Abstract
We present the results of a numerical study designed to address the question
of whether there is a column density threshold for star formation within
molecular clouds. We have simulated a large number of different clouds, with
volume and column densities spanning a wide range of different values, using a
state-of-the-art model for the coupled chemical, thermal and dynamical
evolution of the gas.
We show that for low-mass clouds, around 1000 solar masses and below, star
formation is only possible if the mean cloud column density exceeds 10^21
cm^-2. In more massive clouds, the required mean column density is a factor of
a few lower. We demonstrate that this behaviour is well-described by a simple
Jeans mass argument: clouds must contain multiple Jeans masses in order to form
stars, and hence star-forming clouds cannot have arbitrarily low column
densities.
We have also examined the question of whether there is a column density
threshold for the regions within clouds where star formation occurs. We show
that there is a good correlation between the mass of gas above a K-band
extinction A_K = 0.8 and the star formation rate (SFR), in agreement with
recent observational work. Previously, this relationship has been explained in
terms of a correlation between the SFR and the mass in dense gas. However, we
find that in our simulations, this correlation is weaker and more
time-dependent than that between the SFR and the column density. We argue that
this points to dust shielding as the key process: the true correlation is one
between the SFR and the mass in cold, well-shielded gas, and the latter
correlates better with the column density than the volume density.
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