2009ApJ...699L.165G

We present the first detection of the neutral atomic oxygen (³ P₁-³ P₂ 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 (I₆₃/I_FIR≃ 4x10^–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≃ 10⁴/cm3 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.