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The surroundings of Luminous Blue Variable (LBV) stars are excellentlaboratories to study the effects of their high UV radiation, powerful winds,and strong ejection events onto the surrounding gas and dust. The LBVG79.29+0.46 powered two concentric infrared rings which may interact with theinfrared dark cloud (IRDC) G79.3+0.3. The Effelsberg 100m telescope was used toobserve the NH_3 (1,1), (2,2) emission surrounding G79.29+0.46 and the IRDC. Inaddition, we observed particular positions in the (3,3) transition toward thestrongest region of the IRDC. We report here the first coherent shell-likestructure 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 inboth ammonia lines, and surrounds the ionized nebula. The NH_3 emission in theIRDC is characterized by a low and uniform rotational temperature (T_rot ~ 10K) and moderately high opacities in the (1,1) line. The rest of the observedfield is spotted by warm or hot zones (T_rot > 30 K) and characterized byoptically thin emission of the (1,1) line. The NH_3 abundances are about10^{-8} in the IRDC, and 10^{-10}-10^{-9} elsewhere. The warm temperatures andlow abundances of NH_3 in the shell suggest that the gas is being heated andphoto-dissociated by the intense UV field of the LBV star. An outstandingregion is found to the south-west (SW) of the LBV star within the IRDC. TheNH_3 (3,3) emission at the centre of the SW region reveals two velocitycomponents tracing gas at temperatures > 30K. The northern edge of the SWregion agrees with the border of the ring nebula and a region of continuumenhancement; here, the opacity of the (1,1) line and the NH_3 abundance do notdecrease as expected in a typical clump of an isolated cold dark cloud. Thisstrongly suggests some kind of interaction between the ring nebula and theIRDC.

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