Electron abundance in dense cloud cores Implications for star formation

Abstract

By combining observations of the J=1→0 transitions of C¹⁸O and H¹³CO⁺, and the J=1→0 and 2→1 transitions of DCO⁺, and a model of molecular cloud chemistry, we have obtained the electron abundance (X_e≡n_e/n_H2) in a sample of 20 low-mass and 7 high-mass molecular cloud cores. We find that the electron abundances are confined to a relatively narrow range of -7.5<log(X_e)<-6.5 with very little scatter about the mean values of 〈log(X_e)〉=-7.04; σ=0.22 (low-mass cores) and 〈log(X_e)〉=-7.11; σ=0.15 (high-mass cores). These values are consistent with the standard view that the ionization in dense cloud cores is dominated by cosmic rays, provided that ζ_H2≈5×10^-17 s^-1. These electron abundances also imply that the neutrals are only marginally coupled to the magnetic field (W≈5–8) with only ca. 10% the core radius being cut off from magneto-hydrodynamic (MHD) wave propagation. The coupling parameter (W) also suggests that ambipolar diffusion timescales are about an order of magnitude larger than the freefall timescale.