983 June to 1988 September. We found the primary pulsation mode at 59.731129±0.000002 μHz, in close agreement with the frequency determination of Fitch (1976, IAU Colloquium, 29, 167), but we find our best-fit observed frequencies for other pulsation modes differ by 0.5-2 cycles per year from Fitch's results. In the case of the second strongest pulsation mode, we found a frequency of 61.936104±0.000009 μHz-one cycle per year off of the commonly quoted frequency. All of the other modes not classified as harmonics or beating modes were identified in our data, as well as a new pulsation frequency at 96.21443± 0.00005 μHz discovered in both Strömgren y and b observations. We measured the phase differences between our Strömgren y data and a short string of Strömgren b data taken during the 1987 multisite campaign, and find phase differences ranging from 0 to 0.33 radians, suggesting that there are modes of different spherical harmonic order present in δ Scuti. Finally, we evolved a set of M= 1.8-2.4 M_sun models with solar abundances (X=0.7, Z=0.02) and two (M=2.2 and M=2.4 M_sun) models with solar abundances scaled to (X=0.66, Z =0.06), using recent opacity and reaction rate data, and applied linear, nonadiabatic pulsation analysis to models in the shell hydrogen burning phase. The Z= 0.02 model which best fit the observed spectral type of F2 IH, the Hipparcos absolute magnitude of M_v=1.0, and the radius estimate of Cugier and Monier of R=4.1 R_sun, and which has a pure radial mode at 59.731 μHz has a mass of 2.1 M_sun, with T_eff=6894 K, R=4.14 R_sun, and M_v=1.0. The best-fit Z= 0.06 model has M= 2.4 M0, T_eff=6827 K, R=4.28 R_sun, and M_v=1.0. For the best-fit models, the highest amplitude observed mode is the radial fundamental mode, with several nonradial modes being simultaneously excited. This contradicts the results of Balona et al. that the strongest mode of δ Scuti is first overtone. In addition, the second radial overtone falls at v= 97.4 μHz for both models, thus eliminating the two observed frequencies at 96.2 and 99.4 μHz as candidates for the second radial overtone, unless second-order rotation effects are considered. We are unable to perform a more comprehensive asteroseismological analysis because the theoretical nonradial modes overlap in frequency when rotational splitting is taken into account, thus making it impossible to uniquely determine the mode type without more observational identifications of the pulsation mode types. However, the mode spacing of the Z= 0.06 model is less degenerate than the Z= 0.02 model, so with more observational identifications, it may be possible to actually make definitive model fits. We discuss the implications of our results on S Scuti star modeling in some detail.