2018ApJ...858...57E

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_ex⁶⁵=6.0±0.5 K, T_ex⁶⁷=5.1±0.2 K, and T_ex⁶⁵-T_ex⁶⁷=0.9±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.