THE DELTA-SCUTI STAR FG VIRGINIS .2. A SEARCH FOR HIGH PULSATION FREQUENCIES

Although the delta Scuti and roAp variables occupy similar positions o n and near the main sequence, delta Scuti variables pulsate with lower radial and nonradial overtones and lower frequencies. To test whether high frequencies (as found in the roAp stars) are also present in del ta Scuti stars, a multisite campaign with the Whole Earth Telescope (W ET) was carried out for the star FG Vir. The 96.7 hours of WET photome try were supplemented by measurements made with the Delta Scuti Networ k (DSN), because the DSN technique includes regular measurements of co mparison stars and is better suited to monitoring the low frequencies (less than or equal to 500 mu Hz). This made possible the correction f or low-frequency variability (10 pulsation frequencies from 106 to 395 mu Hz and amplitudes from 0.001 to 0.02) in order to prevent spectral leakage into the high-frequency domain. It is shown that such a corre ction is essential. In the 1 - 10 mHz region of interest (correspondin g to periods between 17 and 1.7 minutes) no significant stellar variab ility could be detected. The highest peaks in the amplitude spectra ra nged from 0.00023 (near 1 mHz) to 0.00012 near 10 mHz, where the ampli tudes are expressed in units of fractional intensity. Statistical test s show that these peaks are caused by noise. These results indicate th at for FG Vir the multimode pulsations in the low-frequency region (wi th individual amplitudes up to 0.02) are not accompanied by photometri cally detectable high overtone pulsation at high frequencies. This res ult is consistent with the hypothesis that high-order p-mode pulsation s in the millimag range require a large magnetic field, as detected in the roAp stars.