Molecular oxygen ( O _ { 2 } ) has been the target of ground-based and space-borne searches for decades .
Of the thousands of lines of sight surveyed , only those toward Rho Ophiuchus and Orion H _ { 2 } Peak 1 have yielded detections of any statistical significance .
The detection of the O _ { 2 } N _ { J } = 3 _ { 3 } – 1 _ { 2 } and 5 _ { 4 } – 3 _ { 4 } lines at 487.249 GHz and 773.840 GHz , respectively , toward Rho Ophiuchus has been attributed to a short-lived peak in the time-dependent , cold-cloud O _ { 2 } abundance , while the detection of the O _ { 2 } N _ { J } = 3 _ { 3 } – 1 _ { 2 } , 5 _ { 4 } – 3 _ { 4 } lines , plus the 7 _ { 6 } – 5 _ { 6 } line at 1120.715 GHz , toward Orion has been ascribed to time-dependent preshock physical and chemical evolution and low-velocity ( 12 km s ^ { { -1 } } ) non-dissociative C -type shocks , both of which are fully shielded from far-ultraviolet ( FUV ) radiation , plus a postshock region that is exposed to a FUV field .
We report a re-interpretation of the Orion O _ { 2 } detection based on new C -type shock models that fully incorporate the significant effects the presence of even a weak FUV field can have on the preshock gas , shock structure and postshock chemistry .
In particular , we show that a family of solutions exists , depending on the FUV intensity , that reproduces both the observed O _ { 2 } intensities and O _ { 2 } line ratios .
The solution in closest agreement with the shock parameters inferred for H _ { 2 } Peak 1 from other gas tracers assumes a 23 km s ^ { { -1 } } shock impacting gas with a preshock density of 8 \times 10 ^ { 4 } cm ^ { -3 } and G _ { o } = 1 , substantially different from that inferred for the fully-shielded shock case .
As pointed out previously , the similarity between the LSR velocity of all three O _ { 2 } lines ( \approx 11 km s ^ { { -1 } } ) and recently measured H _ { 2 } O 5 _ { 32 } – 4 _ { 41 } maser emission at 620.701 GHz toward H _ { 2 } Peak 1 suggests that the O _ { 2 } emission arises behind the same shocks responsible for the maser emission , though the O _ { 2 } emission is almost certainly more extended than the localized high-density maser spots .
Since maser emission arises along lines of sight of low-velocity gradient , indicating shock motion largely perpendicular to our line of sight , we note that this geometry can not only explain the narrow ( \scriptsize { \raisebox { -2.0 pt } { $ \stackrel { \textstyle < } { \sim } $ } } \normalsize 3 km s ^ { { -1 } } ) observed O _ { 2 } line widths despite their excitation behind a shock , but also why such O _ { 2 } detections are rare .