H₂O vapor excitation in dusty AGB envelopes

A PACS view of OH 127.8+0.0^⋆,⋆⋆

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D 2401, 3001 Leuven, Belgium
e-mail: robinl@ster.kuleuven.be
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ Astronomical Institute Utrecht, University Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
⁵ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 7 February 2012
Accepted: 10 April 2013

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.