[delta] Ceti is not monoperiodic: seismic modeling of a [beta] Cephei star from MOST space-based photometry

Abstract

The [beta] Cephei star [delta] 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 [delta] 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 f[subscript 1] = 6.205886 day[superscript minus 1], we have discovered in the MOST data its first harmonic 2f[subscript 1] and three other frequencies (f[subscript 2] = 3.737, f[subscript 3] = 3.673, and f[subscript 4] = 0.318 day [superscript minus 1]), all detected with a signal-to-noise ratio (S/N) [is greater than] 4. In retrospect, f[subscript]2 was also present in archival spectral line-profile data but at lower S/N. We present seismic models whose modes match exactly the frequencies f[subscript 1] and f[subscript 2]. 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, f[subscript 1] is the radial (l = 0) first overtone, and f[subscript 2] is the g[subscript 2] (l = 2, m = 0) mode. This model has a mass of 10.2 [plus or minus] 0.2 M[circled dot] and an age of 17.9 [plus or minus] 0.3 Myr, making [delta] Ceti an evolved [beta] Cephei star. If f[subscript 2] and f[subscript 3] are rotationally split components of the same g[subscript 2] mode, then the star’s equatorial rotation velocity is either 27.6 km s[superscript minus 1] or half this value. Given its v sin i of about 1 km s [superscript minus 1], this implies that we are seeing [delta] Ceti nearly pole-on.