30+frequencies for the delta Scuti variable 4 Canum Venaticorum: results of the 1996 multisite campaign

The evolved delta Scuti variable 4 CVn was observed photometrically for 53 nights on three continents. We found a total of 34 significant and 1 probab le simultaneously excited frequencies. Of these, 16 can be identified as li near combinations of other frequencies. All significant frequencies outside the 4.5 to 10 cd(-1) (52 to 116 mu Hz) range can be identified with freque ncy combinations, f(i) +/- f(j), of other modes with generally high amplitu des. There exists a number of closely spaced frequencies with separations o f similar to 0.06 cd(-1). This cannot be explained in terms of amplitude va riability. The results show that even for stars on and above the main sequence other t han the Sun, a very large number of simultaneously excited nonradial oscill ations can be detected by conventional means. Since all pulsation modes with photometric amplitudes of 1 mmag or larger h ave now been detected for this star, a presently unknown mode selection mec hanism must exist to select between the 1000+ of low-degree modes predicted to be excited for this (and many other) stars. Phase differences and amplitude ratios between the y and v colors are deter mined for the ten main modes. The phase differences indicate p(1) to p(4) m odes of l = 1 for four of these modes. The formulae to determine the uncertainties in the amplitudes and phases of sinusoidal fits to observational data are derived in the Appendix.