Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel

TL;DR

This study provides the first velocity-resolved [CII] emission map of Orion, revealing detailed small-scale gas kinematics, feedback effects, and variations in [CII]/FIR ratios across different cloud regions, enhancing understanding of star-forming environments.

Contribution

It offers the first velocity-resolved [CII] map of Orion, combining multiple data sets to analyze gas dynamics, feedback, and [CII]/FIR ratio variations with unprecedented detail.

Findings

Most [CII] emission originates from extended FUV-illuminated regions and dense PDRs.

Approximately 15% of [CII] emission comes from gas without CO counterpart.

[CII]/FIR ratio decreases from extended cloud to dense star-forming cores.

Abstract

We present the first 7.5'x11.5' velocity-resolved map of the [CII]158um line toward the Orion molecular cloud-1 (OMC-1) taken with the Herschel/HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41alpha hydrogen recombination and CO J=2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [CII] luminosity (~85%) is from the extended, FUV-illuminated face of the cloud G_0>500, n_H>5x10^3 cm^-3) and from dense PDRs (G_0~10^4, n_H~10^5 cm^-3) at the interface between OMC-1 and the HII region surrounding the Trapezium cluster. Around 15% of the [CII] emission arises from a different gas component without CO counterpart. The [CII] excitation, PDR gas turbulence, line opacity (from [13CII]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [CII]/FIR and FIR/M_Gas ratios and show that [CII]/FIR decreases from the extended cloud component (10^-2-10^-3) to the more opaque star-forming cores (10^-3-10^-4). The lowest values are reminiscent of the "[CII] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [CII]/FIR ratio correlates better with the column density of dust through the molecular cloud than with FIR/M_Gas. We conclude that the [CII] emitting column relative to the total dust column along each line of sight is responsible for the observed [CII]/FIR variations through the cloud.