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-2Â FIRÂ 4 obtained in CHESS , we detected several ground-state lines from OH ^ { + } , H _ { 2 } O ^ { + } , 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 H _ { 3 } O ^ { + } .
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 _ { \mathrm { LSR } } = 11.4  km s ^ { -1 } ) .
The results of our modelling indicate that the foreground cloud is composed of predominantly neutral diffuse gas ( n _ { \mathrm { H } } = 100 Â 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 , 2Â pc south of OMC-2Â FIRÂ 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 .