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

The evolvedScuti variable 4 CVn was observed photometrically for 53 nights on three continents. We found a total of 34 significant and 1 probable simultaneously excited frequencies. Of these, 16 can be identified as linear combina- tions of other frequencies. All significant frequencies outside the 4.5 to 10 cd 1 (52 to 116 µHz) range can be identified with frequency combinations, fi ± fj, of other modes with gener- ally high amplitudes. There exists a number of closely spaced frequencies with separations of � 0.06 cd 1 . This cannot be explained in terms of amplitude variability. The results show that even for stars on and above the main sequence other than the Sun, a very large number of simultane- ously excited nonradial oscillations can be detected by conven- tional means. Since all pulsation modes with photometric amplitudes of 1 mmag or larger have now been detected for this star, a presently unknown mode selection mechanism must exist to select be- tween 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 determined for the ten main modes. The phase differences indicate p1 to p4 modes of ` = 1 for four of these modes. The formulae to determine the uncertainties in the ampli- tudes and phases of sinusoidal fits to observational data are derived in the Appendix.