The ionization fraction in OMC-2 and OMC-3

¹ Green Bank Observatory, 155 Observatory Road, Green Bank, WV 24915, USA
e-mail: psalas@nrao.edu
² Max-Planck-Institut fur Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁴ Haystack Observatory, Massachusetts Institute of Technology, Westford, MA 01886, USA
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 26 February 2021
Accepted: 24 June 2021

Abstract

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 (6109.901 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 (v_LSR ≈ 11 km s⁻¹) and widths (σ_v ≈ 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 n_e⁻ = 0.65 ± 0.12 cm⁻³ and 0.95 ± 0.02 cm⁻³, which corresponds to a gas density n_H ≈ 5 × 10³ cm⁻³ and a thermal pressure of p_th ≈ 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.