Journal Article

The peculiar horizontal branch of NGC 2808

E. Dalessandro, M. Salaris, F. R. Ferraro, S. Cassisi, B. Lanzoni, R. T. Rood, F. Fusi Pecci and E. Sabbi

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 1, pages 694-704
Published in print January 2011 | ISSN: 0035-8711
Published online December 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17479.x
The peculiar horizontal branch of NGC 2808

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We present an accurate analysis of the peculiar horizontal branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with the Hubble Space Telescope. We confirm the multimodal distribution of stars along the HB: four sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modelling is possible for three HB groups by assuming different helium abundances in the range 0.24 < ΔY < 0.4 that are consistent with the multiple populations observed in the main sequence; and (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so-called blue hook region. These objects are probably ‘hot flashers’, stars that peel off the red giant branch before reaching the tip and ignite helium at high Teff. Both of these conclusions are based on the luminosity of the HB in the optical and ultraviolet bands and do not depend on specific assumptions about mass loss.

Keywords: stars: evolution; stars: horizontal branch; globular clusters: individual: NGC 2808; ultraviolet: stars

Journal Article.  8796 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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