  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 ~85 arcmin^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 ~ 1e7-1e9 K/cm3, 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 ~ [5e3-8e4] K/cm3 where many PDRs seem
to lie. The CH+ (1-0) emission correlates with the extended emission from
vibrationally excited H2, 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 H2(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.
