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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 C102 α (6109.901 MHz) and C109 α (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 [ 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. 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 [ 13 CII] 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 × 10 3 cm −3 and a thermal pressure of p th  ≈ 4 × 10 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 × 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. 

12CO and 13CO emission maps of the 30 Doradus molecular cloud in the Large Magellanic Cloud, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 7. See the associated article in the Astrophysical Journal, and README file, for details. Please cite the article if you use these data. 

We present results of a wide-field (approximately 60 × 90 pc) Atacama Large Millimeter/submillimeter Array mosaic of CO(2–1) and¹³CO(2–1) emission from the molecular cloud associated with the 30 Doradus star-forming region in the Large Magellanic Cloud (LMC). Three main emission complexes, including two forming a bow-tie-shaped structure extending northeast and southwest from the central R136 cluster, are resolved into complex filamentary networks. Consistent with previous studies, we find that the central region of the cloud has higher line widths at a fixed size relative to the rest of the molecular cloud and to other LMC clouds, indicating an enhanced level of turbulent motions. However, there is no clear trend in gravitational boundedness (as measured by the virial parameter) with distance from R136. Structures observed in¹³CO are spatially coincident with filaments and are close to a state of virial equilibrium. In contrast,¹²CO structures vary greatly in virialization, with low CO surface brightness structures outside of the main filamentary network being predominantly unbound. The low surface brightness structures constitute ∼10% of the measured CO luminosity; they may be shredded remnants of previously star-forming gas clumps, or alternatively the CO-emitting parts of more massive, CO-dark structures.