The ionization fraction in dense molecular gas. II. Massive cores

We present an observational and theoretical study of the ionization fraction in several massive cores located in regions that are currently forming stellar clusters. Maps of the emission from the J = 1-->0 transitions of (CO)-O-18, DCO+, N2H+, and (HCO+)-C-13, as well as the J = 2-->1 and 3-->2 transitions of CS, were obtained for each core. Core densities are determined via a large velocity gradient analysis with values typically of similar to 10(5) cm(-3). With the use of observations to constrain variables in the chemical calculations, we derive electron fractions for our overall sample of five cores directly associated with star formation and two apparently starless cores. The electron abundances are found to lie within a small range, -6.9 < log(10) x(e) < -7.3, and are consistent with previous work. We find no difference in the amount of ionization fraction between cores with and without associated star formation activity, nor is any difference found in electron abundances between the edge and center of the emission region. Thus our models are in agreement with the standard picture of cosmic rays as the primary source of ionization for molecular ions. With the addition of previously determined electron abundances for low-mass cores, and even more massive cores associated with O and B clusters, we systematically examine the ionization fraction as a function of star formation activity. This analysis demonstrates that the most massive sources stand out as having the lowest electron abundances (x(e) < 10(-8)).