Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The High Chromospheres of the Late A Stars   We report the detection of N V 1239 A transition region emission in HST/GHRS
spectra of the A7 V stars, Alpha Aql and Alpha Cep. Our observations provide
the first direct evidence of 1-3 x 10^5 K material in the atmospheres of normal
A-type stars. For both stars, and for the mid-A--type star Tau3 Eri, we also
report the detection of chromospheric emission in the Si III 1206 A line. At a
B-V color of 0.16 and an effective temperature of 8200 K, Tau3 Eri becomes the
hottest main sequence star known to have a chromosphere and thus an outer
convection zone. We see no firm evidence that the Si III line surface fluxes of
the A stars are any lower than those of moderately active, solar-type, G and K
stars. This contrasts sharply with their coronal X-ray emission, which is >100
times weaker than that of the later-type stars. Given the strength of the N V
emission observed here, it now appears unlikely that the X-ray faintness of the
A stars is due to their forming very cool, <= 1 MK coronae. An alternative
explanation in terms of mass loss in coronal winds remains a possibility,
though we conclude from moderate resolution spectra of the Si III lines that
such winds, if they exist, do not penetrate into the chromospheric Si
III--forming layers of the star, since the profiles of these lines are *not*
blueshifted, and may well be redshifted with respect to the star.

31
The categories of this paper are: ao-sci
9
The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
16
The categories of this paper are: stat
20
The categories of this paper are: acc-phys
15
The categories of this paper are: nucl-ex
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The categories of this paper are: q-alg
8
The categories of this paper are: hep-ex
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The categories of this paper are: patt-sol
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The categories of this paper are: comp-gas
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The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: cmp-lg
7
The categories of this paper are: hep-th
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The categories of this paper are: adap-org
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The categories of this paper are: eess