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1. The ionization fraction in OMC-2 and OMC-3

   https://doi.org/10.1051/0004-6361/202140670  

   Salas, P. ; Rugel, M. R. ; Emig, K. L. ; Kauffmann, J. ; Menten, K. M. ; Wyrowski, F. ; Tielens, A. G. ( September 2021 , Astronomy & Astrophysics)

   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. 

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2. An imaging line survey of OMC-1 to OMC-3: Averaged spectra of template regions

   https://doi.org/10.1051/0004-6361/201936885  

   Brinkmann, N. ; Wyrowski, F. ; Kauffmann, J. ; Colombo, D. ; Menten, K. M. ; Tang, X. D. ; Güsten, R. ( April 2020 , Astronomy & Astrophysics)

   Context.Recently, sensitive wide-bandwidth receivers in the millimetre regime have enabled us to combine large spatial and spectral coverage for observations of molecular clouds. The resulting capability to map the distributions of lines from many molecules simultaneously yields unbiased coverage of the various environments within star-forming regions.

   Aims.Our aim is to identify the dominant molecular cooling lines and characteristic emission features in the 1.3 mm window of distinct regions in the northern part of the Orion A molecular cloud. By defining and analysing template regions, we also intend to help with the interpretation of observations from more distant sources which cannot be easily spatially resolved.

   Methods.We analyse an imaging line survey covering the area of OMC-1 to OMC-3 from 200.2 to 281.8 GHz obtained with the PI230 receiver at the APEX telescope. Masks are used to define regions with distinct properties (e.g. column density or temperature ranges) from which we obtain averaged spectra. Lines of 29 molecular species (55 isotopologues) are fitted for each region to obtain the respective total intensity.

   Results.We find that strong sources like Orion KL have a clear impact on the emission on larger scales. Although not spatially extended, their line emission contributes substantially to spectra averaged over large regions. Conversely, the emission signatures of dense, cold regions like OMC-2 and OMC-3 (e.g. enhanced N₂H⁺emission and low HCN/HNC ratio) seem to be difficult to pick up on larger scales, where they are eclipsed by signatures of stronger sources. In all regions, HCO⁺appears to contribute between 3 and 6% to the total intensity, the most stable value for all bright species. N₂H⁺shows the strongest correlation with column density, but not with typical high-density tracers like HCN, HCO⁺, H₂CO, or HNC. Common line ratios associated with UV illumination, CN/HNC and CN/HCO⁺, show ambiguous results on larger scales, suggesting that the identification of UV illuminated material may be more challenging. The HCN/HNC ratio may be related to temperature over varying scales.

    

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3. The role of magnetic fields in the stability and fragmentation of filamentary molecular clouds: two case studies at OMC-3 and OMC-4

   https://doi.org/10.1093/mnras/stac1527  

   Li, Pak Shing ; Lopez-Rodriguez, Enrique ; Soam, Archana ; Klein, Richard I. ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   We present the stability analysis of two regions, OMC-3 and OMC-4, in the massive and long molecular cloud complex of Orion A. We obtained 214 $\mu$m HAWC + /SOFIA polarization data, and we make use of archival data for the column density and C18O (1–0) emission line. We find clear depolarization in both observed regions and that the polarization fraction is anticorrelated with the column density and the polarization-angle dispersion function. We find that the filamentary cloud and dense clumps in OMC-3 are magnetically supercritical and strongly subvirial. This region should be in the gravitational collapse phase and is consistent with many young stellar objects (YSOs) forming in the region. Our histogram of relative orientation (HRO) analysis shows that the magnetic field is dynamically sub-dominant in the dense gas structures of OMC-3. We present the first polarization map of OMC-4. We find that the observed region is generally magnetically subcritical except for an elongated dense core, which could be a result of projection effect of a filamentary structure aligned close to the line of sight. The relative large velocity dispersion and the unusual positive shape parameters at high column densities in the HROs analysis suggest that our viewing angle may be close to axes of filamentary substructures in OMC-4. The dominating strong magnetic field in OMC-4 is unfavourable for star formation and is consistent with much fewer YSOs than in OMC-3.

    

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4. HAWC+/SOFIA Multiwavelength Polarimetric Observations of OMC-1

   https://doi.org/10.3847/1538-4357/aafd37  

   Chuss, David T. ; Andersson, B-G ; Bally, John ; Dotson, Jessie L. ; Darren Dowell, C. ; Guerra, Jordan A. ; Harper, Doyal A. ; Houde, Martin ; Jay Jones, Terry ; Lazarian, A. ; et al ( February 2019 , The Astrophysical Journal)
5. OMC-1 dust polarization in ALMA Band 7: diagnosing grain alignment mechanisms in the vicinity of Orion Source I

   https://doi.org/10.1093/mnras/stab608  

   Pattle, Kate ; Lai, Shih-Ping ; Wright, Melvyn ; Coudé, Simon ; Plambeck, Richard ; Hoang, Thiem ; Tang, Ya-Wen ; Bastien, Pierre ; Eswaraiah, Chakali ; Furuya, Ray S ; et al ( March 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains ($\gt 0.005\!-\!0.1\, \mu$m), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion. 

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