Far-Infrared Detection of Neutral Atomic Oxygen Toward the Horsehead Nebula

We present the first detection of the neutral atomic oxygen ( 3 P 1 – 3 P 2 fine structure line at ∼63 μm) toward the Horsehead photodissociation region (PDR). The cloud has been mapped with the Spitzer Space Telescope at far-IR (FIR) wavelengths using the Multiband Imaging Photometer for Spitzer in the spectral energy distribution mode. The [O i ]63 μm line peaks at the illuminated edge of the cloud at A V ≃ 0.1–0.5 (inward the gas becomes too cold and outward the gas density drops). The luminosity carried by the [O i ]63 μm line represents a significant fraction of the total FIR dust luminosity ( I 63 / I FIR ≃ 4 × 10 −3 ). We analyze the dust continuum emission and the nonlocal O i 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 O i 3 P J level populations and emergent line intensities at such "low" densities is a complex non-LTE problem. FIR radiative pumping, [O i ]63 μm subthermal emission, [O i ]145 μm suprathermal and even maser emission can occur and decrease the resulting [O i ]63/145 intensity ratio. The Herschel Space Observatory , observing from ∼55 to 672 μm, will allow us to exploit the diagnostic power of FIR fine structure lines with unprecedented resolution and sensitivity.