Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Tails of the Unexpected: The Interaction of an Isothermal Shell with a
  Cloud   A new mechanism for the formation of cometary tails behind dense clouds or
globules is discussed. Numerical hydrodynamical models show that when a dense
shell of swept-up matter overruns a cloud, material in the shell is focussed
behind the cloud to form a tail. This mode of tail formation is completely
distinct from other methods, which involve either the removal of material from
the cloud, or shadowing from a strong, nearby source of ionization. This
mechanism is relevant to the cometary tails seen in planetary nebulae and to
the interaction of superbubble shells with dense clouds.

19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas
13
The categories of this paper are: quant-ph
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
5
The categories of this paper are: gr-qc
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
2
The categories of this paper are: math
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th