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

We present an observational and theoretical study of the ionization fractio n in several massive cores located in regions that are currently forming st ellar 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 tran sitions of CS, were obtained for each core. Core densities are determined v ia a large velocity gradient analysis with values typically of similar to 1 0(5) cm(-3). With the use of observations to constrain variables in the che mical calculations, we derive electron fractions for our overall sample of five cores directly associated with star formation and two apparently starl ess 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 n o difference in the amount of ionization fraction between cores with and wi thout associated star formation activity, nor is any difference found in el ectron 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 prev iously determined electron abundances for low-mass cores, and even more mas sive 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)).