The ionization fraction in dense molecular gas. I. Low-mass cores

Observations of (CO)-O-18, (HCO+)-C-13, and DCO+ toward 23 low-mass cores a re used to constrain the fractional ionization (electron abundance) within them. Chemical models have been run over a wide range of densities, cosmic- ray ionization rates, and elemental depletions, and we find that we can fit 20 of the 23 cores for densities of n(H2) = (1-3) x 10(4) cm(-3), moderate C and O abundance variations, and a cosmic-ray ionization rate of zeta(H2) = 5 x 10(-17) s(-1). The derived ionization fractions lie within the range 10(-7.5) to 10(-6.5), with a median value of x(e,m) = 9 x 10(-8) and typic al errors for each individual core equal to a factor of 3. These Values imp ly that the cores are weakly coupled to the magnetic held and that MHD wave s can propagate within them. The ambipolar diffusion timescale is about an order of magnitude greater than the free-fall time, and the cores can be co nsidered to be in quasi-static equilibrium. There is no significant differe nce between the ionization fraction for cores with and without embedded sta rs, which suggests that the molecular ionization in cores is primarily gove rned by cosmic rays alone.