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 similar to 63 mu m) 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 mu m line peaks at the illuminated edge of the cloud at A(V) similar or equal to 0.1-0.5 (inward the gas becomes too cold and outward the gas density drops). The luminosity carried by the [O I] 63 mu m line represents a significant fraction of the total FIR dust luminosity (I(63)/I(FIR) similar or equal to 4 x 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) similar or equal to 10(4) cm(-3) and gas and dust temperatures of T(k) similar or equal to 100 K and T(d) similar or equal to 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 mu m subthermal emission, [O I] 145 mu m suprathermal and even maser emission can occur and decrease the resulting [O I] 63/145 intensity ratio. The Herschel Space Observatory, observing from similar to 55 to 672 mu m, will allow us to exploit the diagnostic power of FIR fine structure lines with unprecedented resolution and sensitivity.