Discovery of warm and dense molecular gas surrounding the ring nebula G79.29+0.46

Abstract

We present for the first time the detection of mid-J CO line emission in the outskirts of an evolved massive star, which indicates the presence of warm and/or high-density molecular gas. Aiming to learn about the interplay between evolved massive stars and their surroundings, we have carried out CO J = 2→ 1, J = 3→ 2, and 13↑CO J = 2→ 1 line observations in a 4' × 4' field around the ring nebula G79.29+0.46, which is illuminated by a strong LBV star candidate. The whole field shows extended predominant emission in both CO and 13↑CO J = 2→ 1 lines, which probably comes from the large cloud which contains the star-forming region DR 15. When this large-scale emission is removed, minor-scale features become evident, particularly in the CO J = 3→ 2 line, strikingly coincident with the ring nebula. The high critical density of CO J = 3→ 2 (some 10↑4 cm↑−3) gives additional support for the association with the massive star, since high-density molecular gas has more chances to survive in such a harsh environment. This structure may have been produced by the cumulative effect of a strong steady wind in the LBV stage or earlier, which has compressed the surviving parent molecular cloud. In addition, immersed within this CO feature, we have also discovered a higher density clump (at least several ~10↑5 cm↑−3), unresolved by the telescope and probably having a higher kinetic temperature. Toward the clump, the presence of a jump of 14-16 km s↑−1 in the gas velocity may indicate the existence of a shock front. This clump may have been created by at least one mass eruption, 10↑3-10↑4 yr ago. Thus, this work shows that not all the molecular gas is destroyed during massive star evolution, and consequently we are dealing with a new laboratory where one can learn about the mass-loss phenomena associated to the brief LBV stage.