Busso, G. and Cassisi, S. and Piotto, G. and Castellani, M. and Romaniello, M. and Catelan, M. and Djorgovski, S. G. and Recio Blanco, A. and Renzini, A. (2007) The peculiar horizontal branch morphology of the Galactic globular clusters NGC 6388 and NGC 6441: new insights from UV observations. Astronomy and Astrophysics, 474 (1). pp. 105-119. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20091020-090102382
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Context. In this paper we present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441. Aims. We investigate the properties of their anomalous horizontal branches in different photometric planes in order to shed light on the nature of the physical mechanism(s) responsible for the existence of an extended blue tail and of a slope in the horizontal branch, visible in all the color-magnitude diagrams. Methods. New photometric data have been collected and carefully reduced. Empirical data have been compared with updated stellar models of low-mass, metal-rich, He-burning structures, transformed to the observational plane with appropriate model atmospheres. Results. We have obtained the first UV color-magnitude diagrams for NGC 6388 and NGC 6441. These diagrams confirm previous results, obtained in optical bands, about the presence of a sizeable stellar population of extremely hot horizontal branch stars. At least in NGC 6388, we find a clear indication that at the hot end of the horizontal branch the distribution of stars forms a hook-like feature, closely resembling those observed in NGC 2808 and Omega Cen. We briefly review the theoretical scenarios that have been suggested for interpreting this observational feature. We also investigate the tilted horizontal branch morphology and provide further evidence that supports early suggestions that this feature cannot be interpreted as an effect of differential reddening. We show that a possible solution of the puzzle is to assume that a small fraction - ranging between 10-20% - of the stellar population in the two clusters is strongly helium-enriched (Y ~ 0.40 in NGC 6388 and Y ~ 0.35 in NGC 6441). The occurrence of a spread in the He abundance between the canonical value (Y ~ 0.26) and the quoted upper limits can significantly help in explaining the "whole" morphology of the horizontal branch and the pulsational properties of the variable stars in the target clusters.
|Additional Information:||© ESO 2007. Received 7 May 2007. Accepted 7 August 2007. We warmly thanks our anonymous referee for her/his pertinent comments and useful suggestions. This work has been partially supported by INAF and MUR. G.B. gratefully acknowledges support from the Deutsche Forschungsgemeinschaft through grant Mo 602/8 and partial support from the ESO Director General Discretionary Fund. S.C. warmly acknowledge the Instituto Astrofisico de Canarias (Tenerife) for the hospitality he experienced during his stay at IAC. M.C. acknowledges support by Proyecto FONDECYT Regular No. 1071002. We warmly thank W. B. Landsman and S. Moehler for detailed readings of the manuscript and for useful comments.|
|Subject Keywords:||stars : horizontal-branch; ultraviolet : stars; globular clusters : individual : NGC 6388 globular clusters : individual : NGC 6441|
|Official Citation:||The peculiar horizontal branch morphology of the Galactic globular clusters NGC 6388 and NGC 6441: new insights from UV observations G. Busso, S. Cassisi, G. Piotto, M. Castellani, M. Romaniello, M. Catelan, S. G. Djorgovski, A. Recio Blanco, A. Renzini, M. R. Rich, A. V. Sweigart and M. Zoccali A&A 474 (1) 105-119 (2007) DOI: 10.1051/0004-6361:20077806|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||26 Oct 2009 17:47|
|Last Modified:||26 Dec 2012 11:29|
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