New measurements and analysis of the β Cephei star V909 Cassiopeiae

2018 September 21

David Boyd & Robert Koff

Beta Cephei variables
β Cep stars are the brightest recognised class of variable stars in the sky, although many other highly luminous stars are intrinsically variable. They are pulsating stars whose brightness varies with periods of 0.1 to 0.3 days and with amplitudes of 0.1 magnitudes or less. Most have spectral types in the range B0 to B2 and luminosity classes III to V.1 They lie within the OB instability strip located on the main sequence in the Hertzsprung-Russell diagram. The pulsation of β Cep stars is due to the kappa mechanism acting on iron-group atoms deep within the star. In some β Cep stars the pulsation amplitude varies because of beating between unresolved pulsation frequencies.

Stankov & Handler review current knowledge about β Cep stars and list 93 confirmed and 77 suspected instances. In the General Catalogue of Variable Stars β Cep stars are designated as BCEP. The AAVSO Variable Star Index (VSX) currently lists 255 BCEP stars, three-quarters of which are south of the celestial equator. This is consistent with stars closer to the galactic centre having the higher metal abundances conducive to the β Cep instability mechanism. OB associations comprise young stars within galactic Population I and are concentrated towards the galactic plane.

V909 Cassiopeiae
In the course of observations of the X-ray source RX J0136.7+6125, Robb et al. serendipitously discovered that the nearby star BD+60 282 (GSC 04031-00631) was a β Cep-type pulsating star. It was subsequently given the designation V909 Cas.

Robb et al. recorded three series of R filtered photometry of the star between 1996 December and 2000 January, during which they measured 18 times of maximum of the lightcurve and 11 times of minimum. From these they derived three independent values for its pulsation period. Calculating a weighted mean of these periods gives 0.2066(2) days, where the figure in parentheses is the uncertainty in the final digit. They also noted that the pulsation amplitude varied between 0.01 and 0.05 magnitudes, and it appears from their Figure 3 that the times of maximum varied slightly with respect to the pulsation phase. V909 Cas is one of the relatively few β Cep stars for which VSX does not currently list a pulsation period. We decided to investigate its current behaviour. (continued…)

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