  We present a catalogue of molecular cloud cores drawn from high latitude,
medium opacity clouds, using the all-sky IRAS Sky Survey Atlas (ISSA) images at
60 and 100~$\mu$m. The typical column densities of the cores are $ N(H_2)\sim
3.8 \times 10^{21} $cm$^{-2}$ and the typical volume densities are $ n(H_2)
\sim 2 \times 10^3$cm$^{-3}$. They are therefore significantly less dense than
many other samples obtained in other ways. Those cloud cores with IRAS point
sources are seen to be already forming stars, but this is found to be only a
small fraction of the total number of cores. The fraction of the cores in the
protostellar stage is used to estimate the prestellar timescale - the time
until the formation of a hydrostatically supported protostellar object. We
argue, on the basis of a comparison with other samples, that a trend exists for
the prestellar lifetime of a cloud core to decrease with the mean column
density and number density of the core. We compare this with model predictions
and show that the data are consistent with star formation regulated by the
ionisation fraction.

Astrophysics