  Context: AGB stars lose a large percentage of their mass in a dust-driven
wind. This creates a circumstellar envelope, which can be studied through
thermal dust emission and molecular emission lines. In the case of high
mass-loss rates, this study is complicated by the high optical depths and the
intricate coupling between gas and dust radiative transfer characteristics. An
important aspect of the physics of gas-dust interactions is the strong
influence of dust on the excitation of several molecules, including H2O. Aims:
The dust and gas content of the envelope surrounding the high mass-loss rate
OH/IR star OH 127.8+0.0, as traced by Herschel observations, is studied, with a
focus on the H2O content and the dust-to-gas ratio. We report detecting a large
number of H2O vapor emission lines up to J = 9 in the Herschel data, for which
we present the measured line strengths. Methods: The treatments of both gas and
dust species are combined using two numerical radiative transfer codes. The
method is illustrated for both low and high mass-loss-rate sources.
Specifically, we discuss different ways of assessing the dust-to-gas ratio: 1)
from the dust thermal emission spectrum and the CO molecular gas line
strengths; 2) from the momentum transfer from dust to gas and the measured gas
terminal velocity; and 3) from the determination of the required amount of dust
to reproduce H2O lines for a given H2O vapor abundance. These three diagnostics
probe different zones of the outflow, for the first time allowing an
investigation of a possible radial dependence of the dust-to-gas ratio.
Results: ... Continued in article.
