2006ApJ...642..470A

The β Cephei star δ Ceti was considered one of the few monoperiodic variables in its class. Despite (or perhaps because of) its apparently simple oscillation spectrum, it has been challenging and controversial to identify this star's pulsation mode and constrain its physical parameters seismically. Broadband time-resolved photometry of δ Ceti spanning 18.7 days with a duty cycle of about 65% obtained by the Microvariability and Oscillations of Stars (MOST) satellite–the first scientific observations ever obtained by MOST–reveals that the star is actually multiperiodic. Besides the well-known dominant frequency of f₁=6.205886/day, we have discovered in the MOST data its first harmonic 2f₁and three other frequencies (f₂=3.737, f₃=3.673, and f₄=0.318/day), all detected with a signal-to-noise ratio S/N>4. In retrospect, f₂was also present in archival spectral line-profile data but at lower S/N. We present seismic models whose modes match exactly the frequencies f₁and f₂. Only one model falls within the common part of the error boxes of the star's observed surface gravity and effective temperature from photometry and spectroscopy. In this model, f₁is the radial (l=0) first overtone, and f₂is the g₂(l=2, m=0) mode. This model has a mass of 10.2±0.2 M_☉and an age of 17.9±0.3 Myr, making δ Ceti an evolved β Cephei star. If f₂and f₃are rotationally split components of the same g₂mode, then the star's equatorial rotation velocity is either 27.6 km/s or half this value. Given its vsini of about 1 km/s, this implies that we are seeing δ Ceti nearly pole-on.