Analysis of the Hot Stellar Population of the Globular Cluster ω Centauri

Whitney, J. H.; Rood, Robert T.; O’Connell, R. W.; D'Cruz, Noella L.; Dorman, Ben; Landsman, W. B.; Bohlin, R. C.; Roberts, M. S.; Smith, Andrew M.; Stecher, T. P.

doi:10.1086/305288

Cited by 50 publications

(58 citation statements)

References 40 publications

(1 reference statement)

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“…As shown in Figure 16d, if the blue-hook stars have envelopes with the normal cluster abundances, the blue-hook stars pile up near the end of the canonical ZAHB, with no gap between the canonical EHB and blue-hook stars. Whitney et al (1998) argued that such an e †ect could explain the gap between the EHB and BHB in u Cen. They only considered blue-hook stars with normal atmospheres, and showed that the range of giving rise to the blue-hook stars was larger g R than the range of populating the HB between the EHB g R and BHB ; this e †ect would be exaggerated for the more metal-rich component of the u Cen metallicity distribution (see DÏCruz et al 1996).…”

Section: Ehb Gapmentioning

confidence: 99%

“…In this paper, we present a new theoretical scenario for understanding these peculiarities of the EHB population in NGC 2808, as well as u Cen. DÏCruz et al (2000) and Whitney et al (1998) claimed that the hot subluminous stars on the HB of u Cen were probably a population of "" blue-hook ÏÏ HB stars. These are stars that experience a late helium-core Ñash.…”

Section: Introductionmentioning

confidence: 99%

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Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Brown

Sweigart

Lanz

et al. 2001

ApJ

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197

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We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we Ðnd a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster u Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core Ñash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium Ñash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "" born again ÏÏ scenario for producing hydrogen-deÐcient stars following a late helium-shell Ñash. The Ñash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB e †ective temperatures at the transition between canonical and Ñash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to conÐrm that a late helium-core Ñash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

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Section: Ehb Gapmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Brown

Sweigart

Lanz

et al. 2001

ApJ

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197

394

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“…This GC has at least three separate, well-defined red giant branches (RGBs), an extended HB morphology, and a large number of subluminous extreme HB stars (e.g., D'Cruz et al 1996D'Cruz et al , 2000Whitney et al 1998;Pulone et al 1998;Pancino et al 2003;Rey et al 2004;Freyhammer et al 2005;Cassisi et al 2009;Bellini et al 2009;Calamida et al 2009Calamida et al , 2010. Hubble Space Telescope (HST) observations have also uncovered a triple MS (Bedin et al 2004;Bellini et al 2009Bellini et al , 2010, in addition to at least four SGBs (Villanova et al 2007).…”

Section: Introductionmentioning

confidence: 99%

Formation of multiple populations in globular clusters: another possible scenario

Valcarce

Catelan

2011

A&A

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Aims. While spreads in chemical composition are now believed to be a universal characteristic of globular clusters (GCs), not all of them display evidence of multiple populations in their color-magnitude diagrams (CMDs). Here we present a new scenario for the formation of GCs, in an attempt to qualitatively explain these otherwise elusive observational results. Methods. Our scenario divides GCs into three groups, depending on the initial mass (M I ) of the progenitor structure (PS), as follows: i) massive PSs can retain the gas ejected by massive stars, including the ejecta of core-collapse SNe. ii) Intermediate-mass PSs can retain at least a fraction of the fast winds of massive stars, but none of the core-collapse SNe ejecta. iii) Low-mass PSs can only retain the slow winds of intermediate-mass stars. The first group would contain members such as ω Centauri (NGC 5139), M 54 (NGC 6715), M 22 (NGC 6656), and Terzan 5, whereas NGC 2808 (and possibly NGC 2419) would be members of the second group. The remaining GCs that have only a spread in light elements, such as O and Na, would be members of the third group. Results. According to our scenario, the different components of ω Cen should not have a sizeable spread in age. We argue that this is consistent with the available observations. We present other simple arguments in favor of our scenario, which can be described in terms of two main analytical relations: i) one between the actual observed ratio of the number of first to second generation stars (R FG SG ) and the fraction of first generation stars that have been lost by the GC (S L ) and ii) another between S L and M I . We also suggest a series of future improvements to and empirical tests of the modeling that may help decide whether the proposed scenario properly describes the chemical evolution of GCs.

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“…The late-hot-flash model has been used to explain the existence of blue-hook stars found in several massive GCs (Whitney et al 1998;D'Cruz et al 2000;Brown et al 2001). In our calculations, however, we did not consider the helium-flash mixing process.…”

Section: Resultsmentioning

confidence: 99%

Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters

Lei

Chen

Zhang

et al. 2013

A&A

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Context. Metallicity is the first parameter to influence the horizontal branch (HB) morphology of globular clusters (GCs). It has been found, however, that some other parameters may also play an important role in affecting the morphology. While the nature of these important parameters remains unclear, they are believed to be likely correlated with wind mass-loss of red giants, since this mass loss determines their subsequent locations on the HB. Unfortunately, the mass loss during the red giant stages of the stellar evolution is poorly understood at present. Aims. The stellar winds of red giants may be tidally enhanced by companion stars if they are in binary systems. We investigate the evolutionary consequences of red giants in binaries by including tidally enhanced stellar winds, and examine the effects on the HB morphology of GCs. Methods. We used Eggleton's stellar evolution code to study the binary evolution. The tidally enhanced stellar-wind model of Tout & Eggleton is incorporated into this code, where the tidal enhancement parameter, B w , has various values (e.g., 10 000 and 500) to examine the dependency of the final results on this parameter. A Monte Carlo simulation was performed to generate a group of binary systems. The position of each primary star on the HB in the Hertzsprung-Russell diagram in this sample is obtained through interpolations among the constructed HB evolutionary tracks. Finally, a synthetic HB in the color-magnitude diagram is obtained by transforming the effective temperature and luminosity of each primary star on the HB into B − V colors and absolute magnitude. Results. We find that red, blue, and extreme horizontal branch stars are all produced under the effects of tidally enhanced stellar wind without any additional assumptions on the mass-loss dispersion. Furthermore, the horizontal branch morphology is found to be insensitive to the tidal enhancement parameter, B w . We compare our theoretical results with the observed horizontal branch morphology of globular cluster NGC 2808, and find that the basic morphology of the horizontal branch can be well reproduced. The number of blue horizontal branch stars in our calculations, however, is lower than that of NGC 2808.

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Analysis of the Hot Stellar Population of the Globular Cluster ω Centauri

Cited by 50 publications

References 40 publications

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Formation of multiple populations in globular clusters: another possible scenario

Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters

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