1995BaltA...4..355S

The Fourier transform of one time-series photoelectric observation of NGC 2452 shows a periodicity of (217±9) s (Silvotti et al. 1995). The amplitude spectrum in the entire range from zero to the Nyquist frequency is shown in Fig. 1. The power of the peak at about 220 s corresponds to 13.4 times the average power of the spectrum, following the definition by Kepler (1993). The average power has been calculated taking into account only the frequencies ≥2 mHz, for which the local average power is almost constant. In Fig. 1, the high peaks in the very low frequency region are probably due to transparency variations of the sky during the run. The period of about 220 s, if real, is very short compared with the typical periods of 10-30 min of the pulsating planetary nebula nuclei (Bond et al. 1993). Variations on a time-scale comparable to the periodicity found in NGC 2452 were observed by Barlow et al. (1980) in the UV spectrum of Sanduleak 3. The O VII emission line at 3886 Å had moved by 1.7 Å to the red (corresponding to a shift of 130 km/s) in a time-scale of 150 s. These variations were interpreted like generated by pulsational waves. The very high ionization potential of O VII (739.3 eV) suggested a coronal origin for these lines. Now we know that Sanduleak 3 is a pulsating planetary nebula nucleus with a rich mode spectrum dominated by a peak near 16 min (Bond & Ciardullo 1993). The fact that our observation of NGC 2452 has been done in the U band, which includes the O VII lines near 3890 Å, let us suspect that we have registered the same kind of phenomenon saw by Barlow et al. (1980) in Sanduleak 3.