... ISM

Appendices A and B are only available in electronic form at http://www.edpsciences.org

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... from

We changed the factor for the case $\sigma_{0}>1$ from $\ensuremath{-3.75 \times 10^{4}}$ to $\ensuremath{-2.75 \times 10^{4}}$ in order to avoid a discontinuity at $\sigma_{0}=1$.

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... instantaneously

We neglect the short time for the SF process itself. This is motivated by recent observations suggesting that SF occurs on the local crossing time for sound waves in a cloud, a time scale that is shorter than a few  $10~\ensuremath{{\rm Myr}}$ (Elmegreen 2000).

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