1996A&A...310..315D

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(H₂)~(1-2)x10⁶cm^–3 and T_kin∼50-100K. 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)x10^–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)x10^–8 in the case of NGC 2264 and x_e=(0.5-1.1)x10^–8 for W 3 IRS5. In the first case, the high abundance of N₂H⁺ requires a rather high cosmic ray ionization rate >10^–16s^–1, even if all nitrogen is assumed to be in gas phase N₂. For W 3 IRS5, ionized metals such as Fe⁺ and Mg⁺ could provide 60% of the electrons.