Far-Infrared detection of neutral atomic oxygen toward the Horsehead Nebula
Abstract
We present the first detection of neutral atomic oxygen (3P1-3P2 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 (I63/IFIR~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 nH~104 cm-3 and gas and dust temperatures of Tk~100 K and Td~30 K. We conclude that the determination of the OI 3PJ 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.