Abstract
The model is presented of H$_2$O maser in the $1_{10}-1_{01}$ 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 took into account rotational levels belonging to the five lowest vibrational states of the H$_2$O molecule. The model predicts the gain values of the 658-GHz maser about $10^{-14} -10^{-13}$ cm$^{-1}$ at H$_2$ molecule concentrations $10^9 - 10^{11}$ cm$^{-3}$ and at high ortho-H$_2$O concentrations $ \gtrsim 10^5$ cm$^{-3}$. The gas temperatures $1000 - 1500$ K are considered to be a necessary condition for the effective maser operation. Results are presented for other maser transitions of the excited vibrational states of the molecule.
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