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Session 44 - Double Stars.
Display session, Thursday, January 08
The study of binary stars and their progeny in globular clusters has been greatly advanced by the imaging and spectroscopic capabilities of HST. We continue this work by reporting Faint Object Spectrograph (FOS) observations of compact binaries in the cores of the dense globular clusters 47 Tucanae and NGC 6397. Using G570H observations we show that a fourth cataclysmic variable (CV) candidate in NGC 6397 has similar properties to the three already known in this cluster, strengthening the hypothesis that these stars all belong to a single class of objects (possibly magnetic CVs). Observations of the brightest of these stars with G160L and the red FOS prism show a significant UV excess but, surprisingly, no strong UV emission lines. By contrast, both UV excess and UV emission lines are found for a 47 Tuc dwarf nova in quiescence, along with weak Balmer emission lines. We also analyze red FOS prism and G570H spectra of other interesting 47 Tuc binaries, including a possible pre-cataclysmic variable (with UV excess and variability but no emission lines) and AKO9, an unusual eclipsing binary known to undergo dramatic UV outbursts. Finally, we analyze G570H spectra of a blue non-variable star in NGC 6397 representing a possible new class of cluster stars with properties reminiscent of white dwarfs. These stars could result from the stripping of red giant envelopes in binary systems. Overall, our results emphasize the exotic outcomes of stellar interactions and binary evolution in the dense stellar environment of globular cluster cores.
Program listing for Thursday