2021A&A...653A.102S

Context. 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.
Aims. 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.
Methods. We observed the C102α (6109.901MHz) and C109α (5011.420MHz) carbon radio recombination lines (CRRLs) using the Effelsberg 100 m 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 µm-[CII] and [¹³CII] 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.
Results. We detect the CRRLs toward four positions, where their velocity (v_LSR≃11km/s) and widths (σ_v≃1km/s) confirms that they trace the envelope of the ISF. Toward two positions we detect the CRRLs, and the 158 µm-[CII] and [¹³CII] lines with a signal-to-noise ratio ≥5, and we find n_e–=0.65±0.12cm^–3 and 0.95±0.02cm^–3, which corresponds to a gas density n_H≃5x10³cm^–3 and a thermal pressure of p_th≃4x10⁵K/cm³. We also constrained the ionization fraction in the denser portions of the molecular cloud using the HCN(1-0) and C₂H(1-0) lines to x(e^–)≤3x10^–6.
Conclusions. 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). This suggests that electron collisional excitation does not play a significant role in setting the excitation of HCN(1-0) toward the region studied, as it is responsible for only ≃10% of the observed emission.