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R. J. Dukes, Jr., S. M. Sonnett, L. R. Mills (Coll. Charleston)
The bright star Spica has long been known to be a spectroscopic binary (Vogel, 1890, AN, 125, 305). Shobbrook et al. (1969, MNRAS, 145, 131) found it to be a Beta Cephei variable. Several studies of the Beta Cephei variation were conducted from 1967 – 1974. Lomb (1978, MNRAS, 185, 325) found that the pulsation amplitude had been decreasing and, by 1972, had become undetectable. While there have been sporadic reports of detection of short period light variation, none of these have been confirmed. In order to monitor the presence or absence of the strong pulsation, we obtained approximately 1000 Stromgren four-color observations on 130 nights during the 1996, 1997, and 2005 seasons. We have good phase coverage of the 4.01-day orbital period, enabling us to both verify the apsidal period and to examine the variation in temperature due to ellipsoidal variation.
A major problem with this star is removing the ellipsoidal variation in order to search the residuals for pulsation; when the pulsation amplitude dominated the ellipsoidal amplitude the problem was the opposite. After fitting the orbital frequency and its harmonics to the photometric data, we examined the residuals for signs of pulsation. We can definitively state that there is no sign of the original 5.75 c/d term. As for the various other frequencies suggested by various observers, the case is not as clear. There is some indication of the 8-hour period found by Smith (1985, ApJ, 297, 224) from the v filter analysis, but this has not yet been confirmed with analyses of the other filter data. This work has been supported by National Science Foundation grants to the College of Charleston and by a South Carolina Space Grant Undergraduate Research Award to one of us (SMS).
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.