AMPLITUDE VARIABILITY OF TAU-PEGASI AND THE NATURE OF SINGLY-PERIODIC DELTA-SCUTI STARS

Tau Peg = HR 8880 shows a dominant pulsation frequency of 18.4052 cycles d-1 with an amplitude changing slowly from 0.005 mag (or less) to 0.012 mag over 13 years. This is accompanied by smooth changes in phase suggesting small period changes. Averaged over 6 years dP/dt approximately 10(-9). The variations in amplitude and phasing (frequency) are similar to those recently discovered in other noradially pulsating delta Scuti stars. A mathematical model of three close frequencies with separations of approximately 0.0003 cycles d-1 fits the data covering six observing seasons very well and also explains the observed constancy of the star during 1966. One or more additional frequencies in the 18 to 28 cycles d-1 range may also be present with an amplitude of less-than-or-equal-to 0.001 mag. The dominant frequency is identified as a nonradial p3 or p4 mode with l = 2 by examining the amplitude ratios and phase differences between observed light and colour variations as well as the derived Q value of 0.016 +/- 0.003 days. Radial pulsation is ruled out. The mode identification and the observed amplitude variability shows that tau Peg is a typical nonradially pulsating delta Scuti star with fast rotation. This star shows that for small amplitudes the detection of only a single frequency is not an indication of radial pulsation. This is in contrast to the results found for large-amplitude delta Scuti stars which are radial pulsators.