A SURVEY FOR RAPID VARIABILITY AMONG EARLY MAIN-SEQUENCE A-STARS

This is a survey of nonpeculiar early (A0-A5) main-sequence A stars for rapid (4-30 min), low-amplitude (< 10 mmag) variability. Peculiar stars (roAp stars) are presently the only objects known to exhibit this behavior on or near the main sequence. There are reasons for suspecting variability in ''nonpeculiar'' stars, however; survey objects are in close proximity, in an HR diagram, to the Cepheid instability strip where many pulsational variables are found (e.g., the delta Scuti and roAp stars), and there is evidence of pulsational variability (at slightly longer periods) in the nonpeculiar delta Sct stars. The survey also is an independent test of the main-sequence mass-loss theory proposed by Willson et al. ( 1986). Finally, surveys of this type may produce objects of asteroseismological interest. All observations were gathered with the University of Wisconsin Two-Star Photometer. This instrument coupled with a computerized high-speed data collection system was used with small (0.41-0.61-m) telescopes at Pine Bluff Observatory and Table Mountain Observatory. Several period-searching methods were used to analyze time series of differential-photometric data. This instrument and data-reduction technique allow millimagnitude light variations to be detected. The survey did produce a few stars that are suspected of rapid variability and should be monitored with time. However, there is apparently no evidence for large-scale rapid variability among the nonpeculiar early main-sequence A stars observed. The survey also discovered several low-amplitude delta Sct stars, all of which are in or blueward of the recognized instability strip. These stars support predictions that delta Sct stars exist at lower amplitudes, and may also indicate they are present at earlier spectral types.