2020A&A...636A..39B

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 1.3mm 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 281.8GHz 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.