The delta Scuti star FG Virginis .2. A search for high pulsation frequencies

Although the delta Scuti and roAp variables occupy similar positions on 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 delta Scuti stars, a multisite campaign with the Whole Earth Telescope (WET) was carried out for the star FG Vir. The 96.7 hours of WET photometry were supplemented by measurements made with the Delta Scuti Network (DSN), because the DSN technique includes regular measurements of comparison stars and is better suited to monitoring the low frequencies (less than or equal to 500 mu Hz). This made possible the correction for 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 correction is essential. In the 1 - 10 mHz region of interest (corresponding to periods between 17 and 1.7 minutes) no significant stellar variability could be detected. The highest peaks in the amplitude spectra ranged from 0.00023 (near 1 mHz) to 0.00012 near 10 mHz, where the amplitudes are expressed in units of fractional intensity. Statistical tests show that these peaks are caused by noise. These results indicate that for FG Vir the multimode pulsations in the low-frequency region (with individual amplitudes up to 0.02) are not accompanied by photometrically detectable high overtone pulsation at high frequencies. This result is consistent with the hypothesis that high-order p-mode pulsations in the millimag range require a large magnetic field, as detected in the roAp stars.