OH as an Alternate Tracer for Molecular Gas: Excitation Temperatures of
  the OH 18-cm Main Lines in W5
  We present excitation temperatures $T_{ex}$ for the OH 18-cm main lines at
1665 and 1667 MHz measured directly in front of the W5 star-forming region,
using observations from the Green Bank Telescope and the Very Large Array. We
find unequivocally that $T_{ex}$ at 1665 MHz is greater than $T_{ex}$ at 1667
MHz. Our method exploits variations in the continuum emission from W5, and the
fact that the continuum brightness temperatures $T_C$ in this nebula are close
to the excitation temperatures of the OH lines in the foreground gas. The
result is that an OH line can appear in emission in one location and in
absorption in a neighboring location, and the value of $T_C$ where the profiles
switch from emission to absorption indicates $T_{ex}$. Absolute measurements of
$T_{ex}$ for the main lines were subject to greater uncertainty because of
unknown effects of geometry of the OH features. We also employed the
traditional "expected profile" method for comparison with our "continuum
background" method, and found that the continuum background method provided
more precise results, and was the one to definitively show the $T_{ex}$
difference. Our best estimate values are: $T^{65}_{ex} = 6.0 \pm 0.5$ K,
$T^{67}_{ex} = 5.1 \pm 0.2$ K, and $T^{65}_{ex} - T^{67}_{ex} = 0.9 \pm 0.5$ K.
The $T_{ex}$ values we have measured for the ISM in front of W5 are similar to
those found in the quiescent ISM, indicating that proximity to massive
star-forming regions does not generally result in widespread anomalous
excitation of OH emission.
