Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observational Constraints on the Ages of Molecular Clouds and the
  Star-Formation Timescale: Ambipolar-Diffusion--Controlled or
  Turbulence-Induced Star Formation?   We revisit the problem of the star formation timescale and the ages of
molecular clouds. The apparent overabundance of star-forming molecular clouds
over clouds without active star formation has been thought to indicate that
molecular clouds are "short-lived" and that star formation is "rapid". We show
that this statistical argument lacks self-consistency and, even within the
rapid star-formation scenario, implies cloud lifetimes of approximately 10 Myr.
We discuss additional observational evidence from external galaxies that
indicate lifetimes of molecular clouds and a timescale of star formation of
approximately 10 Myr . These long cloud lifetimes in conjunction with the rapid
(approximately 1 Myr) decay of supersonic turbulence present severe
difficulties for the scenario of turbulence-controlled star formation. By
contrast, we show that all 31 existing observations of objects for which the
linewidth, the size, and the magnetic field strength have been reliably
measured are in excellent quantitative agreement with the predictions of the
ambipolar-diffusion theory. Within the ambipolar-diffusion-controlled star
formation theory the linewidths may be attributed to large-scale non-radial
cloud oscillations (essentially standing large-amplitude, long-wavelength
Alfven waves), and the predicted relation between the linewidth, the size, and
the magnetic field is a natural consequence of magnetic support of
self-gravitating clouds.

31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
11
The categories of this paper are: hep-lat
15
The categories of this paper are: nucl-ex
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
28
The categories of this paper are: alg-geom
27
The categories of this paper are: comp-gas