Issue |
A&A
Volume 446, Number 2, February I 2006
|
|
---|---|---|
Page(s) | 569 - 577 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20042170 | |
Published online | 13 January 2006 |
On the horizontal branch of the galactic globular cluster NGC 2808
1
INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: vittorio@mporzio.astro.it
2
INFN-Sezione di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
3
Universidad Catolica de Chile, Department of Astronomy & Astrophysics, Casilla 306, Santiago 22, Chile
4
INAF-Osservatorio Astronomico di Teramo, via M. Maggini, 64100 Teramo, Italy
5
Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
Received:
13
October
2004
Accepted:
12
May
2005
We present new UV () data for stars in the central region of the
Galactic globular cluster NGC 2808, collected with the WFPC2 camera on
board the Hubble Space Telescope. These data together with
and
-band data and previous ground-based observations provide a
multifrequency coverage of the cluster stellar population extending up
to a distance of 1.7 times the cluster core radius. We discuss this
complete sample of stars, which includes 764 Red Giant Branch (RGB)
stars brighter than the Horizontal Branch (HB) luminosity level,
1239 HB stars, 119 Asymptotic Giant Branch (AGB), and 22 AGB-manqué
stellar structures. As already known, we find that blue HB stars
separate into three distinct groups. However, our multiband photometry
indicates that several stars in the two hotter HB groups show a flat
spectrum, thus suggesting the binarity of these objects. Artificial
star experiments suggest that at most 50% of them might be
photometric blends. Moreover, at variance with previous claims one
finds that canonical Zero Age Horizontal Branch (ZAHB) models do reach
effective temperatures typical of observed hot HB stars. We also show
that the ratio between HB and RGB stars brighter than the HB
luminosity level steadly increases when moving from the cluster center
to the periphery, passing from
in the cluster core to
in the outer cluster regions. We discuss the possible
origin of such a radial gradient in the context of the Blue Tails
phenomenon, advancing some suggestions concerning the clumpy
stellar distribution along the HB.
Key words: globular clusters: individual: NGC 2808 / stars: evolution / stars: horizontal-branch / global clusters: general
© ESO, 2006
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