  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)e6 cm-3 and T(kin)~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)e-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)e-8 in the case of NGC 2264 and x(e)=(0.5-1.1)e-8 for W 3 IRS5. In the
first case, the high abundance of N2H+ requires a rather high cosmic ray
ionization rate >1e-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.
