4. Numerical experiments
Since additional low-amplitude modes in BT Cnc would be of high importance for asteroseismological applications, we have made several experiments to search for such modes, using different approaches. (i) A typical time-interval between successive observations is 0.0012 d. Since oscillation periods of the order of 0.1 d are relevant here, averages of about 5 observations can be expected to give suppression of high-frequency noise without significant loss of phase information. We have tested schemes using 3-7 neighbouring observations. Using averages, formal errors in amplitudes and phases of the Fourier components of lsq-fits decrease considerably. However, the resulting spectra do not show clear improvements. (ii) Due to the dense sampling we can apply weights by calculation of the deviation of each point from the local mean of the data, following Frandsen et al. (1995). Here we find little or no effect of applying weights. (iii) We have taken slowly varying trends into account using for each night, in addition to a zero-point correction, also terms , where is the time (HJD) of the middle of the night and d and e are determined in each lsq-solution. Tests on synthetic data show that d and e -values can be determined together with standard lsq-coefficients without problems. The linear term often gives some suppression of low-frequency noise, while we usually find no significant effect of the second order term. (iv) Finally, we have excluded all observations from the night of February 15th - 16th, which has significantly larger scatter than the other nights, from the analysis. However, this results in a less favourable window function, and no improvement is obtained.
Comparing results of these numerical experiments, we have not been able to find systematic features in the S-spectra in addition to the two main modes. Because they are very stable against modifications of the lsq-procedure, we can conclude that these modes are definitely present in our data for BT Cnc with characteristics as specified in Table 3. We note that both main modes of BT Cnc are almost sinusoidal with small harmonic components.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998