O$_2$ Emission Toward Orion H$_2$ Peak 1 and the Role of FUV-Illuminated
  C-Shocks
  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 Oph 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) 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 shock impacting gas with a preshock density of 8x10^4 cm^-3 and G_0 =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 (~11 km/s) and recently measured H_2O 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
