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A New Spin for Understanding the Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441In 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 (HB) in different photometric planes in order to shed light on the nature of the physical mechanism(s) responsible for the existence of an extended HB blue tail, and of a slope in the HB, 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 atmosphere models. 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 w Centauri. We briefly review the theoretical scenarios which have been suggested for interpreting this observational feature. We investigate also on the tilt in the horizontal branch morphology, and provide further evidence that supports early suggestions according to which this feature cannot be interpreted as an effect of differential reddening or radiative levitation, though these effects contribute to create the anomaly. We demonstrate that a possible solution of the puzzle is to assume that a small fraction (approx. 13% in NGC 6388 and approx. 8% NGC 6441) of the stellar population in the two clusters is strongly helium enriched (Y approx. 0.40 in NGC6388 and Y approx. 0.35 in NGC 6441). This solution necessarily implies the presence of a double generation of stars in the two clusters.
Document ID
20080045471
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Busso, G.
(Kiel Univ. Germany)
Piotto, G.
(Padua Univ. Italy)
Cassisi, S.
(Osservatorio Astronomico, Collurania Teramo, Italy)
Romaniello, M.
(European Southern Observatory Garching, Germany)
Castelli, F.
(Osservatorio Astronomico Trieste, Italy)
Catelan, M.
(Pontificia Univ. Catolica de Chile Santiago, Chile)
Djorgovski, S. G.
(California Inst. of Tech. Pasadena, CA, United States)
King, I. R.
(Washington Univ. Seattle, WA, United States)
Landsman, W. B.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Blanco, A. Reico
(Observatoire de la Cote d'Azur Nice, France)
Renzini, A.
(Osservatorio Astronomico Padova, Italy)
Rich, M. R.
(California Univ. Los Angeles, CA, United States)
Sweigart, A.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Zoccali, M.
(Pontificia Univ. Catolica de Chile Santiago, Chile)
Date Acquired
August 24, 2013
Publication Date
July 12, 2006
Subject Category
Astronomy
Distribution Limits
Public
Copyright
Other

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