4 HIPPARCOS photometry

M 2-54 was also a target observed by the HIPPARCOS satellite (ESA 1997) and found to be variable. Consequently, these observations may be helpful to constrain the time scale of the light variations of the central star. However, the mean standard deviation of these measurements is more than 0.06 mag per single data point, an order of magnitude higher than the accuracy of our photometric data. For that reason and because of the time distribution of the HIPPARCOS observations one must again be careful when analysing these data.

We first examined the results of the 117 accepted transits. From these data, one extreme outlier (more than 1 mag brighter than the average HIPPARCOS magnitude) and another point with a standard deviation larger than 0.15 mag were rejected. For the given time distribution of the data, we then created data sets consisting of random noise with the same standard deviation as the HIPPARCOS photometry. Both the original and the randomized data were then Fourier analysed. The result is displayed in Fig. 4.

 

Figure 4: Upper panel: the amplitude spectrum of the HIPPARCOS photometric data of M 2-54. Lower panel: an amplitude spectrum of a random data set, sampled exactly as the HIPPARCOS data and having the same standard deviation

The amplitude spectra shown in Fig. 4 supported by some more simulations (e.g. Fourier spectra of zeropoint adjusted data or data with artificially introduced variations) imply that the quality and the quantity of the HIPPARCOS observations are unfortunately too low to be of help in determining secure time scales of light variation of M 2-54. We note that the amplitude of the signal we found in our data was 12 mmag, much smaller than the noise level in the HIPPARCOS data.