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δ Ceti from the Ground and from MOST

Jerzykiewicz M.

Acta Astronomica

2007

Vol. 57, No. 1

33--47

Strömgren b-light curves of δ Cet, well known β Cep type star, obtained in 1990 December and 1991 January at SAAO and in 2003 October at Lowell are presented. The two epochs of maximum light, derived from these data, cannot be reconciled with the constant rate of secular period change found earlier. By a stroke of luck, the Lowell data were taken at the time when δ Cet was observed by MOST. A comparison of these two sets of data reveals discrepancies between the Lowell and MOST light curves, and between the MOST fluxes available on the net and the MOST magnitudes reported by Aerts et al. (2006). This had motivated us to reanalyze the MOST data. We find that the second strongest periodic signal in the MOST photometry of the star has frequency equal to the reciprocal of synodic month. We show further that the three lowest amplitude periodic terms derived by the above-mentioned authors and taken as a proof of multiperiodicity are significant only if one ignores the increase of the noise level toward lower frequencies, seen in the amplitude spectra of the MOST magnitudes. We conclude that the issue of multiperiodicity of δ Cet is debatable. We warn, therefore, that results of seismic modeling based on frequencies of these low amplitude terms, such as the core overshooting parameter or the inclination of the star's axis of rotation, should be treated with caution.