High-Energy Astrophysics American and Soviet Perspectives/Proceedings from the U.S.-U.S.S.R. Workshop on High-Energy Astrophysics (1991) / Chapter Skim
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CLOSE BINARY STARS IN GLOBULAR CLUSTERS
CLOSE BINARY STARS IN GLOBULAR CLUSTERS
Pages 270-284
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From page 270... ...
One of the objects, a close eclipsing system in Cen, is also a blue straggler, thus finally providing firm evidence that globular cluster blue stragglers really are binary stars. INTRODUCTION A growing body of theoretical work indicates that a small number of close binary stars dominate the dynamic evolution of globular star clusters fag, Elson et at 1987~.
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From page 271... ...
, and systems with giant, subgiant, and main sequence components yet more common. SEARCHES FOR EVOLVED GLOBULAR CLUSTER BINARIES White dwarf close binaries in globular clusters are probably most easily found if they are undergoing mass exchange.
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From page 272... ...
1989~. It is sometimes not amply enough stressed that all of the evidence which identifies these systems as white dwarf close binaries is quite indirect, based primarily on the fact that these sources have considerably lower Xray luminosities than the highly luminous sources located within a few arcseconds of the cluster cores; the latter are now widely agreed to be neutron star binary systems (Lewin 1980; Lewin and Joss 1983; Grindlay et al.
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From page 273... ...
AN ECLIPSING BLUE STRAGGLER IN A GLOBULAR CLUSTER Blue stragglers, especially in globular clusters, have long rankled both 1 Multicolor photometry for MS V101 appears in Richer and Fahlman (1987) , although the object is anonymous in that paper; it is the most extreme ultraviolet point in their Figure 11.
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From page 274... ...
Alternative theones, involving long-lived single stars undergoing odd evolution, are also viable (Wheeler 1979~. Some evidence that blue stragglers in NGC 5466 and NGC 5053 have a peculiarly concentrated spatial distnbution, and thus a higher primordial mass than other evolved stars in the cluster, has been provided recently (Nemec and Harris 1987; Nemec and Cohen 1989~.
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From page 275... ...
This work on NJL 5 was done collaboratively with Russell Cannon, and will be described more fully elsewhere (Margon and Cannon 1989~. FAINT MAIN SEQUENCE BINARIES IN GLOBULAR CLUSTERS What are the prospects for detection of main sequence binaries of later spectral type than blue stragglers, and thus at a less odd stage of evolution?
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From page 277... ...
The displaced stars are spatially well separated by our reduction programs, so we have no particular reason to suspect that their distinguishing characteristic is merely poor quality photometry. As an interpretive aid, we show with the solid line where the fiducial main sequence would fall if displaced upward in luminosity by 0.75 may, as expected for a sequence of equal mass binaries.
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From page 278... ...
I-he solid line is a locus off~;et by 0.75 mag ~om a fiducial main sequence. The dozen or so candidate main-sequence binaries, Iying quite close to this line, are readily apparent.
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From page 279... ...
The colors of the candidates indicate spectral Apes near solar, and so we may anticipate ample absorption lines for radial velocity determination. We are conducting our observations using the fiber-optic coupled multiobject spectrograph (FOCAP)
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From page 280... ...
Upper panel: one of the bright radial velocity standard stars in the cluster, AL80, a giant with V = 13.89, (B-V) = 1.14 (AIcaino and Liller l980~; a precise (~0.7 km s-1)
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From page 281... ...
Thus we are encouraged by the prospect that current instrumentation is indeed adequate to reveal radial velocity variations in main sequence binaries in this and similar clusters in the near future. CONCLUSION In an lateral of only a few years, we have progressed from knowing of literally no non-degenerate binary stars in globular clusters, to a short list of candidates of a variety of luminosities.
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1983a. X-ray evidence for white dwarf binaries in globular clusters Astrophysical Joumal (Letters)
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19~. We nalum of the 1~-l~in~ globular duster <~ _ ~1 ~ ~: ~1~.
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1984. X-ray sources in globular clusters Months Notices of the Royal Astronomical Society 210: 899 914.
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