Blue Stragglers in the Core of the Globular Cluster 47 Tucanae
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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.Keywords:
Blue straggler
Horizontal branch
Blue straggler
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Stellar collision
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Horizontal branch
Blue straggler
Variable star
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We study the post-main-sequence evolution of products of collisions between main-sequence stars (blue stragglers), with particular interest paid to the horizontal branch and asymptotic giant branch phases. We found that the blue straggler progeny populate the color-magnitude diagram slightly blueward of the red giant branch and between 0.2 and 1 magnitudes brighter than the horizontal branch. We also found that the lifetimes of collision products on the horizontal branch is consistent with the numbers of so-called "evolved blue straggler stars" (E-BSS) identified by various authors in a number of globular clusters, and is almost independent of mass or initial composition profile. The observed ratio of the number of E-BSS to blue stragglers points to a main-sequence lifetime for blue stragglers of approximately 1–2 Gyr on average.
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Sequence (biology)
Red-giant branch
Stellar collision
Asymptotic giant branch
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Abstract The presence of blue straggler stars (BSs) as secure members of star clusters poses a major challenge to the conventional picture of simple stellar population (SSP) models. The models are based on the stellar evolution theory of single stars, while the major formation mechanisms of BSs are all correlated with stellar interactions. Based on a sufficient working sample including 100 Galactic open clusters, one Galactic globular cluster, and seven Magellanic Cloud star clusters, we discuss the modifications of the properties of broad-band colors and Lick indices of the standard SSP models due to BS populations.
Blue straggler
Large Magellanic Cloud
Stellar collision
Stellar population
Star (game theory)
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We present BV photometry centered on the globular cluster M54 (NGC 6715). The color-magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero-age horizontal-branch theoretical models. These kinds of horizontal-branch stars (also called "blue hook" stars), which go beyond the lower limit of the envelope mass of canonical horizontal-branch hot stars, have so far been known to exist in only a few globular clusters: NGC 2808, ω Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indicating a possible correlation between the existence of these types of horizontal-branch stars and the total mass of the cluster. A gap in the observed horizontal branch of M54 around Teff = 27,000 K could be interpreted within the late helium flash theoretical scenario, which is a possible explanation for the origin of blue hook stars.
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Blue straggler
Instability strip
Asymptotic giant branch
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I present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC6388 and NGC6441, in order to investigate the nature of the physical mechanism(s) responsible for the existence of an extended blue tail and of a slope in the horizontal branch. Further evidence that the horizontal branch tilt cannot be interpreted as an effect of differential reddening is provided, while I show that a possible solution of the puzzle is to assume that a small fraction of the stellar population in the two clusters is strongly helium enriched.
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Stellar population
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It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts helium enhancements of ≳0.02 in virtually all blue HB stars. Using high-precision Strömgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the cluster's blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature.
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Blue straggler
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Using CCD color-magnitude diagrams of Galactic globular clusters in the B and V filter bands, we focus on the main sequence turnoff region and its usefulness in studying blue straggler stars, relative cluster ages, and the luminosity of the horizontal branch. In addition, we have measured several basic photometric parameters for the clusters in our program; these include NGC 288, NGC 3201, NGC 4372, NGC 4590 (M68), IC 4499, NGC 5897, NGC 6101, NGC 6171 (M107), NGC 6496, NGC 6584, NGC 6723, NGC 6809 (M55), NGC 6934, and NGC 6981 (M72). Of the 14 globular clusters in our sample, 11 appear to have definite blue straggler sequences. In the majority of cases, the blue stragglers are found to be more centrally concentrated and therefore more massive than the subgiant branch stars. Furthermore, the characteristics of the clusters which contain blue stragglers span practically the entire observed range for Galactic globular clusters. It therefore seems that all such systems must contain blue stragglers. The HR Diagram of the blue stragglers reveals that all of these stars occupy the same general region of this diagram, but the mean location of the individual cluster sequences do scale with metal abundance in the expected fashion. Finally, the luminosity function indicates the possible identification of blue stragglers which have evolved away from their initial positions. There exists a range in age among the Galactic globular clusters of 3 to 5 gigayears. Such a range suggests a relatively slow formation timescale for the Galactic halo. In addition, from the variation of age as a function of horizontal branch type, we conclude that age is the 'second parameter' which, along with metallicity, governs the behavior of the horizontal branch. Finally, we derive a simple algebraic expression that describes the slope of the horizontal branch luminosity - metallicity relation in terms of three relative cluster parameters. Using available values of these quantities in 8 globular clusters, we compute a slope of 0.15 ± 0.05 (mag/dex) for the variation of horizontal branch luminosity with metallicity.
Blue straggler
Horizontal branch
RR Lyrae variable
Distance modulus
Red-giant branch
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The Strömgren four-color system has been used to classify blue horizontal-branch stars in globular clusters and in the general field. A relation between δ log g (log g main sequence –log g star ) and δ c 1 ( c 1 observed – c 1 main sequence ) has been determined which gives log g to within ±0.2 for stars with log g 's between 4.4 and 2.0. Newell (1970) has determined a relation between ( B–V ) 0 and θ e . Thus the gravities and effective temperatures of blue horizontal-branch stars can be determined from photometric measures alone. Five globular clusters have been studied at the present time. The BHB stars in the globular cluster M3 and M13 have lower surface gravities than BHB stars in the other clusters studied. Iben and Rood (1970) have recently suggested that the BHB stars of M3 may have lost more mass than BHB stars in more metal poor clusters.
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Blue straggler
Stellar collision
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