The optical gravitational lensing experiment. Variable stars in globular clusters
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We present V-band photometry for 141 variable stars from the field of ω Cen. All but one variables are either RR Lyr stars or Pop. II Cepheids. One object is a distant background RR Lyr star from the galactic halo. The presented sample includes 33 newly identified variables, mostly low amplitude RR Lyr stars. We identified also two Pop. II Cepheids with exceptionally low amplitudes of light variations . The presented data may be used to study period changes of ω Cen variables and/or for Fourier decomposition of light curves.Keywords:
Variable star
Blue straggler
Blue straggler
Stellar collision
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Blue straggler
Horizontal branch
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We have used high resolution WFPC2-HST and wide field ground-based observations to construct a catalog of blue straggler stars (BSS) which spans the entire radial extent of the globular cluster NGC 6752. The BSS sample is the most extensive ever obtained for this cluster. Though NGC 6752 is a high density cluster with a large binary population, we found that its BSS content is surprisingly low: the specific number of BSS is among the lowest ever measured in a cluster. The BSS distribution is highly peaked in the cluster center, shows a rapid decrease at intermediate radii, and finally rises again at larger distances. This distribution closely resembles those observed in M3 and 47Tuc by Ferraro et al. (1993), Ferraro et al. (2003c). To date, BSS surveys covering the central regions with HST and the outer regions with wide field CCD ground-based observations have been performed for only these three clusters. Despite the different dynamical properties, a bimodal radial distribution has been found in each. A detailed comparison of observed BSS luminosity and temperature distributions with theoretical models reveals a population of luminous, hot BSS which is not easily interpreted.
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Star (game theory)
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Preliminary results are presented for 3 southern globular clusters that have been searched via CCD photometry for new short-period variables that might be associated with blue stragglers. The clusters considered here are NGC6121, NGC3201 and NGC6809.
Blue straggler
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We investigate a new interaction of globular clusters with galactic field stars: the deceleration (by dynamical friction) of high-velocity field stars diffusing through individual globular clusters. This frictional interaction contributes to cluster heating and, in conjunction with disc shocking and other mechanisms, helps to regulate the evolution of globular clusters. Moreover, penetrating field stars of low relative velocity can even be captured by globular clusters. Our calculated rate of capture suggests that there is a modest population of stars having an origin external to the clusters in which they now reside. Intriguing candidates for this ‘immigrant’ population include some blue stragglers and short-period pulsars.
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Stellar dynamics
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There are several observed phenomena in globular clusters that are thought to be the result of dynamical processes or binary star evolution. This review examines these manifestations of the interaction between globular cluster dynamics and stellar evolution. Blue stragglers may be formed by the evolution of primordial binaries or by collisions. Current evidence suggests that both processes are likely to occur, and that the observed blue straggler sequences can place dynamically interesting limits on rates. Color gradients in globular clusters are thought to be caused by the stripping of giants by collisions, although the creation of blue subdwarfs by the same process may also be required to explain the observations. The observed X-ray sources and radio pulsars are apparently also made by a variety of dynamical processes that are still not fully understood.
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Abstract High resolution imaging observations of the core of the globular cluster 47 Tucanae with the Faint Object Camera (FOC) on the Hubble Space Telescope (HST) reveal a very high surface density of blue stragglers. This discovery supports the hypothesis that they are among the most massive objects in globular clusters, concentrated in the core by mass segregation. Taken together with the presence in 47 Tue of an X-ray source, eleven millisecond and binary pulsars, and two high-velocity stars ejected out of the core, these observations favour the scenario that blue stragglers are either mergers formed through coalescent collisions, or binaries formed through close encounters.
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Two of the three long-period red variables in the globular cluster NGC 6656 have been found to show Ba star characteristics. This observation provides the first conclusive evidence of the late evolutionary status of red variables in globular clusters.
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Barium
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Observations of variable stars near = 295 , b = 0 are described, including a new cepheid for which the distance is estimated to be 11.8 kpc. Key words: cepheids - variable stars - spiral structure
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