Herschel CHESS discovery of the fossil cloud that gave birth to the
  Trapezium and Orion KL
  As part of the Herschel Guaranteed Time Key Programme CHESS, we present the
discovery of a diffuse gas component in the foreground of the intermediate-mass
protostar OMC-2 FIR 4, located in the Orion A region. Making use of the full
HIFI spectrum of OMC-2 FIR 4 obtained in CHESS, we detected several
ground-state lines from OH+, H2O+, HF, and CH+, all of them seen in absorption
against the dust continuum emission of the protostar's envelope. The lines peak
at a velocity of 9 km/s, which is blue-shifted by 2 km/s with respect to the
systemic velocity of OMC-2 FIR 4 (V = 11.4 km/s). We derived column densities
for each species, as well as an upper limit to the column density of the
undetected H3O+. In order to model and characterise the foreground cloud, we
used the Meudon PDR code to run a homogeneous grid of models that spans a
reasonable range of densities, visual extinctions, cosmic ray ionisation rates
and far-ultraviolet (FUV) radiation fields. The results of our modelling
indicate that the foreground cloud is composed of predominantly neutral diffuse
gas (n(H) = 100 cm-3) and is heavily irradiated by an external source of FUV
that most likely arises from the nearby Trapezium OB association. The cloud is
6 pc thick and bears many similarities with the so-called C+ interface between
Orion-KL and the Trapezium cluster, 2 pc south of OMC-2 FIR 4. We conclude that
the foreground cloud we detected is an extension of the C+ interface seen in
the direction of Orion KL, and interpret it to be the remains of the parental
cloud of OMC-1, which extends from OMC-1 up to OMC-2.
