Far-Infrared detection of neutral atomic oxygen toward the Horsehead Nebula

TL;DR

This paper reports the first detection of neutral atomic oxygen at 63 micrometers in the Horsehead Nebula's PDR, analyzing its emission, excitation, and radiative transfer to understand the physical conditions of the region.

Contribution

It provides the first detection and detailed analysis of the [OI] 63um line in the Horsehead Nebula, highlighting complex non-LTE effects and the potential of FIR lines for astrophysical diagnostics.

Findings

[OI]63um line peaks at the cloud edge.

Line luminosity is a significant fraction of FIR dust luminosity.

Complex non-LTE effects influence line ratios.

Abstract

We present the first detection of neutral atomic oxygen (3P_1-3P_2 fine structure line at ~63um) toward the Horsehead photodissociation region (PDR). The cloud has been mapped with the Spitzer Space Telescope at far-IR (FIR) wavelengths using MIPS in the spectral energy distribution (SED) mode. The [OI]63um line peaks at the illuminated edge of the cloud at AV~0.1-0.5 (inward the gas becomes too cold and outward the gas density drops). The luminosity carried by the [OI]63um line represents a significant fraction of the total FIR dust luminosity (I_63/I_FIR~4x10^-3). We analyze the dust continuum emission and the nonlocal OI excitation and radiative transfer in detail. The observations are reproduced with a gas density of n_H~10^4 cm^-3 and gas and dust temperatures of T_k~100 K and T_d~30 K. We conclude that the determination of the OI 3P_J level populations and emergent line intensities at such ``low'' densities is a complex non-LTE problem. FIR radiative pumping, [OI]63um subthermal emission, [OI]145um suprathermal and even maser emission can occur and decrease the resulting [OI]63/145 intensity ratio. The Herschel Space Observatory, observing from ~55 to 672um, will allow us to exploit the diagnostic power of FIR fine structure lines with unprecedented resolution and sensitivity.