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          Institute: MPI für Astrophysik     Collection: Stellar Astrophysics     Display Documents

ID: 13502.0, MPI für Astrophysik / Stellar Astrophysics
On the observational properties of He-burning stars: Some clues on the tilt of the horizontal branch in metal-rich clusters
Authors:Raimondo, G.; Castellani, V.; Cassisi, S.; Brocato, E.; Piotto, G.
Date of Publication (YYYY-MM-DD):2002-04-20
Title of Journal:The Astrophysical Journal
Journal Abbrev.:Astrophys. J.
Issue / Number:2
Start Page:975
End Page:983
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:We investigate the predicted color-magnitude distribution of metal-rich horizontal-branch (HB) stars, discussing selected theoretical models computed under various assumptions about the star metallicity and the efficiency of superadiabatic convection. We find that canonical zero-age horizontal branches with metallicity larger or of the order of Z = 0.002 should all be affected by a tilt by an amount that increases when the metallicity is increased and/or the mixing length is decreased, reaching a tilt of DeltaV similar to0.2 mag in the case of solar metallicity when a mixing-length value alpha = 1.6 is assumed (DeltaV is the magnitude difference between the top of the blue HB and the fainter magnitude reached by the red HB). Uncertainties in the luminosity of the red HB due to uncertainty in the mixing-length value are discussed. We finally discuss the much larger tilt observed in the clusters NGC 6441 and NGC 6388, reporting additional evidence against suggested noncanonical evolutionary scenarios. Numerical experiments show that differential reddening could produce such sloped HBs. Furthermore, Hubble Space Telescope planetary- camera imaging of NGC 6441 gives clear indications of the occurrence of differential reddening across the cluster. However, the same imaging shows that the observed slope of the red HB is not an artifact of differential reddening. We finally show that sloping red HBs in metal-rich clusters are a common occurrence not necessarily correlated with the appearance of extended blue HBs.
Free Keywords:globular clusters : general; globular clusters : individual (NGC 6388; NGC 6441); stars : evolution; stars : horizontal- branch
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astrophysik
External Affiliations:Osservatorio Astron Collurania, Via Mentore Maggini, I-64100; Teramo, Italy; Osservatorio Astron Collurania, I-64100 Teramo, Italy; Univ Roma La Sapienza, Astron Dipartimento Fis, I-00185 Rome, Italy; Univ Pisa, Dipartimento Fis, I-56100 Pisa, Italy; Ist Nazl Fis Nucl, I-56100 Pisa, Italy; Max Planck Inst Astrophys, D-85740 Garching, Germany; Ist Nazl Fis Nucl, Lab Nazl Gran Sasso, I-67100 Laquila, Italy; Univ Padua, Dipartimento Astron, I-35122 Padua, Italy
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