Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The co-existence of hot and cold gas in debris discs   Debris discs have often been described as gas-poor discs as the gas-to-dust
ratio is expected to be considerably lower than in primordial,protoplanetary
discs. However, recent observations have confirmed the presence of a
non-negligible amount of cold gas in the circumstellar (CS) debris discs around
young main-sequence stars.This cold gas has been suggested to be related to the
outgassing of planetesimals and cometary-like objects. The aim of the paper is
to investigate the presence of hot gas in the surroundings of stars bearing
cold-gas debris discs. High-resolution optical spectra of all currently known
cold-gas-bearing debris-disc systems, with the exception of $\beta$ Pic and
Fomalhaut, have been obtained from different observatories.We have analysed the
Ca II H & K and the Na I D lines searching for non-photospheric absorptions of
CS origin, usually attributed to cometary-like activity. Narrow, stable Ca II
and/or Na I absorption features have been detected superimposed to the
photospheric lines in 10 out of the 15 observed cold-gas-bearing debris
disc.Features are found at the radial velocity of the stars, or slightly blue-
or red-shifted, and/or at the velocity of the local interstellar medium (ISM).
Some stars also present transient variable events or absorptions extended
towards red wavelengths. These are the first detections of such Ca II features
in 7 out of the 15 observed stars. In some of these stars, results suggest that
the stable and variable absorptions arise from relatively hot gas located in
the CS close-in environment. This hot gas is detected in at least ~80%, of
edge-on cold-gas-bearing debris discs, while in only ~10% of the discs seen
close to face-on. We interpret this as a geometrical effect, and suggest that
the non-detection of hot gas absorptions is due to the disc inclination rather
than to the absence of the hot-gas component.

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The categories of this paper are: dg-ga
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The categories of this paper are: cmp-lg
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The categories of this paper are: supr-con
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The categories of this paper are: gr-qc
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The categories of this paper are: patt-sol
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The categories of this paper are: cs
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The categories of this paper are: cond-mat
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: chem-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-ph
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The categories of this paper are: funct-an
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The categories of this paper are: math-ph
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The categories of this paper are: mtrl-th