The ionization fraction in dense clouds

We present submillimeter observations of various molecular ions toward two dense clouds, NGC 2264 IRS1 and W 3 IRS5, in order to investigate their ionization fraction. Analysis of the line intensity ratios by the way of statistical equilibrium calculations allows determination of the physical parameters: n(H-2) similar to (1 - 2) 10(6) cm(-3) and T-kin similar to 50 - 100 K. Column densities and abundances are also derived. Together, the abundances of the observed ions provide a lower limit to the ionization fraction, which is (2 - 3) 10(-9) in both clouds. In order to better constrain the electron abundance, a simple chemical model is built which calculates the steady state abundances of the major positive ions, using the observed abundances wherever available. With reasonable assumptions, good agreement within a factor of two with the observations can be achieved. The calculated electron fraction is x(e) = (1.0 - 3.3) 10(-8) in the case of NGC 2264 and x(e) = (0.5 - 1.1) 10(-8) for W 3 IRS5. In the first case, the high abundance of N2H+ requires a rather high cosmic ray ionization rate > 10(-16) s(-1), even if all nitrogen is assumed to be in gas phase N-2. For W 3 IRS5, ionized metals such as Fe+ and Mg+ could provide 60% of the electrons.