H₂O vapor excitation in dusty AGB envelopes

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 H 2 O. 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 H 2 O content and the dust-to-gas ratio. We report detecting a large number of H 2 O 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 H 2 O lines for a given H 2 O 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 H 2 O emission lines in OH 127.8+0.0 simultaneously. We find a dustmass-loss rate of (0.5 ± 0.1) × 10 −6 M yr −1 and an H 2 O 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 H 2 O ice fraction leads to a minimum required H 2 O vapor abundance with respect to H 2 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 H 2 O 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 ...

Herschelobservations of extreme OH/IR stars

“…4 and Appendix C). These lines have previously been reported by Sylvester et al (1997) and Lombaert et al (2013) and are thought to be the pumping line for the OH masers seen in these objects. Although the archived spectra from the short-wavelength spectrometer (LWS, Clegg et al 1996) aboard the Infrared Space Observatory (ISO, Kessler et al 1996) of some of these stars are very noisy, there may be a possible hint of an absorption of the infrared pumping line at 53 μm.…”

“…Together with the PACS data taken by the MESS guaranteed time program (Groenewegen et al 2011) coverage from 50 to 670 μm for all the stars in our sample. For OH 127.8+0.0, the data were taken as part of the calibration time (Lombaert et al 2013). We used the calibration files PACS_CAL_48_0 for our targets.…”

Nucleosynthesis in AGB stars traced by oxygen isotopic ratios

“…In order to determine the extent of these effects, a grid of model circumstellar envelopes was computed for an appropriate range of L , M, C/O and 17 O/ 18 O input abundances using the non-local thermodynamic equilibrium (NLTE) radiative-transfer code GASTRoNOoM (Decin et al 2006(Decin et al , 2010aLombaert et al 2013). Required optical-depth corrections on the observational 17 O/ 18 O ratios as determined through Eq.…”

The wind of W Hydrae as seen byHerschel

“…We calculate the dust temperature and emissivity as a function of radius with MCMax, a Monte Carlo dust continuum radiative transfer code presented by Min et al (2009). The two codes are combined to provide a consistent physical description of the gaseous and dusty components of the circumstellar envelope (Lombaert et al 2013).…”

“…They took the grain size distribution to be the same as found by Mathis et al (1977) for interstellar grains. Following Lombaert et al (2013), we instead represent the shape distribution of the dust particles by a continuous distribution of ellipsoids (CDE, Bohren & Huffman 1998;Min et al 2003). In the CDE approximation, the mass-extinction coefficients are determined for homogeneous particles with constant volume.…”