Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Measuring helium abundance difference in giants of NGC 2808   Multiple populations have been detected in several globular clusters (GC)
that do not display a spread in metallicity. Unusual features of their CMD can
be interpreted in terms of differences in the Helium content of the stars
belonging to the sub-populations. Differences in He abundance have never been
directly observed. We attempt to measure these differences in two giant stars
of NGC 2808 with very similar parameters but different Na and O abundances,
hence that presumably belong to different sub-populations, by directly
comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the
upper chromosphere. Our detailed models derive the chromospheric structure
using the Ca II and H$\alpha$, and simulate the corresponding He I 10830 line
profiles. We show that, at a given value of He abundance, the He I 10830
equivalent width cannot significantly change without a corresponding much
larger change in the Ca II lines. We have used the VLT-CRIRES to obtain
high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain
spectra of the Ca II and H$\alpha$ lines of our target stars. The two target
stars have very similar Ca II and H$\alpha$ lines, but different appearances in
the He region. One line, blueshifted by 17 km s$^{-1}$ with respect to the He
10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas
the Na-poor star spectrum is consistent with a non-detection. The difference in
the spectra is consistent and most closely explained by an He abundance
difference between the two stars of $\Delta Y \ge$ 0.17.We provide direct
evidence of a significant He line strength difference in giant stars of NGC
2808 belonging to different sub-populations, which had been previously detected
by other photometric and spectroscopic means.

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The categories of this paper are: alg-geom
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-lat
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-alg
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The categories of this paper are: atom-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: funct-an
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The categories of this paper are: hep-th
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The categories of this paper are: chem-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: cmp-lg
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The categories of this paper are: acc-phys
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The categories of this paper are: eess
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-th