Observations of multiple NH3 transitions in W33

At a distance of 2.4 kpc, W33 is an outstanding massive and luminous 10 pc-sized star forming complex containing quiescent infrared dark clouds as well as highly active infrared bright cloud cores heated by young massive stars. We report measurements of ammonia (NH3) inversion lines in the frequency range 18-26 GHz obtained with the 40 '' resolution of the 100 m Effelsberg telescope. We detect the (J, K) = (1,1), (2,2), (3,3), (4,4), (5,5), (6,6), (2,1), and (3,2) transitions. There is a maser line in the (3,3) transition towards W33 Main. Brightness temperature and line shape indicate no significant variation during the last similar to 36 yr. We determined kinetic temperatures, column densities, and other physical properties of NH3 and the molecular clouds in W33. For the total-NH 3 column density inside 40 '' (0.5 pc) regions, we find 6.0 (+/- 2.1) x 10(14), 3.5 (+/- 0.1) x 10(15) , 3.4 (+/- 0.2) x 10(15), 3.1 (+/- 0.2) x 10(15), 2.8 (+/- 0.2) x 10(15), and 2.0 (+/- 0.2) x 10(15) cm(-2) at the peak positions of W33 Main, W33 A, W33 B, W33 Mainl, W33 A1, and W33 B1, respectively. W33 Main has a total-NH3 fractional abundance of 1.3 (+/- 0.1) x 10(-9) at the peak position. High values of 1.4 (+/- 0.3) x 10(-8), 1.6 (+/- 0.3) x 10(-8), 3.4 (+/- 0.5) x 10(-8), 1.6 (+/- 0.5) x 10(-8), and 4.0 (+/- 1.2) x 10(-8) are obtained at the central positions of W33 A, W33 B, W33 Mainl, W33 A1, and W33 B1. From this, we confirm the previously proposed variation in the evolutionary stages of the six W33 clumps and find that there is no hot core in the region approaching the extreme conditions encountered in W51-IRS2 or Sgr B2. The ortho-to-para-NH3 abundance ratios suggest that ammonia should have been formed in the gas phase or on dust grain mantles at kinetic temperatures of greater than or similar to 20 K. We determine kinetic temperatures only using NH3 (1,1) and (2,2), and from this we provide gas volume densities for the six main sources in the W33 region. With our new T-kin values, we find that our volume densities are similar to those estimated by Immer et al. (2014, A&A, 572, A63), suggesting that ammonia beam-filling factors are close to unity.

Automated summary

This study investigates the W33 star forming complex, which is a massive and luminous region containing infrared dark clouds and bright cloud cores. By measuring ammonia (NH3) inversion lines, the researchers determined the physical properties and evolutionary stages of the molecular clouds in W33. The results show variations in temperature, column densities, and abundance ratios, indicating different stages of star formation.