Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Phase-resolved HST/STIS spectroscopy of the exposed white dwarf in the
  high-field polar AR UMa   Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR
UMa confirms that the WD photospheric Ly alpha Zeeman features are formed in a
magnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+
components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes
``enabled'' in the presence of both strong magnetic and electric fields. Our
attempt in fitting the overall optical+UV low state spectrum with single
temperature magnetic WD models remains rather unsatisfactory, indicating either
a shortcoming in the present models or a new physical process acting in AR UMa.
As a result, our estimate of the WD temperature remains somewhat uncertain,
Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A and
present throughout the entire binary orbit, and a second bump near ~1650A,
which appears only near the inferior conjunction of the secondary star. These
are suggestive of low harmonic cyclotron emission produced by low-level
(M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is no
evidence in the power spectrum of light variations for accretion in gas blobs.
The observed Ly alpha emission line shows a strong phase dependence with
maximum flux and redshift near orbital phase phi~0.3, strongly indicating an
origin on the trailing hemisphere of the secondary star. An additional Ly alpha
absorption feature with similar phasing as the Ly alpha emission, but a
\~700km/s blueshift could tentatively be ascribed to absorption of WD emission
in a moderately fast wind. We derive a column density of neutral hydrogen of
NH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.

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The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
11
The categories of this paper are: hep-lat
7
The categories of this paper are: hep-th
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The categories of this paper are: plasm-ph
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
19
The categories of this paper are: eess
12
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: atom-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: dg-ga
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The categories of this paper are: q-alg
0
The categories of this paper are: hep-ph