Abstract
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 NH3 (1, 1) and (2, 2) emission in a field of view of 7' x 7' including the infrared rings and a part of the IRDC. In addition, we observed particular positions in the NH3 (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 NH3 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 NH3 emission in the IRDC is characterized by a low and uniform rotational temperature (T-rot similar to 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 NH3 abundances are about 10(-8) in the IRDC, and 10(-10)-10(-9) elsewhere. The warm temperatures and low abundances of NH3 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 NH3 (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 NH3 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 NH3 evaporation from the dust grain mantles due to the already known presence of low-velocity shocks in the area.
Conclusions. The detection of the NH3 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 constitute an exemplary scenario, which merits to be studied in detail by other molecular tracers and higher angular resolutions.