Large-Scale NH3 Observations toward the Galactic Star-Forming Regions I. W 51 Molecular Cloud Complex

Abstract

Abstract Large-area observations of dense molecular gas were made in NH$_{3}$ ($J$, $K$) $=$ (1, 1), (2, 2), and (3, 3) inversion lines with 4'.5 resolution toward the molecular cloud complex of the W51 region. The observations were a part of a NH$_{3}$ survey of the Galactic star-forming regions using the Tomakomai 11-m radio telescope. NH$_{3}$ ($J$, $K$) $=$ (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 W51 B. The rotation temperature was $\sim$40K in the central part of the W51 A complex, while being $\sim$20K in the other positions. A weak correlation is found that the ortho-to-para ratio decreases with increasing the rotation temperature, the far-infrared luminosity and the index of the star-formation efficiency. This tendency is explained if star formation has continued for more than the time scale of the transformation between ortho- and para-NH$_{3}$: 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 $\sim$10pc, and the proceeding transformation has made the ortho-to-para ratio lower.