We present the first detection of neutral atomic oxygen ( ^ { 3 } P _ { 1 } – ^ { 3 } P _ { 2 } fine structure line at \sim 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 ( MIPS ) in the spectral energy distribution ( SED ) mode .
The [ O i ] 63 \mu m line peaks at the illuminated edge of the cloud at A _ { V } \simeq 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 _ { \mathrm { FIR } } \simeq 4 \times 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 _ { \mathrm { H } } \simeq 10 ^ { 4 } cm ^ { -3 } and gas and dust temperatures of T _ { k } \simeq 100 K and T _ { d } \simeq 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 \sim 55 to 672 \mu m , will allow us to exploit the diagnostic power of FIR fine structure lines with unprecedented resolution and sensitivity .