Large-scale NH₃ observations toward the galactic star-forming regions I. W 51 molecular cloud complex

Large-area observations of dense molecular gas were made in NH₃ (J, K) = (1, 1), (2, 2), and (3, 3) inversion lines with 4′.5 resolution toward the molecular cloud complex of the W 51 region. The observations were a part of a NH₃ survey of the Galactic star-forming regions using the Tomakomai 11-m radio telescope. NH₃ (J, K) = (1, 1) and (2, 2) emission was detected in the W51 A and B complexes, while the detection of (J, K) = (2, 2) emission was marginal in W 51 B. The rotation temperature was ∼40 K in the central part of the W 51 A complex, while being ∼20 K in the other positions. A weak correlation is found that the ortho-to-para ratio decreases with increasing the rotation temperature, the far-infrared luminosity and the index of the star-formation efficiency. This tendency is explained if star formation has continued for more than the time scale of the transformation between ortho- and para-NH₃: active star formation on a large scale, such as the interaction of molecular clouds with a spiral arm, has made the molecular gas warmer, even in a scale of ∼ 10 pc, and the proceeding transformation has made the ortho-to-para ratio lower.