1998A&A...334..678R

We show that the fractional ionization in a wide range of models of cloud chemistry undergoes a steep decline at extinctions of A_V≃2-3. We identify this extinction with the critical value for clumps in the Rosette Molecular Cloud: clumps with A_V above ∼3 may contain embedded stars, while those with extinction below this value do not. We argue that the ionization decline at this critical extinction is directly related to the extent of turbulent support in the cloud. This leads us to investigate for the first time the chemical evolution of a cloud that is initially magnetically supported against collapse perpendicular to the field lines, but is collapsing along the field lines in a 1-D mode, up to an unknown but critical density. The fractional ionization corresponding to this critical density then allows ambipolar diffusion to control a collapse across the large-scale magnetic field as well as parallel to it. The results of the chemical evolution of a cloud undergoing this two-stage collapse are presented, and are shown to be sensitive tracers of the details of the collapse mode.