Large-Scale NH_3Observations toward the Galactic Star-Forming Regions I.W 51 Molecular Cloud Complex

摘要

Large-area observations of dense molecular gas were made in NH3 (J, K) = (1, 1), (2, 2), and (3, 3) inversionlines with 43 resolution toward the molecular cloud complex of the W 51 region. The observations were a partof 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 W 51 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 withincreasing the rotation temperature, the far-infrared luminosity and the index of the star-formation efficiency. Thistendency is explained if star formation has continued for more than the time scale of the transformation betweenortho- and para-NH3: active star formation on a large scale, such as the interaction of molecular clouds with a spiralarm, has made the molecular gas warmer, even in a scale of — 10 pc, and the proceeding transformation has madethe ortho-to-para ratio lower.