Context : 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 .

Aims : We aim at determining the physical parameters of the dense gas near G79.29+0.46 , an LBV-candidate located at the centre of two concentric infrared rings which may interact with the infrared dark cloud ( IRDC ) G79.3+0.3 .

Methods : The Effelsberg 100 m telescope was used to observe the NH _ { 3 } ( 1,1 ) , ( 2,2 ) emission in a field of view of 7 ^ { \prime } \times 7 ^ { \prime } including the infrared rings and a part of the IRDC .
In addition , we observed particular positions in the NH _ { 3 } ( 3,3 ) transition toward the strongest region of the IRDC , which is also closest to the ring nebula .

Results : We report here the first coherent ring-like structure of dense NH _ { 3 } gas associated with an evolved massive star .
It is well traced in both ammonia lines , surrounding an already known infrared ring nebula ; its column density is two orders of magnitude lower than the IRDC .
The NH _ { 3 } emission in the IRDC is characterized by a low and uniform rotational temperature ( T _ { \mathrm { rot } } \sim 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 _ { \mathrm { 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 ring 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 > 30 K. Of particular interest is the northern edge of the SW region , which coincides with the border of the ring nebula and a region of strong 6 cm continuum emission ; 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 ( powered by the LBV star ) and the IRDC .
We finally discuss the possibility of NH _ { 3 } evaporation from the dust grain mantles due to the already known presence of low-velocity shocks in the area .

Conclusions : The detection of the NH _ { 3 } associated with this LBV ring nebula , as well as the special characteristics of the northern border of the SW region , confirm that the surroundings of G79.29+0.46 constitutes an exemplary scenario , which is worth to be studied in detail by other molecular tracers and higher angular resolutions .