A NEW PULSATION SPECTRUM AND ASTEROSEISMOLOGY OF DELTA-SCUTI

We present the results of a five-year Stromgren y photometric campaign on delta Scuti. Our data set consists of 6515 discrete differential m agnitudes, and spans the period of 1983 June to 1988 September. We fou nd the primary pulsation mode at 59.731129+/-0.000002 mu Hz, in close agreement with the frequency determination of Fitch (1976, IAU Colloqu ium, 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 freque ncy of 61.936104+/-0.000009 mu Hz-one cycle per year off of the common ly quoted frequency. All of the other modes not classified as harmonic s or beating modes were identified in our data, as well as a new pulsa tion frequency at 96.21443+/-0.00005 mu HZ discovered in both Stromgre n y and b observations. We measured the phase differences between our Stromgren y data and a short string of Stromgren b data taken during t he 1987 multisite campaign, and find phase differences ranging from 0 to 0.33 radians, suggesting that there are modes of different spherica l harmonic order present in delta Scuti. Finally, we evolved a set of M=1.8-2.4 M. models with solar abundances (X=0.7, Z=0.02) and two (M=2 .2 and M=2.4 M.) models with solar abundances scaled to (X=0.66,Z =0.0 6), using recent opacity and reaction rate data, and applied Linear, n onadiabatic pulsation analysis to models in the shell hydrogen burning phase. The Z=0.02 model which best fit the observed spectral type of F2 III, the Hipparcos absolute magnitude of M-v=1.0, and the radius es timate of Cugier and Monier of R=4.1 R., and which has a pure radial m ode at 59.731 mu Hz has a mass of 2.1 M., with T-eff=6894 K, R=4.14 R. , and M-v=1.0. The best-fit Z=0.06 model has M=2.4 M., T-eff=6827 K, R =4.28 R., and M-v=1.0. For the best-fit models, the highest amplitude observed mode is the radial fundamental mode, with several nonradial m odes being simultaneously excited, This contradicts the results of Bal ona et al. that the strongest mode of delta Scuti is first overtone. I n addition, the second radial overtone falls at nu=97.4 mu Hz for both models, thus eliminating the two observed frequencies at 96.2 and 99. 4 mu Hz as candidates for the second radial overtone, unless second-or der rotation effects are considered. We are unable to perform a more c omprehensive asteroseismological analysis because the theoretical nonr adial modes overlap in frequency when rotational splitting is taken in to account, thus making it impossible to uniquely determine the mode t ype without more observational identifications of the pulsation mode t ypes. 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 th e implications of our results on delta Scuti star modeling in some det ail. (C) 1997 American Astronomical Society.