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Ronald Mennickent |
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"Unveiling the Phenomenon
of Double Periodic Variables" |
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We describe the main phenomenological characteristics of an interesting group of 100 blue close binary stars that we have recently found in the Magellanic Clouds, including new and unpublished results. These stars are the Double Periodic Variables (DPVs), characterized by a short periodicity (1-16 days) and a long periodicity (50-600 days) in their light curves. We present examples of DPV light curves and show how both periodicities are linked through an approximately constant period ratio. However, we also show examples indicating that the long-term cycle is not strictly periodic. We give sound arguments favoring the binary hypothesis for DPVs. We show how the data for one DPV indicates that it consists of a B-type giant in a close orbit with an under-massive later type companion. Infrared data is examined for the first time indicating a strong infrared excess for all SMC DPVs with available 2MASS data. We also present high resolution spectra and reports from old H-alpha image surveys indicating the presence of Balmer emission in some bright DPVs. We examine possible interpretations for the phenomenon, and present our hypothesis of the propeller. In this view, DPVs are semi-detached binaries in a stage of mass transfer, probably at the end of Case-A mass exchange. The B-type star is spinning up while accretes matter from the later type donor in an Algol-like configuration. Matter is also ejected from the binary system forming a reservoir of circumbinary gas. The long term periodicity could be produced by some kind of global motion of this circumbinary gas envelope, possibly triggered by binary tidal forces and influenced by a variable mass ejection rate. Sensible observational tests for this scenario and incoming observational programs are described. |