The ionization fraction in OMC-2 and OMC-3

The electron density ($n_{e^{-}}$) plays an important role in setting the chemistry and physics of interstellar medium. However, measurements $n_{e^{-}}$ neutral clouds have been directly obtained only toward a few lines sight or they rely on indirect determinations. We use carbon radio recombination far-infrared C$^{+}$ to measure gas temperature envelope integral shaped filament (ISF) Orion A molecular cloud. observed C$102\alpha$ C$109\alpha$ (CRRLs) using Effelsberg 100m telescope at ~2' resolution five positions OMC-2 OMC-3. Since CRRLs similar line properties, we averaged them increase signal-to-noise ratio spectra. compared intensities CRRLs, 158 {\mu}m-[CII] [$^{13}$CII] predictions homogeneous model for C$^{+}$/C interface cloud from this comparison determined density, column gas. detect four positions, where their velocity widths (FWHM 2.3 km s$^{-1}$) confirms that trace ISF. Toward two [CII] with >5, find $n_{e^{-}}=0.65\pm0.12$ cm$^{-3}$ $0.95\pm0.02$ cm$^{-3}$, which corresponds $n_{H}\approx5\times10^{3}$ thermal pressure $p_{th}\approx4\times10^{5}$ K cm$^{-3}$. also constrained ionization fraction denser portions HCN(1-0) C$_{2}$H(1-0) $x(e^{-})<3\times10^{-6}$. derived densities imply $x(e^{-})$ drops by factor >100 between layer regions probed HCN(1-0).