The Be Star Newsletter, Volume 36 - August 2002

[Be Star Newsletter Home]

Monitoring the Period Changes in Beta Cephei Stars using Hipparcos Epoch Photometry John R. Percy & Rebecca Dehmassi Department of Astronomy & Astrophysics, University of Toronto, Toronto ON Canada M5S 3H8 email:  jpercy@utm.utoronto.ca Received: August 22, 2002

ABSTRACT The purpose of this short note is to point out that the Hipparcos epoch photometry can be useful, in retrospect, for monitoring the period changes in Beta Cephei stars.

Monitoring period changes in pulsating variables such as Beta Cephei stars is important for studying the evolution of these stars (e.g. Jerzykiewicz & Pigulski 1998), but many of these bright stars are not being monitored regularly. We have used the large-amplitude Beta Cephei star BW Vul (HD 199140, SpT B2III, V = 6.52) as an example of how Hipparcos} epoch photometry can be used, in retrospect, for this purpose. Sterken (1993) has recently published a detailed study of the period history of this star. The most dominant feature is an almost-parabolic (O-C) diagram, indicating a linear period change of 2.8 seconds/century; this feature has been known for several decades. Sterken (1993) has also called attention to the systematic deviations from this parabolic trend. In some stars, these deviations have provided information about binarity (through light-time effects) or on random cycle-to-cycle fluctuations in period. Because of the specific observing schedule of the Hipparcos satellite, the photometric measurements were made in clusters of a few measurements in a few hours; the clusters are separated by 20-30 days. For some applications, this time distribution may be inconvenient. For Beta Cephei stars, which have periods of 0.1 to 0.3 day, it can be quite adequate. We first divided the Hipparcos measurements of BW Vul into three sets, each approximately a year long. We then constructed phase diagrams for each set, using the period 0.201038 day and the epoch 2428802.5487 (Sterken 1993). From each phase diagram, we determined the phase of minimum brightness using Hertzsprung's method of bisected chords. Following Sterken (1993), we used the time of minimum because it tends to be better-defined. The results are as follows: set I (median JD 2495000): phase 0.480; set II (median JD 2496800): phase 0.530; set III (median JD 2498600): phase 0.575. Translated into (O-C) values in days, these are: -0.105, -0.094, and -0.085, respectively. These agree exactly with the trend in the (O-C) diagram presented by Sterken (1993) in his Figure 1. This demonstrates that Hipparcos measurements can be used to track the period changes in this Beta Cephei star, and presumably in others. There are at least two dozen other Beta Cephei stars which could be analyzed. We strongly recommend using the Hipparcos epoch photometry to check the periods of these stars during the 1989-1993 interval of the mission. This is straightforward for monoperiodic stars such as BW Vul. It becomes more complicated for multiperiodic Beta Cephei stars, but it should be possible to check the constancy of the one or two largest-amplitude modes by using a phase-shift analysis for each mode. Acknowledgement. We thank Dr. C. Sterken for sending the times of minima of BW Vul to us in electronic form. Rebecca Dehmassi was a participant in the University of Toronto Mentorship Program, which enables outstanding senior high school students to work on research projects at the university. REFERENCES Jerzykiewicz, M. & Pigulski, A. 1998, in A Half-Century of Stellar Pulsation Interpretations, ed. P.A. Bradley & J.A. Guzik, ASP Conf. Ser. 135, 43 Sterken, C. 1993, A&A, 270, 259