SIMBAD references

The maser emission of the para-H₂O 3₁₃⟶2₂₀ line at 183 GHz in O-rich evolved stars has been modeled to account for the empirical characteristics of this line reported by González-Alfonso et al. Likewise, efforts have been made to derive water vapor abundance in these sources. The Sobolev or large velocity gradient (LVG) method has been employed to study the intensity of this line as a function of source properties and physical conditions (i.e., mass-loss rate, p-H₂O abundance, velocity field, kinetic temperature profile, stellar luminosity, and the set of collisional rates adopted in the calculations). It has been found that the intensity of the 3₁₃⟶2₂₀ line is sensitive to the mass-loss rate, the p-H₂O abundance, and the terminal velocity of the envelope, but it is rather insensitive to the rest of the parameters in stars with high mass-loss rates (M{dot}>10^–6 M_☉.yr^–1). The models reproduce the main spectral characteristics of the emission at 183 GHz in the latter sources. A global fit to the data proves that the observational luminosities can be explained by assuming an H₂O abundance relative to H₂ [x(H₂O)] of 1-2x10^–4. Detailed fitting to the line profile in five selected objects yields a similar value for x(H₂O). The validity of the LVG approach has been verified by modeling the maser emission at 183 GHz through a nonlocal radiative transfer code. The model calculations with both methods lead to similar results. The pumping of the first bending mode of water vapor through absorption of photons emitted by the dust and the star has been also simulated. This effect is found to be important in the pumping of the H₂O rotational levels. Hence, in order to recover LVG results, the water abundance must be increased by a factor of ~2 for stars with high mass-loss rates. Consequently, x(H₂O) has been estimated to be 3x10^–4 within a factor ≃2. With this value for x(H₂O), the expected near- and far-infrared spectra of the circumstellar envelopes of O-rich stars for several mass-loss rates have also been computed. Hence, it is possible to predict that, in some stars, the ro-vibrational lines of the 6 µm water vapor band with wavelengths longer that 6.3 µm–the P-branch–can be observed in emission, rather than in absorption.