Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters

Rich, R. Michael; Sosin, C.; Djorgovski, S. G.; Piotto, G.; King, Ivan R.; Renzini, A.; Phinney, E. S.; Dorman, Ben; Liebert, James; Meylan, G.

doi:10.1086/310758

Cited by 244 publications

(318 citation statements)

References 28 publications

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“…First, our B and V photometry was compared with the HST B, V photometry obtained by Rich et al (1997). Unfortunately, even the outermost stars in the Ðeld of view of WFPC2 tended to be crowded on the images obtained at CTIO.…”

Section: Observations and Reductionsmentioning

confidence: 99%

“…One might therefore have expected that the color-magnitude diagram (CMD) of NGC 6441 would show the stubby red horizontal branch (HB) typical of metal-rich globular clusters such as 47 Tuc. The CMD of NGC 6441 does indeed show a strong, red HB component, but Rich et al (1997), using Hubble Space T elescope (HST ) observations, discovered that the CMD of NGC 6441 also has a pronounced blue HB component, which stretches across the location of the instability strip. Not only does the HB have a blue component, but it also slopes upward as one goes blueward in a (V , B[V ) CMD.…”

Section: Introductionmentioning

confidence: 99%

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Variable Stars in the Unusual, Metal-rich, Globular Cluster NGC 6441

Pritzl

Smith

Catelan

et al. 2001

The Astronomical Journal

130

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We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6441 using time-series BV photometry. The total number of variables found near NGC 6441 has been increased to D104, with 48 new variables being found in this survey. A signiÐcant number of the variables are RR Lyrae stars (D46), most of which are probable cluster members. As was noted by Layden et al. in 1999, the periods of the fundamental-mode RR Lyrae stars are unusually long compared with Ðeld stars of similar metallicity. The existence of these long-period RRab stars is consistent with Sweigart & CatelanÏs prediction that the horizontal branch of NGC 6441 is unusually bright. This result implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal. We discuss the difficulty in determining the Oosterho † classiÐcation of NGC 6441 because of the unusual nature of its RR Lyrae stars. A number of ab-type RR Lyrae stars are found to be both brighter and redder than the other probable RRab found along the horizontal branch, which may be a result of blending with stars of redder color. A smaller than usual gap is found between the shortest period fundamental-mode and the longest period Ðrst-overtoneÈmode RR Lyrae stars. We determine the reddening of the cluster to be E(B[V ) \ 0.51^0.02 mag, with substantial di †erential reddening across the face of the cluster. The mean V magnitude of the RR Lyrae is found to be 17.51^0.02 mag, resulting in a distance of 10.4È11.9 kpc, for a range of assumed values of for RR Lyrae stars. The possibility that stars in NGC 6441 SM V T may span a range in [Fe/H] is also discussed.

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Section: Observations and Reductionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Variable Stars in the Unusual, Metal-rich, Globular Cluster NGC 6441

Pritzl

Smith

Catelan

et al. 2001

The Astronomical Journal

130

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“…However, the existence of clusters with similar metallicities but very distinct HB morphologies has led to the "" second parameter ÏÏ debate. Indeed, Rich et al (1997) have recently shown that even metal-rich globular clusters can have extended blue HBs. It is well known that multiple parameters can potentially act as second parameters on the HB (e.g., age, mass loss, helium abundance, rotation, cluster dynamics ; Fusi Pecci & Bellazzini 1997).…”

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

197

394

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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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“…The metal-rich bulge globular cluster NGC 6388 shows a distinct blue horizontal-branch tail in its colour-magnitude diagram (Rich et al 1997) and is thus a strong case of the well-known 2nd Parameter Problem. In addition, its horizontal branch (HB) shows an upward tilt toward bluer colours, which cannot be explained by canonical evolutionary models.…”

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confidence: 99%

Observations of the hot horizontal-branch stars in the metal-rich bulge globular cluster NGC 6388

Moehler¹,

Sweigart

2006

A&A

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Context. The metal-rich bulge globular cluster NGC 6388 shows a distinct blue horizontal-branch tail in its colour-magnitude diagram (Rich et al. 1997) and is thus a strong case of the well-known 2nd Parameter Problem. In addition, its horizontal branch (HB) shows an upward tilt toward bluer colours, which cannot be explained by canonical evolutionary models. Aims. Several noncanonical scenarios have been proposed to explain these puzzling observations. In order to test the predictions of these scenarios, we have obtained medium resolution spectra to determine the atmospheric parameters of a sample of the blue HB stars in NGC 6388. Methods. Using the medium resolution spectra, we determine effective temperatures, surface gravities and helium abundances by fitting the observed Balmer and helium lines with appropriate theoretical stellar spectra. As we know the distance to the cluster, we can verify our results by determining masses for the stars. During the data reduction we took special care in subtracting the background, which is dominated by the overlapping spectra of cool stars. Results. The cool blue tail stars in our sample with effective temperatures T eff ≈ 10 000 K have lower than canonical surface gravities, suggesting that these stars are, on average, ≈0. m 4 brighter than canonical HB stars in agreement with the observed upward slope of the HB in NGC 6388. Moreover, the mean mass of these stars agrees well with theoretical predictions. In contrast, the hot blue tail stars in our sample with T eff ≥ 12 000 K show significantly lower surface gravities than predicted by any scenario, which can reproduce the photometric observations. Their masses are also too low by about a factor of 2 compared to theoretical predictions. Conclusions. The physical parameters of the blue HB stars near 10 000 K support the helium pollution scenario. The low gravities and masses of the hot blue tail stars, however, are probably caused by problems with the data reduction, most likely due to remaining background light in the spectra, which would affect the fainter hot blue tail stars much more strongly than the brighter cool blue tail stars. Our study of the hot blue tail stars in NGC 6388 illustrates the obstacles which are encountered when attempting to determine the atmospheric parameters of hot HB stars in very crowded fields using ground-based observations. We discuss these obstacles and offer possible solutions for future projects.

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Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters

Cited by 244 publications

References 28 publications

Variable Stars in the Unusual, Metal-rich, Globular Cluster NGC 6441

Variable Stars in the Unusual, Metal-rich, Globular Cluster NGC 6441

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

Observations of the hot horizontal-branch stars in the metal-rich bulge globular cluster NGC 6388

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