The ionization fraction in OMC-2 and OMC-3

Abstract

Context. The electron density (ne−) plays an important role in setting the chemistry and physics of the interstellar medium. However, measurements of ne− 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 ne− and the gas temperature in the envelope of the integral shaped filament (ISF) in the Orion A molecular cloud. Methods. We observed the C102α (6109.901 MHz) and C109α (5011.420 MHz) 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 (vLSR ≈ 11 km s−1) and widths (σv ≈ 1 km s−1) 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 ne− = 0.65 ± 0.12 cm−3 and 0.95 ± 0.02 cm−3, which corresponds to a gas density nH ≈ 5 × 103 cm−3 and a thermal pressure of pth ≈ 4 × 105 K cm−3. 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−) ≤ 3 × 10−6. 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.