Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: ROSAT X-ray Spectral Properties of Nearby Young Associations: TW Hydrae,
  Tucana-Horologium, and the beta Pic Moving Group   We present archival ROSAT data for three recently identified, nearby (D<70
pc), young (~10-40 Myr) stellar associations: the TW Hydrae Association, the
Tucana-Horologium Association, and the beta Pic Moving Group. The distributions
of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose
membership is dominated by low-mass, weak-lined T Tauri stars, are tightly
clustered and very similar to one another. The value of HR1 for TW Hya itself
-- the only bona fide classical T Tauri star in any of the nearby groups -- is
clearly anomalous among these nearby young stars. We compare the hardness ratio
distributions of stars in the three nearby groups with those of T Tauri stars,
the Hyades, and main sequence dwarfs in the field. This comparison demonstrates
that the X-ray spectra of F through M stars soften with age, and that F and G
stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It
is as yet unclear whether this trend can be attributed to age-dependent changes
in the intrinsic X-ray spectra of stars of type F and later, to a decrease in
the column density of circumstellar gas (e.g., in residual protoplanetary
disks), or to the diminishing contributions of star-disk interactions to X-ray
emission. Regardless, these results demonstrate that analysis of archival ROSAT
X-ray spectral data can help both to identify nearby, young associations and to
ascertain the X-ray emission properties of members of known associations.

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The categories of this paper are: hep-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: q-alg
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: stat
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The categories of this paper are: chem-ph
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The categories of this paper are: econ
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The categories of this paper are: q-bio
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The categories of this paper are: q-fin
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The categories of this paper are: physics
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The categories of this paper are: gr-qc
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The categories of this paper are: nlin
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The categories of this paper are: ao-sci
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The categories of this paper are: cs