Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Is molecular gas necessary for star formation?   On galactic scales, the surface density of star formation appears to be well
correlated with the surface density of molecular gas. This has lead many
authors to suggest that there exists a causal relationship between the chemical
state of the gas and its ability to form stars -- in other words, the
assumption that the gas must be molecular before star formation can occur. We
test this hypothesis by modelling star formation within a dense cloud of gas
with properties similar to a small molecular cloud using a series of different
models of the chemistry, ranging from one in which the formation of molecules
is not followed and the gas is assumed to remain atomic throughout, to one that
tracks the formation of both H2 and CO. We find that presence of molecules in
the gas has little effect on the ability of the gas to form stars: star
formation can occur just as easily in atomic gas as in molecular gas. At low
densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure
emission almost as efficiently as via CO rotational line emission, while at
higher densities, the main cooling process involves the transfer of energy from
gas to dust, meaning that the presence of molecules is again unimportant.
Cooling by H2 is particularly inefficient, accounting for as little as 1
percent of the overall cooling in the cloud. Rather than the chemical makeup,
we find that the most important factor controlling the rate of star formation
is the ability of the gas to shield itself from the interstellar radiation
field. As this is also a prerequisite for the survival of molecules within the
gas, our results support a picture in which molecule formation and the
formation of cold gas are both correlated with the column density of the cloud
-- and thus its ability to shield itself -- rather than being directly
correlated with each other.

16
The categories of this paper are: stat
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
2
The categories of this paper are: math
7
The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg
1
The categories of this paper are: cs
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio