Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the Structure of the Orion A Cloud and the Formation of the Orion
  Nebula Cluster   We suggest that the Orion A cloud is gravitationally collapsing on large
scales, and is producing the Orion Nebula Cluster due to the focusing effects
of gravity acting within a finite cloud geometry. In support of this
suggestion, we show how an elliptical rotating sheet of gas with a modest
density gradient along the major axis can collapse to produce a structure
qualitatively resembling Orion A, with a fan-shaped structure at one end,
ridges or filaments along the fan, and a narrow curved filament at the other
end reminiscent of the famous integral-shaped filament. The model produces a
local concentration of mass within the narrow filament which in principle could
form a dense cluster of stars like that of the Orion Nebula. We suggest that
global gravitational contraction might be a more common feature of molecular
clouds than previously recognized, and that the formation of star clusters is a
dynamic process resulting from the focusing effects of gravity acting upon the
geometry of finite clouds.

11
The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
16
The categories of this paper are: stat