Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Non-LTE line formation for Si I-II-III in A-B stars and the origin of Si
  II emission lines in iota Her   A comprehensive model atom was developed for Si I-II-III using the most
up-to-date atomic data available so far. Based on the non-local thermodynamic
equilibrium (non-LTE = NLTE) line formation for Si I, Si II, and Si III and
high-resolution observed spectra, we determined the NLTE abundances for a
sample of nine unevolved A9 to B3-type stars with well determined atmospheric
parameters. For each star, NLTE reduces substantially the line-to-line scatter
for Si II compared with the LTE case and leads to consistent mean abundances
from lines of different ionisation stages. In the hottest star of our sample,
$\iota$ Her, Si II is subject to overionisation that drives emission in the
lines arising from the high-excitation (above 12.5 eV) doublet levels. Our NLTE
calculations reproduced 10 emission lines of Si II observed in $\iota$ Her. The
same overionisation effect leads to greatly weakened Si II lines, which are
observed in absorption in $\iota$ Her. Large positive NLTE abundance
corrections (up to 0.98 dex for 5055 A) were useful for achieving consistent
mean abundances from lines of the two ionisation stages, Si II and Si III. It
was found that the NLTE effects are overestimated for the Si II 6347, 6371 A
doublet in $\iota$ Her, while the new model atom works well for the cooler
stars. At this stage, we failed to understand this problem. We computed for a
grid of the NLTE abundance corrections for lines of Si I, Si II, and Si III in
the model atmospheres with effective temperatures and surface gravities
characteristic of unevolved A-B type stars.

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The categories of this paper are: cs
21
The categories of this paper are: adap-org
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
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The categories of this paper are: econ
24
The categories of this paper are: dg-ga
27
The categories of this paper are: comp-gas
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The categories of this paper are: math
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The categories of this paper are: nlin
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The categories of this paper are: physics
31
The categories of this paper are: ao-sci
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The categories of this paper are: gr-qc
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The categories of this paper are: q-fin
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The categories of this paper are: hep-lat