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Context. The star-formation rates and stellar densities found in young massive clusters suggest that these stellar systems originate from gas at densities of n(H2) > 106 cm−3. Until today, however, the physical characterization of this ultra high density material remains largely unconstrained in observations. Aims. We investigate the density properties of the star-forming gas in the OMC-1 region located in the vicinity of the Orion Nebula Cluster (ONC). Methods. We mapped the molecular emission at 652 GHz in OMC-1 as part of the APEX-SEPIA660 Early Science. Results. We detect bright and extended N2H+ (J = 7–6) line emission along the entire OMC-1 region. Comparisons with previous ALMA data of the (J = 1–0) transition and radiative transfer models indicate that the line intensities observed in this N2H+ (7–6) line are produced by large mass reservoirs of gas at densities n(H2) > 107 cm−3. Conclusions. The first detection of this N2H+ (7–6) line at parsec-scales demonstrates the extreme density conditions of the star-forming gas in young massive clusters such as the ONC. Our results highlight the unique combination of sensitivity and mapping capabilities of the new SEPIA660 receiver for the study of the ISM properties at high frequencies. [ABSTRACT FROM AUTHOR]

Aims. We investigate the increase of the differential emission measure (DEM) towards the chromosphere due to small and cool magnetic loops (height ≲8 Mm, T ≲ 105 K). In a previous paper, we analysed the conditions of existence and stability of these loops through hydrodynamic simulations, focussing on their dependence on the details of the optically thin radiative loss function. Methods. In this paper, we extend those hydrodynamic simulations to verify if this class of loops exists and is stable when using an optically thick radiative loss function. We study two cases: constant background heating and a heating depending on the density. The contribution to the transition region extreme-UV output of these loops is also calculated and presented. Results. We find that stable, quasi-static cool loops can be obtained using an optically thick radiative loss function and a background heating depending on the density. The DEMs of these loops, however, fail to reproduce the observed DEM for temperatures between 4.6 < log T < 4.8. We also show the transient phase of a dynamic loop obtained by considering constant heating rate and find that its average DEM, interpreted as a set of evolving dynamic loops, reproduces the observed DEM very well. [ABSTRACT FROM AUTHOR]