Hot water around late-type stars - Detection of two millimeter-wave emission lines from the nu2 vibrationally excited state

The first astronomical detection of two millimeter-wave emission lines from vibrationally excited water is reported. The 96 GHz 4(40)-5(33) and the 232 GHz 5(50)-6(43) rotational transitions within the nu2 vibrationally excited state of H2O have been discovered toward the supergiant star VY CMa. The latter transition was also detected toward the semiregular variable W Hya. The 4(40)-5(33) transition shows definite signs of maser action while the 5(50)-6(43) emission may be of (quasi-)thermal nature. Based on a comparison of the line profiles of the 22 GHz H2O masers, various SiO maser lines, and the nu2 H2O emission, it is concluded that the vibrationally excited H2O and the SiO masers probably arise from a region in which the expansion of the circumstellar envelope has not yet begun and some infalling motions are present. This region is closer to the star than the part of the envelope that gives rise to the 22 GHz H2O masers. 18 refs