H₂O vapor excitation in dusty AGB envelopes. A PACS view of OH 127.8+0.0

Abstract

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: We modeled the infrared continuum and the CO and H₂O emission lines in OH 127.8+0.0 simultaneously. We find a dust-mass-loss rate of (0.5 ± 0.1) × 10^-6 M_⊙ yr^-1 and an H₂O ice fraction of 16% ± 2% with a crystalline-to-amorphous ratio of 0.8 ± 0.2. The gas temperature structure is modeled with a power law, leading to a constant gas-mass-loss rate between 2 × 10^-5 M_⊙ yr^-1 and 1 × 10^-4 M_⊙ yr^-1, depending on the temperature profile. In addition, a change in mass-loss rate is needed to explain the J = 1-0 and J = 2-1 CO lines formed in the outer wind, where the older mass-loss rate is estimated to be 1 × 10^-7 M_⊙ yr^-1. The dust-to-gas ratio found with method 1) is 0.01, accurate to within a factor of three; method 2) yields a lower limit of 0.0005; and method 3) results in an upper limit of 0.005. The H₂O ice fraction leads to a minimum required H₂O vapor abundance with respect to H₂ of (1.7 ± 0.2) × 10^-4. Finally, we report detecting 1612 MHz OH maser pumping channels in the far-infrared at 79.1, 98.7, and 162.9 μm.
Conclusions: Abundance predictions for a stellar atmosphere in local thermodynamic equilibrium yield a twice higher H₂O vapor abundance (~3 × 10^-4), suggesting a 50% freeze-out. This is considerably higher than current freeze-out predictions. Regarding the dust-to-gas ratio, methods 2) and 3) probe a deeper part of the envelope, while method 1) is sensitive to the outermost regions. The latter diagnostic yields a significantly higher dust-to-gas ratio than do the two other probes. We offer several potential explanations for this behavior: a clumpy outflow, a variable mass loss, or a continued dust growth.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Appendix A is available in electronic form at http://www.aanda.org