The Ionization Fraction in Dense Molecular Gas. I. Low‐Mass Cores

Observations of C18O, H13CO+, and DCO+ toward 23 low-mass cores are 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 nH2=(1-3)×104 cm-3, moderate C and O abundance variations, and a cosmic-ray ionization rate of ζH2=5×10−17 s-1. The derived ionization fractions lie within the range 10-7.5 to 10-6.5, with a median value of xe,m = 9 × 10-8 and typical errors for each individual core equal to a factor of 3. These values imply that the cores are weakly coupled to the magnetic field and that MHD waves 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 considered to be in quasi-static equilibrium. There is no significant difference between the ionization fraction for cores with and without embedded stars, which suggests that the molecular ionization in cores is primarily governed by cosmic rays alone.