Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The emission nebula associated with V1974 Cyg: a unique object?   Through a program of narrow band imaging, we have observed the changing
structure of the H alpha emission line around Nova Cyg 1992 (V1974 Cyg) at
regular intervals from 1996 to 1999. Between 1994 and 1996, the nebular
boundary advanced to the southwest at nearly the speed of light, implying that
the nebula was created by an expanding wave of radiation originating in the
explosion interacting with surrounding material. The expansion speed dropped to
0.35c during 1996-1999. We have taken spectra of the nebula in 1998 and 1999.
Only Balmer lines are detected, no He I, [OIII], [OII], [NII], or [SII]. There
is also no trace of the high excitation nova lines (He II, NeV, etc). These
spectra show that the nebula is NOT a reflection nebula, a conventional HII
region, or a shock involving motions of the gas. The integrated H alpha
luminosity of the nebula between 1996 and 1999 is in the range =~1.3-2.2 x
10(35) erg s(-1). The density is poorly determined, but is probably very large
(10(4) cm(-3)) in order to explain the brightest region of the nebula. The
dynamical timescale is about a year and the recombination timescale of the same
order. Bright patches are observed to fade in these times. The energy required
to ionize the nebula is the bolometric luminosity of the nova for 30 days,
smaller than the time during which the temperature of the nova photosphere was
in the right range to produce the ionizing photons. We have also undertaken
sensitive surveys of H alpha nebulae around recent novae but find no evidence
of other such nebulae, so this type of object must be rare.

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The categories of this paper are: chem-ph
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The categories of this paper are: math-ph
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The categories of this paper are: adap-org
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The categories of this paper are: nucl-ex
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The categories of this paper are: nlin
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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: dg-ga
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ph
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The categories of this paper are: physics
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The categories of this paper are: q-alg
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an