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Description
Young massive stars regulate the physical conditions, ionization, and fate of their natal molecular cloud. It is important to find tracers that help quantifying the stellar feedback processes that take place at different scales. We present ~85arcmin^2^ velocity-resolved maps of several submm molecular lines toward the closest high-mass star-forming region, OMC-1. The observed rotational lines include probes of warm and dense molecular gas that are difficult to detect from ground-based telescopes: CH^+^ (1-0), CO (10-9), HCO^+^ (6-5), and HCN (6-5). These lines trace an extended but thin layer of molecular gas at high thermal pressure, P_th_~10^7^-10^9^K/cm^3^, associated with the FUV-irradiated surface of OMC-1. The intense FUV field, emerging from massive stars in the Trapezium cluster, heats, compresses and photoevaporates the cloud edge. It also triggers the formation of reactive molecules such as CH^+^. The CH^+^ (1-0) emission spatially correlates with the flux of FUV photons impinging the cloud: G_0 from 1e3 to 1e5. This correlation is supported by isobaric PDR models in the parameter space P_th_/G_0_~[5*10^3^-8*10^4^]K/cm^3^ where many PDRs seem to lie. The CH^+^ (1-0) emission correlates with the extended emission from vibrationally excited H_2_, and with that of [CII]158um and CO 10-9, all emerging from FUV-irradiated gas. These correlations link the presence of CH^+^ to the availability of C^+^ ions and of FUV-pumped H_2_(v>0) molecules. The parsec-scale CH^+^ emission and narrow-line (dv~3 km/s) mid-J CO emission arises from extended PDRs and not from fast shocks. PDR line tracers are the smoking gun of the stellar feedback from young massive stars. The PDR component in OMC-1 represents 5 to 10 % of the total gas mass, however, it dominates the emitted line luminosity. These results provide insights into the source of submm CH+ and mid-J CO emission from distant star-forming galaxies.
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