The water vapor abundance in circumstellar envelopes

The maser emission of the para-H2O 3(13) --> 2(20) line at 183 GHz in O-ric h evolved stars has been modeled to account for the empirical characteristi cs of this line reported by Gonzalez-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 intensi ty 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 t he calculations). It has been found that the intensity of the 3(13) --> 2(2 0) line is sensitive to the mass-loss rate, the p-H2O abundance, and the te rminal velocity of the envelope, but it is rather insensitive to the rest o f the parameters in stars with high mass-loss rates (M > 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 obs ervational luminosities can be explained by assuming an H2O abundance relat ive to H-2 [x(H2O)] of 1-2 x 10(-4). Detailed fitting to the line profile i n 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 GH z through a nonlocal radiative transfer code. The model calculations with b oth 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 st ar has been also simulated. This effect is found to be important in the pum ping of the H2O rotational levels. Hence, in order to recover LVG results, the water abundance must be increased by a factor of approximate to 2 for s tars with high mass-loss rates. Consequently, x(H2O) has been estimated to be 3 x 10(-4) within a factor similar or equal to 2. With this value for x( H2O), the expected near- and far-infrared spectra of the circumstellar enve lopes of O-rich stars for several mass-loss rates have also been computed. Hence, it is possible to predict that, in some stars, the re-vibrational li nes of the 6 mu m water vapor band with wavelengths longer that 6.3 mu m-th e P-branch-can be observed in emission, rather than in absorption.