Matthias Maercker

Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris.

The asymptotic-giant-branch star R Sculptoris is surrounded by a detached shell of dust and gas. The shell originates from a thermal pulse during which the star underwent a brief period of increased mass loss. It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse—parameters that determine the lifetime of the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3″. What was previously thought to be only a thin, spherical shell with a clumpy structure is revealed to also contain a spiral structure. Spiral structures associated with circumstellar envelopes have been previously seen, leading to the conclusion that the systems must be binaries. Combining the observational data with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse about 1,800 years ago, lasting approximately 200 years. About 3 × 10−3 solar masses of material were ejected at a velocity of 14.3 km s−1 and at a rate around 30 times higher than the pre-pulse mass-loss rate. This shows that about three times more mass was returned to the interstellar medium during and immediately after the pulse than previously thought.

A chemical inventory of the S-type AGB star χ Cygni based on Herschel/HIFI observations of circumstellar line emission The importance of non-LTE chemical processes in a dynamical region

Context. S-type AGB stars (C/O ≈ 1) are thought to be transition objects from M-type (O-rich) AGB stars to carbon stars and as such are interesting objects in themselves. Of particular interest is to determine accurate circumstellar properties and molecular abundances, due to their predicted sensitivity to the photospheric C/O-ratio. Aims. Presented here are new sensitive sub-millimetre line observations of molecules towards the S-type AGB star χ Cyg, using the HIFI instrument on-board the Herschel Space Observatory. The observed lines predominantly probe warm gas relatively close to the central star. Methods. Detailed, non-LTE, radiative transfer modelling has been used in order to interpret the circumstellar molecular line observations performed using HIFI, assuming a spherically symmetric, smooth, accelerating wind. Results. Lines from common molecules such as H2O, CO and SiO, which are expected to be abundant in an S-type AGB star, are clearly detected (as well as some of their isotopologues) in the HIFI spectra. In addition, we detect lines from carbon-bearing molecules such as HCN and CN. The CO line modelling indicates that the mass-loss rate has not undergone any significant modulations during the past ≈1000 yr. The derived o-H2O fractional abundance is ≈7 × 10 −6 , i.e., lower than those obtained for a small sample of M-type AGB stars but higher than what has been derived for a few carbon stars. We further obtain a p-H2O fractional abundance of ≈5 × 10 −6 giving an o/p-ratio of ≈1.4. Molecular line cooling is dominated by H2O only in a region close to the star (6 × 10 14 cm). The SiO abundance is estimated to be ≈1 × 10 −5 .T he 12 CO/ 13 CO ratio is 43 ± 6. The high-excitation rotational lines clearly probe the acceleration region of the stellar wind (2 × 10 15 cm) and put constraints on dynamical wind models. We are unable to fit consistently the combined ground-based and HIFI data for HCN and CN. Conclusions. The derived H2O abundance is reasonably consistent with recent chemical model predictions and so is the SiO abundance. The o/p-ratio of ≈1.4 supports a chemical formation under non-LTE conditions for the H2O molecules, and the presence of carbon-bearing molecules at relatively large abundances is also indicative of the importance of non-LTE chemical processes in regulating the circumstellar chemistry. The velocity field derived from the molecular line modelling is consistent with that obtained from solving for the wind dynamics through the coupled momentum equations of the dust and gas particles.

Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae

Context. S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. Aims. We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. Methods. We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H2O line cooling as part of the solution of the energy balance. Results. We detect circumstellar molecular lines from CO, H2O, SiO, HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0 x 10(-6) M-circle dot yr(-1) based on the (CO)-C-12 lines. The estimated (CO)-C-12/(CO)-C-13 ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H2O abundance of 1.5 x 10(-5), which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H2O that is consistent with formation at warm temperatures. We find an HCN abundance of 3 x 10(-6), and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6 x 10(-6), and distribution of CN in W Aqls circumstellar envelope using ground-based data. We find an SiO abundance of 3 x 10(-6), and an NH3 abundance of 1.7 x 10(-5), confined to a small envelope. If we include uncertainties in the adopted circumstellar model - in the adopted abundance distributions, etc. - the errors in the abundances are of the order of factors of a few. The data also suggest that, in terms of HCN, S-type and M-type AGB stars are similar, and in terms of H2O, S-type AGB stars are more like C-type than M-type AGB stars. We detect excess blue-shifted emission in several molecular lines, possibly due to an asymmetric outflow. Conclusions. The estimated abundances of circumstellar HCN, SiO and H2O place W Aql in between M-and C-type AGB stars, i.e., the abundances are consistent with an S-type classification.

Circumstellar water vapour in M-type AGB stars: constraints from H2O(110-101) lines obtained with Odin

Context. A detailed radiative transfer code has been previously used to model circumstellar ortho-H2O line emission towards six M-type asymptotic giant branch stars using Infrared Space Observatory ...

Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL

We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study the photosphere, the warm molecular layer, and the inner wind of the close-by oxygen-rich AGB star R Doradus. We present observations in filters V, cntH

The wind of W Hydrae as seen by Herschel II. The molecular envelope of W Hydrae

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The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI ⋆

, and cnt820 and investigate the surface brightness distribution of the star and of the polarised light produced in the inner envelope. Thanks to second-epoch observations in cntH

Polarization of thermal molecular lines in the envelope of IK Tauri

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A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

, we are able to see variability on the stellar photosphere. We find that in the first epoch the surface brightness of R Dor is asymmetric in V and cntH

Dissecting the AGB star L-2 Puppis: a torus in the making

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