THE IONIZATION FRACTION IN DENSE CLOUD CORES

The degree of ionization, x(e)= n(e)/n(H-2), and the cosmic-ray ioniza tion rate, zeta, in 24 cloud cores have been determined by comparing o bservational data from Butner et al. on the abundance ratios R-D = [DC O+]/[HCO+] and R-H = [HCO+]/[CO] with a simple analytical chemical mod el and with a detailed ''pseudo-time-dependent'' chemical model. The r esults are dependent on the depletion of elemental carbon and oxygen f rom their cosmic abundances, especially for cores with a low degree of ionization. We determine the depletion of C and O from the measured H C3N/CO abundance ratios using model results. We find that the range of fractional ionization In the dark cores extends from similar to 10(-6 ) to similar to 10(-8), with inferred cosmic ray ionization rates in t he range of 10(-16)-10(-18) s(-1). This corresponds to ambipolar diffu sion timescales of between 3 x 10(7) and 3 x 10(5) yr, with a median v alue of 5 x 10(6) yr. The ratio of ambipolar diffusion to the free-fal l timescales varies between 3 and 200, with a median value of 50. We f ind, rather surprisingly, no clear segregation in the ambipolar diffus ion timescales between cores with embedded stars and those without. An interesting by-product of our results is the conclusion that the cyan opolyyne-rich core in TMC-1 is atypical in its abundance distribution and may be unusually young.