Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: VLT / Infrared Integral Field Spectrometer Observations of Molecular
  Hydrogen Lines in the Knots in the Planetary Nebula NGC 7293 (the Helix
  Nebula)   Knots are commonly found in nearby planetary nebulae (PNe) and star forming
regions. Within PNe, knots are often found to be associated with the brightest
parts of the nebulae and understanding the physics involved in knots may reveal
the processes dominating in PNe. As one of the closest PNe, the Helix Nebula
(NGC 7293) is an ideal target to study such small-scale (~300 AU) structures.
We have obtained infrared integral spectroscopy of a comet-shaped knot in the
Helix Nebula using SINFONI on the Very Large Telescope at high spatial
resolution (50-125 mas). With spatially resolved 2 micron spectra, we find that
the H2 rotational temperature within the cometary knots is uniform. The
rotational-vibrational temperature of the cometary knot (situated in the
innermost region of the nebula, 2.5 arcmin away from the central star), is 1800
K, higher than the temperature seen in the outer regions (5-6 arcmin from the
central star) of the nebula (900 K), showing that the excitation temperature
varies across the nebula. The obtained intensities are reasonably well fitted
with 27 km s-1 C-type shock model. This ambient gas velocity is slightly higher
than the observed [HeII] wind velocity of 13 km s-1. The gas excitation can
also be reproduced with a PDR (photo dominant region) model, but this requires
an order of magnitude higher UV radiation. Both models have limitations,
highlighting the need for models that treats both hydrodynamical physics and
the PDR.

20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
22
The categories of this paper are: chao-dyn
18
The categories of this paper are: econ