Astron. Astrophys. 361, 685-694 (2000)

5. Conclusions

The NH₃(J,K) = (1,1) spectra from the NGC 6334 region exhibit hyperfine structure intensity anomalies, suggesting departures from LTE conditions. We have used the numerical model of SW85, based on the qualitative model of Matsakis et al. (1977) to calculate the physical parameters of the ammonia clouds. This model takes into account the non-LTE effects through selective trapping in the far IR transition NH₃(J,K) = (2,1) (1,1) and hyperfine selective collisional excitation. Its main limitation is that it does not include the effects of an IR continuum at the NH₃(J,K) = (2,1) (1,1) transition frequency. As it was stressed by SW85, its influence on the anomalies is difficult to predict, but they argued that if the IR radiation field had significant influence, variations in their strength with the distance from the exciting source would be seen, which is not supported by observations, at least in the regions S106 and W48.

The existence of clumps with very small dispersion velocity was questioned by Gaume et al. (1996), based on the NH₃ absorption spectra in the direction of the continuum source DR21. They proposed, instead, inflows and outflows of matter in different regions of the cloud. However, the mechanism producing the non-LTE level population is the same as in Matsakis et al. (1977), that is, the selective trapping of photons in the NH₃(J,K) = (2,1) (1,1) transition. Since this model was not developed quantitatively, it is not clear if it will work to explain the observed anomalies.

Although our observations do not have high angular resolution, our high quality spectra allow the identification of three distinct sources. The first one is NGC 6334 I, a compact source mapped in NH₃(J,K) = (1,1) transition by JHH88, reveling the presence of a possible bipolar molecular outflow in NH₃ and CO (Bachiller & Cernicharo 1990), having also a cluster of IR sources (Tapia et al. 1996). The second region is NGC 6334 I(N)w, which also presents a bipolar molecular outflow in SiO (Megeath & Tieftrunk 1999), is the most intense ammonia source in the sky, but is not spatially resolved in our observations. It is formed by clumps with diameters of 0.007 pc, masses of 0.2 , kinetic temperatures of 24 K and with a total mass of about 7600 . This result suggests larger masses for this region than estimated by Kuiper et al. (1995), assuming LTE conditions. It also exihibits velocity gradients almost perpendicular to the Galactic plane, and line-width gradients parallel to the Galactic plane, showing larger line-widths towards the more evolved sources. Finally, the region NGC 6334 I(N)e is formed by clumps with physical conditions similar to those observed in I(N)w, and spread through an area larger than the telescope beam.

The ammonia abundances in both regions are similar. Their values indicate that the regions have either ages smaller than 10⁶ years or that depletion effects are important.

Among the three regions identified, NGC 6334 I is the most evolved while NGC 6334 I(N)e is in a very early star formation stage.

Comparison between physical parameters determined under LTE and non-LTE conditions showed that the NH₃ column density and ammonia abundance are the only quantities which are similar between them. Other parameters, as rotational and kinetic temperatures, exhibit differences that can reach a factor of three.

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000
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