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