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 \rightarrow 0 transitions of C ^ { 18 } O , DCO ^ { + } , N _ { 2 } H ^ { + } , and H ^ { 13 } CO ^ { + } , as well as the J = 2 \rightarrow 1 and J = 3 \rightarrow 2 transitions of CS , were obtained for each core .
Core densities are determined via a large velocity gradient analysis with values typically \sim 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 5 cores directly associated with star formation and 2 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 } ) .