Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Birth of Molecular Clouds: Formation of Atomic Precursors in
  Colliding Flows   Molecular Cloud Complexes (MCCs) are highly structured and ``turbulent''.
Observational evidence suggests that MCCs are dynamically dominated systems,
rather than quasi-equilibrium entities. The observed structure is more likely a
consequence of the formation process rather than something that is imprinted
after the formation of the MCC. Converging flows provide a natural mechanism to
generate MCC structure. We present a detailed numerical analysis of this
scenario. Our study addresses the evolution of a MCC from its birth in
colliding atomic hydrogen flows up until the point when H$_2$ may begin to
form. A combination of dynamical and thermal instabilities breaks up coherent
flows efficiently, seeding the small-scale non-linear density perturbations
necessary for local gravitational collapse and thus allowing (close to)
instantaneous star formation. Many observed properties of MCCs come as a
natural consequence of this formation scenario. Since converging flows are
omnipresent in the ISM, we discuss the general applicability of this mechanism,
from local star formation regions to galaxy mergers.

37
The categories of this paper are: supr-con
3
The categories of this paper are: physics
13
The categories of this paper are: quant-ph
10
The categories of this paper are: q-bio
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
2
The categories of this paper are: math
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
34
The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
29
The categories of this paper are: funct-an