Context. The Orion A molecular complex is a nearby (420 pc), very well studied stellar nursery that is believed to contain examples of triggered star formation.
Aims: 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.
Methods: 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. 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, and studied the implications of adopting the Orion Nebula extinction properties instead of the standard interstellar medium ones.
Results: The detected absorption lines peak at a velocity of 9 km s^-1, which is blue-shifted by 2 km s^-1 with respect to the systemic velocity of OMC-2 FIR 4 (V_LSR = 11.4 km s^-1). 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.
Conclusions: 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.