In the sixth observational campaign made by the STEPHI network during January-February of 1995, two Scuti stars in Praesepe cluster, BQ and BW Cnc, were monitored for a period of 29 days, with a total observing time of 219.1 hours with an overlapping between sites of 8.9 hours. Low noise level (mag at 200 µHz) and a good spectral window (sidelobes at 38% of the main lobe), were reached thanks to the continuous three-continental monitoring of the target stars.
Three and nine frequencies were found significant for BQ and BW Cnc, respectively, as shown in the paper. We have also shown that the 3.7-mean amplitude level in boxes of 100 µHz along the spectrum can reproduce fairly well the 99% significance level from statistical tests such as Fischer's or Scargle's test.
The good spectral resolution achieved during this campaign ( = 0.60 µHz) has permitted to resolve two pairs of very close frequencies in BW Cnc. Close inspection on these modes revealed that their amplitude changed considerably along the observations, due to mutual interference between the elements corresponding to each pair. That interference produced the cancellation of the amplitude of each pair of modes in different epochs of the observing period. In this way, situations like the ones presented here, could produce different detection of modes depending on the observing period, if this is not long enough. We suggest that long campaigns are necessary to avoid this phenomenon. At the same time, any study of secular amplitude changes should take into account this effect, if short observing periods have been used.
The binarity of BQ Cnc offers a good scenario for Astero-seismology
since, potentially, further information about the star can be
obtained. Unfortunately, no detailed radial velocity measurements were
found in the literature. Considering the possible positions of the
system components in the HR diagram in order to recover the observed
color and luminosity, the mode at =
136.9 µHz seems to be a g-mode, after
comparison with models within the requirements for the cluster.
© European Southern Observatory (ESO) 1998
Online publication: November 3, 1998