2022A&A...658A..34T

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 ∼36 yr. We determined kinetic temperatures, column densities, and other physical properties of NH₃ and the molecular clouds in W33. For the total-NH₃ column density inside 40'' (0.5 pc) regions, we find 6.0 (±2.1) x 10¹⁴, 3.5 (±0.1) x 10¹⁵, 3.4 (±0.2) x 10¹⁵, 3.1 (±0.2) x 10¹⁵, 2.8 (±0.2) x 10¹⁵, and 2.0 (±0.2) x 10¹⁵ cm^–2 at the peak positions of W33 Main, W33 A, W33 B, W33 Main1, 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 Main1, 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 ≥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. (2014A&A...572A..63I), suggesting that ammonia beam-filling factors are close to unity.