Herschel CHESS discovery of the fossil cloud that gave birth to the Trapezium and Orion KL

TL;DR

This study used Herschel data to identify a diffuse, heavily FUV-irradiated foreground cloud in Orion, likely the parental cloud of OMC-1, revealing its composition, structure, and relation to nearby star-forming regions.

Contribution

First detection and characterization of a fossil cloud in Orion's foreground, linking it to the parental cloud of OMC-1 through detailed spectral and modeling analysis.

Findings

Foreground cloud is diffuse and neutral with n(H) = 100 cm-3.

Cloud is heavily irradiated by FUV from Trapezium OB stars.

The cloud is an extension of the C+ interface near Orion KL.

Abstract

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.