2015MNRAS.449.2875N

The model is presented of the H2O maser in the 110-101 line within the first excited vibrational state of the molecule around oxygen-rich asymptotic giant branch stars. It is suggested that the maser cloud is located in the inner layers of the circumstellar envelope where intense dust formation takes place. The calculations take into account rotational levels belonging to the five lowest vibrational states of the H2O molecule. The model predicts the gain values of the 658-GHz maser about 10^–14-10^–13/ cm at H2 molecule concentrations 10⁹-10¹¹/ cm3 and at high ortho-H2O concentrations ≳ 10⁵/ cm3. The gas temperatures 1000-1500 K are considered to be a necessary condition for the effective maser operation.
Result. are presented for other maser transitions of the excited vibrational states of the molecule.