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Ammonia observations in the LBV nebula G79.29+0.46. Discovery of a cold ring and some warm spots

The surroundings of Luminous Blue Variable (LBV) stars are excellent laboratories to study the effects of their high UV radiation, powerful winds, and strong ejection events onto the surrounding gas and dust. The LBV G79.29+0.46 powered two concentric infrared rings which may interact with the infrared dark cloud (IRDC) G79.3+0.3. The Effelsberg 100m telescope was used to observe the NH_3 (1,1), (2,2) emission surrounding G79.29+0.46 and the IRDC. In addition, we observed particular positions in the (3,3) transition toward the strongest region of the IRDC. We report here the first coherent shell-like structure of dense NH_3 gas associated with an evolved massive star. The shell, two or three orders of magnitude more tenuous than the IRDC, is well traced in both ammonia lines, and surrounds the ionized nebula. The NH_3 emission in the IRDC is characterized by a low and uniform rotational temperature (T_rot ~ 10 K) and moderately high opacities in the (1,1) line. The rest of the observed field is spotted by warm or hot zones (T_rot > 30 K) and characterized by optically thin emission of the (1,1) line. The NH_3 abundances are about 10^{-8} in the IRDC, and 10^{-10}-10^{-9} elsewhere. The warm temperatures and low abundances of NH_3 in the shell suggest that the gas is being heated and photo-dissociated by the intense UV field of the LBV star. An outstanding region is found to the south-west (SW) of the LBV star within the IRDC. The NH_3 (3,3) emission at the centre of the SW region reveals two velocity components tracing gas at temperatures > 30K. The northern edge of the SW region agrees with the border of the ring nebula and a region of continuum enhancement; here, the opacity of the (1,1) line and the NH_3 abundance do not decrease as expected in a typical clump of an isolated cold dark cloud. This strongly suggests some kind of interaction between the ring nebula and the IRDC.