Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the Horizontal Branch of the Galactic Globular NGC2808   We present new UV (F218W) data for stars in the central region of the
Galactic globular cluster NGC2808, collected with the WFPC2 camera on board of
the Hubble Space Telescope. These data together with F439W and F555W-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-manque'
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
R=1.37 +/- 0.14 in the cluster core to R=1.95 +/- 0.26 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.

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The categories of this paper are: patt-sol
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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The categories of this paper are: adap-org
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The categories of this paper are: math
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-th
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The categories of this paper are: funct-an
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The categories of this paper are: eess
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The categories of this paper are: nucl-th
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-lat
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The categories of this paper are: gr-qc
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The categories of this paper are: bayes-an
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The categories of this paper are: acc-phys